Frequently Asked Questions About West Nile Virus
and Mosquito Management

Frank B. Peairs, Professor of Entomology and Extension Specialist (reviewed 3/09)
Department of Bioagricultural Sciences and Pest Management
Colorado State University

Questions on West Nile Virus
Questions on Mosquito Biology
Questions on West Nile Virus and Human Health
Questions on West Nile Virus and Children
Questions on West Nile Virus and Horses
Questions on West Nile Virus and Pets
Questions on West Nile Virus and Birds
Questions on Personal Protection from Mosquito Bites
Questions on Mosquito Repellents
Questions on Devices used to Repel or Trap Mosquitoes
Questions on Mosquito Control Programs
Questions on Mosquito Larvicides
Questions on Mosquito Adulticides

Questions About West Nile Virus

What is West Nile virus?

West Nile virus is a mosquito-borne virus of the encephalitis group. The term West Nile virus is also now popularly used to describe the illness that can result from infection by the virus.

West Nile virus recently entered the United States.

What are West Nile virus, West Nile fever, and West Nile encephalitis?

These are different names for the same disease that indicate different levels of severity. West Nile virus is the name most commonly used.

What is encephalitis?

Encephalitis is an inflammation of the brain.

How do mosquitoes get and spread West Nile virus?

West Nile virus occurs mostly in birds. Mosquitoes pick the virus up when they feed on infected birds and transmit it when they feed on uninfected birds. When the number of infected birds and mosquitoes reach a certain level then transmission of the virus to other animals, including humans and horses, starts to occur.

Where did West Nile virus come from?

The first case of this disease was found in the West Nile area (Uganda) of Africa in 1937.

Historically, where has West Nile virus occurred worldwide?

Cases of West Nile virus have been found in Africa, Europe, west and Central Asia, and the Middle East. Outbreaks have occurred recently in Romania, the Czech Republic, Russia and Israel.

Is West Nile virus now established in the Western Hemisphere?

Yes, it has been found in Canada, the United States and Mexico.

How long has West Nile virus been in the U.S.?

The first US case of West Nile virus was detected in 1999.

Where was the first outbreak of West Nile virus in the US?

The first outbreak of West Nile virus in the United States occurred in New York City.

How long has West Nile virus been in Colorado?

The first Colorado case of West Nile virus was detected in 2002.

What is the basic transmission cycle of West Nile virus?

An infected mosquito feeds on an uninfected bird. The virus builds up in the bird. Another mosquito feeds on this bird and becomes infected. As this cycle is repeated the number of infected birds and mosquitoes increases. At some point the number of infected mosquitoes becomes great enough that transmission to other animals, including horses and humans, starts to occur.

Besides mosquitoes, can you get West Nile virus directly from other insects or ticks?

No.

How many types of animals have been found to be infected with West Nile virus?

West Nile virus has been detected in more than 100 kinds of birds and many mammals, including horses, dogs, and cats.

Can squirrels infected with West Nile virus transmit the virus to humans?

No.

Is West Nile virus seasonal in its occurrence?

Yes, most cases are contracted in July, August, and early September.

I understand West Nile virus was found in "overwintering" mosquitoes in the New York City area in early 2000. What does this mean?

This means that the virus is capable of surviving the winter months in North America. It is unknown what this means in terms of future outbreaks.

Will West Nile virus successfully overwinter in Colorado?

Yes.

Is West Nile virus here to stay in Colorado?

It seems likely that this disease is here to stay. It can overwinter here in mosquitoes, and it can be reintroduced each spring by migratory birds.

What measures are now being taken to protect the public from West Nile virus?

Measures vary greatly from community to community, but they usually involve monitoring mosquito and virus activity, controlling mosquito larvae, controlling mosquito adults, and encouraging individuals to protect themselves against mosquito bites.

Where can I get more information on West Nile virus?

Information on West Nile virus in Colorado can be obtained from at Fight the Bite Colorado (www.fightthebitecolorado.com/), the Colorado Department of Health and the Environment (www.cdphe.state.co.us/dc/Zoonosis/wnv/index.html) and from the Centers for Disease Control (www.cdc.gov/ncidod/dvbid/westnile/index.htm).

Questions on Mosquito Biology

How many kinds of mosquitoes are there?

There are about 3,500 different kinds of mosquitoes. About 40 of these are found in Colorado.

What is the life cycle of a mosquito?

There are four distinct stages in the mosquito life cycle: egg, larva, pupa and adult. Eggs are laid in or near water and hatch into larvae. The larval stage feeds on suspended organic material and a variety of small aquatic organisms. The pupae also are found in water, and this is the stage in which transformation from the larva to adult occurs. Adults emerge from pupae. Males feed on nectar and pollen. Females feed on nectar, pollen and blood from various hosts.

Where do mosquitoes live?

The egg stage may be found in soil (floodwater types) or on the surface of standing water. The larval and pupal stages are always found in water. Adults are found in vegetation or other protected resting areas (harborages) when they are not searching for hosts to feed on.

Where do mosquitoes live and breed?

Mosquito larvae are found in a variety of standing water habitats. Examples include old tires, tree holes, storm sewers, and vegetated edges of ponds. Adults rest in protected areas near larval habitats, but females may fly some distance in search of a blood meal.

What is the egg stage of a mosquito?

The egg is the first stage of the mosquito life cycle. Mosquito eggs are laid either near water or on water. Eggs laid near water do not hatch until they are covered with water. Eggs laid on water start hatching soon after they are placed.

How do mosquitoes lay their eggs?

There are two main type of egg-laying. The floodwater mosquitoes lay their eggs on moist soil near the edges of bodies of water. Mosquitoes in the other group lay floating "rafts" of 100 or more eggs on the surface of standing water.

What is the larval stage of a mosquito?

The larval stage is the second step in the mosquito life cycle. This is a stage of feeding and growth and takes place in standing water. Larvae feed on suspended organic material and a variety of small aquatic organisms. The larvae are often referred to as "wrigglers" because of the way they move. Some species live at the water surface, breathing through a specialized body part called a siphon. Others surface occasionally and a few remain submerged throughout the larval stage. Since controlling larvae prevents the occurrence of adults, the larval stage is often targeted by mosquito management programs.

What is the pupal stage of a mosquito?

Pupae do not feed and occur in water, usually at the surface. This is the stage in which transformation from the larva to adult occurs. Pupae are often referred to as "tumblers" because of the way they move.

What is the adult stage of a mosquito?

The adult stage is the final stage of the mosquito life cycle. It is when reproduction occurs for the next generation. Both males and females fly, but only females feed on blood and transmit diseases, including West Nile virus. Males feed only on pollen and nectar and are not involved in disease transmission.

What do female mosquitoes feed on?

Female mosquitoes feed on blood during their breeding season. They also may feed on nectar and other sources of moisture.

What do male mosquitoes feed on?

Male mosquitoes feed on nectar and other sources of moisture. They are not blood feeders. They can be distinguished from the female most easily by having more prominent, feathery antennae.

How far can an adult mosquito travel?

A few kinds of mosquitoes will fly only a few yards from their larval habitat to take a blood meal. More typically adults will fly a mile or two to feed.

How far do mosquitoes typically fly?

A few kinds of mosquitoes will fly only a few yards from their larval habitat to take a blood meal. More typically adults will fly a mile or two to feed. However, some mosquitoes have been observed to fly 10 miles or more.

How long do adult mosquitoes live?

During the summer, adult mosquitoes have a life span of a few weeks. However, some kinds of mosquitoes spend the winter as adults. These overwintering individuals have a life span of several months.

When can we expect mosquitoes to become active?

There are many kinds of mosquitoes in Colorado. Some emerge from snow pools early in the year. Others are common during the summer. The kinds expected to be important in transmission of West Nile virus are most active during July and August.

Why do mosquitoes make a buzzing or whining noise?

The female of each mosquito species has a unique wing beat frequency that is attractive to the male of the same species. We hear this sound as a buzz or whine. The noise is more audible in Culex mosquitoes than in Aedes mosquitoes.

Why do mosquitoes bite?

Adult females bite to obtain a blood meal, which provides them the necessary nutrition to produce eggs.

Why do mosquitoes leave welts when they bite?

When a mosquito bites she injects saliva containing anticoagulants and other substances into the wound to insure a continuous flow of blood while feeding. The welt is a local allergic reaction to some of the proteins found in the saliva.

Do all mosquitoes bite humans?

No. Mosquitoes tend to concentrate on certain animal groups such as birds. Humans are rarely first or second choice. Many, but not all, kinds of mosquitoes will feed on humans.

Why are some people bitten more by mosquitoes than others?

Mosquitoes use a variety of cues to find their hosts, including volatile chemicals (e.g., carbon dioxide, lactic acid, octenol), shape, color, heat, and humidity. Individuals vary in the type and amount of these cues they produce, so they also vary in their attractiveness to mosquitoes.

When are female mosquitoes most active?

Each kind of mosquito has a specific daily cycle of activity. There are usually one or two periods of flight per day. These can occur during the day, at night, or at dawn and dusk.

What time of year do mosquitoes stop taking blood meals?

This occurs in late summer as the weather cools and days start to shorten.

How long do mosquitoes live?

This depends on the kind of mosquito and the stage. Eggs laid on water hatch within a few days, while eggs laid near water may remain dormant for several years until they are flooded. The larval stage lasts one to three weeks at ideal temperatures, while the pupal stage lasts a week or less. During the summer, adult mosquitoes have a life span of a few weeks. However, some kinds of mosquitoes spend the winter as adults. These overwintering individuals have a life span of several months.

Where do mosquitoes go in the winter?

Some species overwinter as adults, which seek protected areas such as buildings to spend the winter. Other species overwinter as eggs on soil near bodies of water.

In what stage do mosquitoes spend the winter in Colorado?

The common "floodwater" mosquitoes of the genus Aedes spend the winter in the egg stage. Mosquitoes in the genera Culex, Culiseta, and Anopheles winter in the adult stage.

What other insects look like mosquitoes?

The non-biting midges are the insects most commonly mistaken for adult mosquitoes. These may resemble mosquitoes in many features but do not feed on blood and pose no human health risks. The non-biting midges are very common insects that develop in the bottom of ponds. Adults produce conspicuous mating swarms, which are commonly mistaken for "mosquito swarms".

Several other gnat-like flies also may sometimes be mistaken for mosquitoes. In spring the march flies (Bibionidae) may attract attention. A common group of insects that resemble very large mosquitoes are the crane flies (Tipulidae). These are sometimes called "mosquito hawks". Both march flies and crane flies develop in damp, organic matter-rich soils. Adults do not bite.

Are those swarms of insects I see at the golf course all mosquitoes?

Those swarms of small gnat-like flies you may be seeing about the golf course look a lot like mosquitoes but are harmless, non-biting midges. Like mosquitoes, the immature stages of the midges develop in water. Adults emerge when the water surface rises in the spring. Midges are most familiar due to the curious habit of the adult midges to aggregate about various landscape marks, such as rocks, vehicles, small shrubs - or even sometimes humans, where they engage in mating swarms. Swarming is most common in the spring, although they can occur in the summer and fall as well.

What kinds of diseases can be carried by mosquitoes?

Mosquitoes transmit many important diseases, such as yellow fever and malaria. In Colorado, important mosquito borne diseases include West Nile virus, western equine encephalitis, Saint Louis encephalitis, and heartworm in dogs.

Can mosquitoes carry other diseases besides West Nile virus?

Mosquitoes transmit many important diseases, such as yellow fever and malaria. In Colorado, important diseases include West Nile virus, western equine encephalitis, Saint Louis encephalitis, and heartworm in dogs.

Can mosquitoes transmit AIDS?

No. Several studies have been done to answer this question, with negative results.

Do all mosquitoes transmit disease?

Only a few of the several thousand kinds of mosquitoes in the world are considered to be important transmitters of human diseases.

Do all mosquitoes transmit West Nile virus?

No, for example, of the 40 or so kinds of mosquitoes in Colorado only two or three are expected to be important transmitters of West Nile virus.

What types of mosquitoes are known to transmit the West Nile Virus?

West Nile virus has been detected in more than 30 kinds of mosquitoes. However, only a very few species are expected to be important in West Nile virus transmission. The most prominent include the northern house mosquito, Culex pipiens, and Culex tarsalis, both of which occur in Colorado. Aedes vexans, is another potentially important transmitter that occurs locally.

How does the mosquito transmit the West Nile virus?

West Nile virus occurs mostly in birds. Mosquitoes pick the virus up when they feed on infected birds and transmit it when they feed on uninfected birds. When the number of infected birds and mosquitoes reaches a certain level then transmission to other animals, including humans and horses, occurs.

Can a mosquito acquire West Nile virus from a horse or human and subsequently transmit it to another animal?

West Nile virus does not build up in sufficient amounts in the blood of humans or horses to be acquired by a mosquito for subsequent transmission. West Nile virus occurs in much higher concentrations in blood of certain birds, which is why they are so important in the transmission cycle.

What is a "dead-end" host?

The virus usually does not build up to high enough levels to be picked up by further mosquitoes, except in birds. For this reason, humans, horses and other mammals that can't serve as a source of the virus are often referred to as "dead-end" disease hosts.

How are mosquitoes tested for West Nile virus?

Adult mosquitoes are collected and subjected to laboratory tests, such as PCR, capable of detecting the virus.

Questions on West Nile Virus and Human Health

How do people get infected with West Nile virus?

West Nile virus occurs mostly in birds. Mosquitoes pick the virus up when they feed on infected birds and transmit it when they feed on uninfected birds. When the number of infected birds and mosquitoes reaches a certain level then transmission to other animals, including humans and horses, starts to occur.

If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?

No. Only a small percentage of mosquitoes carry the virus, and only one person in five bitten by infectious mosquitoes will develop symptoms. Only one person out of 150 will develop severe or life-threatening symptoms.

What are the symptoms of West Nile virus?

About four out of five people infected with West Nile virus will not have any symptoms. About one out of five will develop the mild symptoms of West Nile fever: fever, headache, and body aches. Occasionally there will also be a skin rash on the trunk of the body and swollen lymph glands. About 1 in 150 people infected with West Nile virus develop the more severe symptoms of West Nile encephalitis or meningitis: headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. Mortality occurs in less than 1% of clinical cases.

Are there different kinds of symptoms of West Nile virus infection?

Mild symptoms of West Nile fever include: fever, headache, and body aches. The more severe symptoms of West Nile encephalitis or meningitis include: headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.

What is the incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile virus?

Four out of five infected persons do not develop symptoms. If symptoms develop, they appear within 3 to 14 days of infection.

How long do West Nile virus symptoms last?

The mild symptoms of West Nile fever may last about a week, while the severe symptoms of West Nile encephalitis may last two weeks or longer. Some nerve damage may be permanent.

How does West Nile virus actually cause severe illness and death in humans?

After infection the virus multiplies and moves to the brain and other parts of the nervous system. This interferes with the normal functioning of the nervous system and causes brain inflammation.

How long does the West Nile virus remain in a person's body after they are infected?

There is no evidence for chronic or long term West Nile virus infection.

If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?

Yes, infected persons develop immunity whether they develop symptoms or not. This is thought to be a life long immunity.

I think I have symptoms of West Nile virus. What should I do?

See your doctor if you are concerned that you may be infected with West Nile virus. See the doctor immediately if you develop any of the severe symptoms of this disease (high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis).

How do health care providers test for West Nile virus?

A laboratory blood test is used to test for West Nile virus. You should be tested if you have symptoms and are at risk of severe disease.

Is there a human vaccine against West Nile virus?

No. The only available vaccine is for use in horses.

How is West Nile virus treated?

There are no specific treatments for West Nile virus. However, the severe form of the disease may require hospitalization and treatment of symptoms.

How many cases of West Nile virus in humans have occurred in the U.S.?

In 2002 there were 4,161 cases with 277 fatalities. This was the first year the disease occurred in Colorado, with 13 reported cases.

What proportion of people infected with West Nile virus become severely ill?

About four out of five people infected with West Nile virus will not have any symptoms. About one out of five will develop the mild symptoms of West Nile fever: fever, headache, and body aches. Occasionally there will also be a skin rash on the trunk of the body and swollen lymph glands. About 1 in 150 people infected with West Nile virus develop the more severe symptoms of West Nile encephalitis or meningitis: headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. Mortality occurs in 3 - 15% of cases with severe symptoms.

What proportion of people die when infected with West Nile virus?

About 1 in 150 people infected with West Nile virus develop the more severe symptoms of West Nile encephalitis or meningitis. Mortality occurs in 3 to 15% of these cases.

Who is at risk for getting West Nile encephalitis?

Everyone in an area where the disease is prevalent is at risk of contracting West Nile virus. However, persons over the age of 50 are most at risk of developing the severe symptoms of West Nile encephalitis.

Who is at highest risk for becoming seriously ill?

Anyone in an area where the virus is active is at some risk of getting West Nile virus. However, persons over 50 years of age have the highest risk of severe disease. It is unknown if people with impaired immune systems have increased risk for West Nile virus disease.

Are infants at higher risk than other groups for illness with West Nile virus?

No, West Nile virus is rare in children under the age of one year. For example, only four of the 2500 confirmed cases in the US in 2002 were in this age group. Part of the reason for this low number, however, may be that infants are less exposed to mosquitoes than other age groups.

Are laboratory workers in contact with West Nile virus-positive specimens at risk for infection?

There are two reports of laboratory workers contracting the illness while handling West Nile virus-positive specimens.

Are duck and other wild game hunters at risk for West Nile virus infection?

They are at risk because hunting is an outdoor activity. There is no evidence for increased risk from handling or consuming game, as long as normal precautions are taken.

Can you get West Nile virus directly from birds?

No, there are no reports of transmission through handling live or dead infected birds. However, gloves or similar protection should be worn to handle dead birds or other animals.

Can you get West Nile virus from eating game birds or animals that have been infected?

No. However, game animals always should be prepared and cooked properly before consumption.

Can you get West Nile virus directly from crows?

No. There is no evidence for direct crow-to-person West Nile virus transmission. However, you should wear gloves if it is necessary to handle a dead or dying crow. There is some evidence for direct crow-to-crow transmission.

Can you get West Nile virus from another person?

No, there are no reports of West Nile virus transmission through normal contact with infected persons. There are a few reports of transmission through blood transfusions, donated organs, mother to child during pregnancy, and breast feeding.

Is West Nile virus transmitted by blood transfusion or organ donation?

There are several confirmed cases of people contracting West Nile virus from organ donations or blood transfusions.

What is being done about the possibility of transfusion-related West Nile virus transmission?

Blood products from persons known to be infected with West Nile are being withdrawn from the blood supply. Doctors have been requested to report any cases of West Nile virus in persons who have received organ donations or blood transfusions within the previous four weeks. These reports are used to locate additional potential sources of contaminated blood or organs.

Should people avoid donating blood or getting blood transfusions or organ transplants?

No. Donating blood save lives, and donors have no risk of contracting West Nile virus while giving blood. The potential West Nile virus health risks from receiving blood transfusions or organ donations are very low and are far outweighed by the benefits.

How can blood banks avoid collecting blood from donors who may have West Nile virus?

Blood banks do not accept donors who indicate that they have had any West Nile virus symptoms in the last few weeks. Also, the blood banks test their supplies for the presence of West Nile virus.

If a person has had West Nile virus, can they still donate blood?

Once the potential donor is fully recovered, there should be no risk in giving blood.

If I recently had a transfusion or transplant, should I be concerned about getting West Nile virus?

Contact your physician if you develop any of the symptoms associated with West Nile virus. However, the likelihood of contracting the disease in this manner is extremely low; you are much more likely to contract it from a mosquito bite.

Is there any evidence that West Nile virus is transmitted from mother to child during pregnancy or during birth?

There is one documented case of West Nile virus being transmitted from mother to child during pregnancy. The child was severely ill at birth, but it is not known whether the problems were related to the infection.

Pregnant women should follow standard recommendations to avoid mosquito bites if there is local virus activity. Wear shoes, socks, long pants and a long-sleeved shirt made from tightly-woven, light-colored fabric if you need to be outdoors for an extended period of time, especially in the evening. Also, use an effective mosquito repellent according to the instructions on the label.

Is a woman's pregnancy at risk if she gets infected with West Nile virus?

There is one documented case of West Nile virus being transmitted from mother to child during pregnancy. The child was severely ill at birth, but it is not known whether the problems were related to the infection.

Pregnant women should follow standard recommendations to avoid mosquito bites if there is local virus activity. Wear shoes, socks, long pants and a long-sleeved shirt made from tightly-woven, light-colored fabric if you need to be outdoors for an extended period of time, especially in the evening. Also, use an effective mosquito repellent according to the instructions on the label.

Can West Nile virus be transmitted through breast milk?

There is good evidence for one case of West Nile virus transmission through breast feeding. However, the child did not develop symptoms of the disease.

Should I continue breast-feeding if I am symptomatic for West Nile virus?

The benefits of breast feeding are well understood. The health risks, if any, of West Nile virus transmission through breast milk are unknown. Based on current information, the benefits of breast-feeding while symptomatic for West Nile virus seem to outweigh the risks.

Should I continue breast-feeding if I am not symptomatic for West Nile virus?

The benefits of breast-feeding are well understood. The health risks, if any, of West Nile virus transmission through breast milk are unknown. Based on current information, the benefits of breast-feeding if you do not have West Nile virus symptoms seem to outweigh the risks.

If I am breast-feeding, should I be tested for West Nile virus?

No, there is no need for testing.

If I am breast-feeding, should I use insect repellents containing DEET?

Yes. There are no known adverse health effects from using DEET based mosquito repellents to pregnant or breast feeding women.

Should outdoor activities in August and September be cancelled when there is evidence of West Nile virus activity?

No, it isn't necessary to limit any outdoor activities. However, you should take precautions to avoid mosquito bites. Follow recommendations to reduce mosquito activity around your home. If West Nile virus is active in your area wear shoes, socks, long pants and a long-sleeved shirt made from tightly-woven, light-colored fabric if you need to be outdoors for an extended period of time, especially in the evening. Also, use an effective mosquito repellent according to the instructions on the label.

I like camping and other activities in the mountains. Do the mosquitoes that transmit West Nile virus occur at high altitudes?

Culex tarsalis, one of the main West Nile virus vectors, commonly occurs up to altitudes of 8,500 feet and is found as high as 10,000 feet above sea level. Other potential vectors are more common above 8,500 feet and can be found well above this elevation. On the other hand, the transmission season becomes shorter as elevation increases, which probably reduces risk significantly. Given the lack of knowledge and experience with this disease in Colorado, it is prudent to use an effective repellent when mosquitoes are active, even at these higher elevations.

What should wild game hunters do to protect against West Nile virus infection?

Insect repellents should be applied to clothing and skin while hunting during mosquito season and periods of West Nile virus activity.

Who should wild game hunters contact for information about the risk for West Nile virus infection in specific geographic areas?

The Colorado Department of Public Health and the Environment maintains current information about West Nile virus and other outdoor health risks at http://www.cdphe.state.co.us/dc/zoonosis/index.html.

Questions on West Nile Virus and Children

Are children or infants at greater risk for becoming infected with West Nile Virus?

No, all age groups are equally likely to become infected, if their exposure to infective mosquitoes is similar. However, infected individuals older than 50 are more likely to develop the severe symptoms of West Nile encephalitis.

If an infected mosquito bites my child, will he or she get sick?

It is very unlikely. Only about one person in five bitten by an infectious mosquito develops symptoms. Furthermore, the chance of severe symptoms is lower in children than in older adults.

If a mosquito bites my child, should he or she be tested for West Nile Virus?

No. Only a very small percentage of mosquitoes carry the virus, and only one person in five bitten by infectious mosquitoes will develop symptoms. It your child was bitten when West Nile virus is known to be active and you are concerned about symptoms that are developing, then consult a physician.

What is the treatment for West Nile Virus in children?

There are no specific treatments for West Nile virus. However, severe disease (more likely in older adults) may require hospitalization and treatment of symptoms.

Is my child at risk for becoming infected with West Nile Virus while attending school?

No. Generally, the risk of infection is quite low. The key transmitters of West Nile virus (Culex mosquitoes do not bite during the day. Also, in Colorado the summer school vacation months of July and August are when most infections are expected to occur.

Should children avoid outdoor field trips and playing outdoors because of West Nile virus?

No, but precautions against mosquito bites should be taken if West Nile virus is known to be active in the area. Wear shoes, socks, long pants and a long-sleeved shirt when outdoors for long periods of time, especially during the evenings, which is the period when Culex mosquitoes are most active. Use an effective mosquito repellent according to the directions on the label.

What can I do around my home to help protect my children from exposure to mosquitoes?

Mosquitoes breed in standing water. The most effective means of reducing the number of mosquitoes around your home is to reduce or eliminate standing water. If standing water, such as in a fish pond, is permanent then it can be treated with floating Bti briquets or similar larvicides.

Questions on West Nile Virus and Horses

How do horses become infected with West Nile Virus?

West Nile virus occurs mostly in birds. Mosquitoes pick the virus up when they feed on infected birds and transmit it when they feed on uninfected birds. When virus levels in birds reach a certain level then mosquitoes can transmit the virus to other animals, especially horses.

What are the symptoms of West Nile virus infection in horses?

Symptoms include listlessness, stumbling, in coordination, weakness of the limbs, partial paralysis, and death. Usually there is not a fever.

How does the virus cause severe illness or death in horses?

After infection the virus multiplies and moves to the brain and other parts of the nervous system. This interferes with the normal functioning of the nervous system and causes brain inflammation.

Has West Nile virus caused severe illness or death in horses?

Yes, in 2002 there were nearly 15,000 cases of West Nile virus reported (378 in Colorado) in horses, with a mortality rate of 30-40% in cases where the animal developed severe symptoms.

How long will a horse infected with West Nile virus be infectious?

Most evidence indicates that the virus does not build to high enough levels in horses to be acquired by mosquitoes. In other words, horses become ill from West Nile virus, but they do not become infectious.

Can a horse infected with West Nile virus infect horses in neighboring stalls?

No. West Nile virus is transmitted by infectious mosquitoes. There is no evidence for horse-to-horse transmission of West Nile virus.

Can you get infected with West Nile virus by caring for an infected horse?

No. West Nile virus is transmitted by infectious mosquitoes. There is no evidence for horse-to-person transmission of West Nile virus.

Is there anything I can do to protect my horse against West Nile Virus?

The best way to protect your horse is to make sure that it has been properly vaccinated. The first time immunization takes two doses 3 to 6 weeks apart. The second dose should be completed before the peak season for mosquito activity (July-August). An annual booster is required every year thereafter. The vaccine is only available through a licensed veterinarian.

My horse is vaccinated against eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE). Will these vaccines protect my horse against West Nile virus infection?

No. There is a specific vaccine for West Nile virus that is necessary to protect your horse. The first time immunization takes two doses 3 to 6 weeks apart. The second dose should be completed before the peak season for mosquito activity (July-August). An annual booster is required every year thereafter. The vaccine is only available through a licensed veterinarian.

Can I vaccinate my horse against West Nile virus infection?

Yes. There is a specific vaccine for West Nile virus. The first time immunization takes two doses 3 to 6 weeks apart. The second dose should be completed before the peak season for mosquito activity (July-August). An annual booster is required every year thereafter. The vaccine is only available through a licensed veterinarian.

What is the treatment for a horse infected with West Nile virus?

Sick horses should be in the care of a veterinarian. Treatments address the symptoms displayed by the individual animal. There is no cure.

Should a horse with a West Nile virus infection be destroyed?

No. Since sick horse do not become infectious (mosquitoes can acquire the virus from the horse) and since there is no evidence for horse to horse transmission, there is no reason to destroy infected animals.

Where can I get more information on horses and West Nile virus?

APHIS Veterinary Services, www.aphis.usda.gov/vs/nahss/equine/wnv, and the Centers for Disease Control, http://www.cdc.gov/ncidod/dvbid/westnile/index.htm, have good West Nile virus web sites.

Questions on West Nile Virus and Pets

What signs of West Nile virus infection should I look for in domestic animals?

Except for horses most domestic animals are not expected to have noticeable symptoms of West Nile virus infection.

Can pets and livestock get West Nile virus infections?

More than 99% of the animal cases of West Nile virus infection have been detected in horses. A few infections have been reported from dogs, cats, squirrels and several other wild and domestic animal species.

What kinds of pets have been reported infected with West Nile virus?

More than 99% of the animal cases of West Nile virus infection have been detected in horses. A few infections have been reported from dogs, cats, squirrels and several other wild and domestic animal species.

Can West Nile virus cause illness in dogs and cats?

There have been a few reports of West Nile virus infection in dogs, but more than 99% of animal cases have been detected in horses. There is experimental evidence for infections in cats, cows, pigs, sheep, chickens and several other species.

Can sick dogs or cats be carriers and transmit West Nile Virus to people and other dogs or cats?

No, and there is no evidence that mosquitoes can acquire the virus from infected dogs or cats.

Can you get West Nile virus directly from birds, game or domestic animals?

This virus is transmitted mostly by mosquitoes. There is no evidence for animal-to-person transmission of West Nile virus, although there is some evidence for crow-to-crow transmission.

How can I protect pets and livestock from West Nile virus infection?

The best way to protect them is to reduce the amount of mosquito activity in the area. The most effective means of doing this is to reduce or eliminate all standing water. If standing water, such as in a fish pond or stock tank, is permanent then it can be treated with floating Bti briquets or similar larvicides. The use of insecticides to control adults also may be warranted in certain high risk situations. If possible, animals should be kept indoors at night.

Is there a vaccine for pets and livestock for West Nile virus?

There is a vaccine available for horses. There is little risk to other domestic animals.

What should I do if I suspect my pet has West Nile virus?

Contact your veterinarian if you are concerned that your pet might have West Nile virus infection.

How is West Nile virus infection in domestic animals treated?

Except for horses, most domestic animals have not shown symptoms of West Nile virus. Sick horses should be in the care of a veterinarian. Treatments address the symptoms displayed by the individual animal.

What is the treatment for a pet infected with West Nile Virus?

Except for horses, most domestic animals have not shown symptoms of West Nile virus. Sick horses should be in the care of a veterinarian. Treatments address the symptoms displayed by the individual animal.

Should a dog or cat infected with West Nile Virus be euthanized?

No. There is no evidence that mosquitoes can acquire the virus from infected dogs or cats. Also, the chance for fatal cases in dogs and cats is thought to be extremely low.

Questions on West Nile Virus and Birds

What bird(s) can carry West Nile virus?

During the first three years of West Nile virus in the United States most cases of infected birds involved American crows. However, the number of bird species affected by West Nile virus expanded greatly in 2002. There are now records of West Nile virus mortality in 127 native bird species and 11 exotic species.

How many birds have become infected with West Nile virus?

The number of individual birds sickened or killed is unknown, although it is thought that crows and related species have been most affected. The kinds of birds affected by West Nile virus expanded greatly in 2002.

What species of birds have become infected with West Nile virus?

There are now records of West Nile virus mortality in 127 native bird species and 11 exotic species. A list is kept at http://www.cdc.gov/ncidod/dvbid/westnile/birdspecies.htm.

Are pet birds or poultry at risk for West Nile virus?

Many different kinds of birds were reported killed by West Nile virus in 2002. We are still learning which kinds of birds are susceptible and what their risks are from West Nile virus. Assume that your pet birds and poultry are at some risk and keep them indoors, if possible, during high risk periods. If your birds cant be kept indoors then take steps to reduce the amount of local mosquito activity. The most effective means of reducing the number of mosquitoes around your home is to reduce or eliminate all standing water. If standing water, such as in a fish pond, is permanent then it can be treated with floating Bti briquets or similar larvicides.

Where in Colorado have birds infected with West Nile virus been reported?

West Nile virus arrived in the state late in 2002. Most infected birds were found in river valleys in the eastern part of the state. It is likely that infected birds will be found over a much wider area in the future.

Do birds infected with West Nile virus die or become ill?

Many different kinds of birds were reported killed by West Nile virus in 2002. We are still learning which kinds of birds are susceptible and what their risks are from West Nile virus.

Can a dead bird pose a risk to my pet?

There is no evidence to date for animal to animal transmission of West Nile virus.

Do I risk exposure to West Nile virus by handling a dead bird?

No, there are no reports of transmission through handling live or dead infected birds. However, gloves or similar protection should be worn to handle dead birds or other animals.

If I see a lot of crows roosting in an area, should I be concerned about West Nile virus?

No. However, if you see several dead crows you should report the location to your local health department.

How can I report a sighting of dead bird(s) for West Nile virus testing?

Contact your local health department for information on reporting and collecting dead birds. A list of Colorado health departments is found at www.cdphe.state.co.us/opp/locallist.html.

Why are crows, but not most other dead birds, tested for West Nile virus?

Crows are very susceptible to the virus, which builds to very high concentrations in their blood. A die-off of crows is a strong indicator of local West Nile virus activity. Other kinds of birds may not be as susceptible and thus not as useful in detecting the presence of the virus.

Why have some areas stopped collecting dead birds for West Nile virus testing?

Birds are collected to test for the presence of West Nile virus. Once it is certain that the virus is established in an area there is little to be gained from additional testing, which is expensive. However some public health agencies may monitor reports of dead birds, particularly highly susceptible species such as crows. This information can be useful in tracking seasonal activity of the virus.

What information about the dead bird do I need to give my local health department for West Nile virus testing?

Early in the season your health department may want information on how to get the bird from you for West Nile virus testing. Later in the season they will just want the location.

What should I do with the dead bird until it is picked up for testing for West Nile virus?

Your local health department will give you instructions on how to handle the bird. It is most likely that they will ask you to store it in a non-food refrigerator or freezer. Use gloves to handle the carcass. If a freezer is not available, you may be asked to cover the bird in ice and then cover the ice with a bucket.

What should I do if I'm told the dead bird does not require testing for West Nile virus?

The bird may be picked up with a gloved hand or shovel, double bagged in plastic and placed in the trash.

How long does it take to test a dead bird for West Nile virus?

This will change with the priorities of the health departments and the pattern of West Nile virus activity, so it is not possible to provide a specific answer for a given locality.

When will I receive the test results of the bird (s) that I submitted for testing for West Nile virus?

The amount of time required for testing and reporting results is variable and depends on the current priorities of the health departments and the pattern of West Nile virus activity.

How will West Nile virus test results be used?

Test results are used to monitor the activity of West Nile virus and to help decide whether it is necessary to implement mosquito control activities.

Questions on Personal Protection from Mosquito Bites

How can I help protect my family and myself from mosquitoes and the diseases, such as West Nile virus, they may carry?

If West Nile virus or other mosquito borne illnesses are known to be active in the area then precautions against mosquito bites should be taken. Wear shoes, socks, long pants and a long-sleeved shirt when outdoors for long periods of time, especially during the evenings. Use an effective mosquito repellent according to the directions on the label. Also, take steps to reduce mosquito activity around the home. The most effective means of doing this is to reduce or eliminate all standing water. If standing water, such as in a fish pond or stock tank, is permanent then it can be treated with floating Bti briquets or similar larvicides.

What can I do to reduce my risk of becoming infected with West Nile virus?

If West Nile virus or other mosquito borne illnesses are known to be active in the area then precautions against mosquito bites should be taken. Wear shoes, socks, long pants and a long-sleeved shirt when outdoors for long periods of time, especially during the evenings. Use an effective mosquito repellent according to the directions on the label. Also, take steps to reduce mosquito activity around the home. The most effective means of doing this is to reduce or eliminate all standing water. If standing water, such as in a fish pond or stock tank, is permanent then it can be treated with floating Bti briquets or similar larvicides.

What else can we do to protect ourselves from mosquito bites and West Nile virus?

If West Nile virus or other mosquito borne illnesses are known to be active in the area then precautions against mosquito bites should be taken. Wear shoes, socks, long pants and a long-sleeved shirt when outdoors for long periods of time, especially during the evenings. Use an effective mosquito repellent according to the directions on the label.

Should we stay indoors to protect ourselves from mosquito bites and West Nile virus?

The risk of contracting West Nile virus is low. Only a very small percentage of mosquitoes carry the virus, and only one person in five bitten by infectious mosquitoes will develop symptoms. If West Nile virus or other mosquito borne illnesses are known to be active in the area then precautions against mosquito bites should be taken. Wear shoes, socks, long pants and a long-sleeved shirt when outdoors for long periods of time, especially during the evenings. Use an effective mosquito repellent according to the directions on the label.

Can I fog my backyard for mosquitoes?

Yes, fogging will provide temporary relief from mosquitoes. However, new mosquitoes will quickly reinvade your backyard from nearby untreated areas, often later that same evening. The best way to control adult mosquitoes is through community or government sponsored programs that control adults over a wide area.

Can I treat a depression that floods in my backyard for mosquitoes?

Yes, the flooded area can be treated with floating Bti briquets or other approved larvicides. A longer term solution would be to fill in the depression, if feasible, or improve the drainage in that spot.

Is my farm a mosquito-breeding site?

If your farm, particularly the irrigation system, has water that stands for more than a week then it is a potential mosquito breeding site. Maintain your irrigation system so that water moves freely. Avoid leaks that allow ponding, and control vegetation that may impede water movement. If standing water, such as in a fish pond or stock tank, is permanent then it can be treated with floating Bti briquets or similar larvicides.

Questions on Mosquito Repellents

Why should I use insect repellent?

Mosquito repellents are an effective means of personal protection against mosquito bites. Mosquito repellents lower the risk of contracting West Nile virus and other diseases transmitted by mosquitoes. They allow you to continue to work and play outdoors even when mosquitoes are active. Use of a mosquito repellent is particularly important in areas where mosquitoes are active and management programs are not available or are not ongoing.

When should I use mosquito repellent?

The most important transmitters of West Nile virus are active at dusk and dawn. However, other kinds of mosquitoes, which also may be able to transmit the disease, bite during the day. To be on the safe side, use a mosquito repellent whenever you plan on being outdoors for extended periods of time, but especially during the evening hours.

What time of day should I wear mosquito repellent?

The most important transmitters of West Nile virus are active at dusk and dawn. However, other kinds of mosquitoes, which also may be able to transmit the disease, bite during the day. To be on the safe side, use a mosquito repellent whenever you plan on being outdoors for extended periods of time, especially during the evening hours.

How often should mosquito repellent be reapplied?

Follow the instructions on the label. Mosquito repellent products differ in how long they last depending on the amount and type of active ingredient, as well as how it is formulated.

Should I wear mosquito repellent while I am indoors?

It is generally not necessary to use mosquito repellents indoors. If you are getting bitten indoors there are other steps to eliminate mosquito activity that should be taken first. The most important is to make sure all windows and doors are screened and that the screens are in good repair.

How does mosquito repellent work?

Mosquito repellents interfere with the female mosquito's ability to find its host. They work over fairly short distances so the mosquitoes remain in the area but do not land on your skin.

Which mosquito repellent works the best?

Most studies have shown that the mosquito repellent called DEET is the most effective, and most public health agencies recommend the use of mosquito repellent products with this active ingredient. There are products with other active ingredients available, which generally have to be applied much more frequently than DEET products in order to be effective.

How does the percentage of DEET in a product relate to the amount of protection it gives?

The percentage DEET affects the amount of time that a single application of the product will last. Lower percentage products give the same level of protection as more concentrated products, but do not last as long. Some studies indicate that the optimum concentration is around 30%, with higher concentration products not providing much additional protection.

Why does the Centers for Disease Control recommend using DEET?

DEET is the safest and most effective mosquito repellent known. It has been used billions of times over the last 50 years with only a few reports of adverse effects. The risk of adverse effects from DEET use is substantially lower than the risk of contracting West Nile virus or other mosquito-borne illness while using a less effective mosquito repellent.

Are non-DEET mosquito repellents effective (e.g. Skin-So-Soft, plant-based repellents)?

Generally alternatives to DEET are effective for relatively short periods of time, often ½ hour or less, which means they must be reapplied frequently. Also, alternative products are made from ingredients that have not been tested so thoroughly for safety and effectiveness as DEET.

I'm confused. None of the mosquito repellent products in the store says "DEET".

Until recently, repellent labels were required to use the approved chemical names of active ingredients, which for DEET is either N,N-diethyl-m-toluamide or N,N-diethly-3-methylbenamide. EPA now allows labels to use DEET as well.

Is DEET safe?

It has been used billions of times over the last 50 years with only a handful of reports of adverse effects. The risk of adverse effects from DEET use is substantially lower than the risk of contracting West Nile virus or other mosquito-borne illness while using a less effective mosquito repellent. DEET is the safest and most effective mosquito repellent known.

What are some general considerations to remember in order to use mosquito repellent products containing DEET safely?

Always follow the label instructions on the product container. Generally these precautions will include:

  • Treat only exposed skin.
  • Do not treat cuts, wounds, or irritated skin.
  • Wash treated skin with soap and water when the mosquito repellent is no longer needed.
  • Apply sprays only in well ventilated areas.
  • Do not spray mosquito repellent directly on your face. Treat hands and then rub mosquito repellent on face.

How should mosquito repellent products containing DEET be used on children?

The American Academy of Pediatrics recommends against treating infants younger than two months with DEET. They recommend products containing about 30% DEET for both adults and children. Parents concerned about possible adverse effects of DEET can use lower concentration products (10 - 15%) to protect their children from mosquito bites if the local risk of West Nile virus and other mosquito borne illnesses is low.

DEET-free mosquito repellent products are also available, but are not considered to be as effective as those that contain DEET. Also, non-DEET mosquito repellents have not been tested nearly as much as DEET for child safety.

Is DEET safe for pregnant or nursing women?

There are no reports of adverse effects from DEET use by pregnant or nursing women.

Are there any risks due to using mosquito repellents containing DEET?

Adverse reactions are rare, with the most common one being skin rash. The very few cases of more severe reactions have been associated mostly with failure to follow label precautions. If you suspect an adverse reaction, stop using the product and consult your physician. Be sure to bring the product container with you to the physician's office or the hospital.

Should parents spray insect repellent on their children before they go to school?

This will depend on the amount of time the child will be outdoors during the day, as well as the amount of local mosquito activity and risks from West Nile virus or other mosquito borne illnesses.

Should children be given mosquito repellent to use during the day?

This will depend partly on school policy. It is also important to be sure that the child is capable of applying the mosquito repellent safely and according to the label instructions.

Is there a plant that repels mosquitoes?

The "mosquito plant" is a geranium that produces small amounts of citronella, a known mosquito repellent. The citronella is released only when leaves are wounded or crushed.

Where can I get more information about mosquito repellents?

The EPA has DEET information at http://www.epa.gov/pesticides/factsheets/chemicals/deet.htm.

The most recent American Academy of Pediatrics information on mosquito repellents is found at http://www2.aap.org/family/wnv.htm.

Colorado State University has information on mosquito repellents at

http://www.ext.colostate.edu/westnile/mosquito_mgt.html.

Questions on Devices used to Repel or Trap Mosquitoes

What are "attract and kill" mosquito control devices?

Attract and kill devices use different combinations of ultraviolet light, CO2, and octenol to attract mosquitoes to an electrocuting grid. "Bug zappers" rely on just ultraviolet light for attraction, are ineffective in reducing mosquito biting rates and kill many harmless or beneficial insects, as well. In one study, just 0.13% of the insects killed by one of these devices were biting flies, including female mosquitoes. Another study estimated that up to 350 billion beneficial insects are killed annually in the United States by these devices.

Why does carbon dioxide make "attract and kill" mosquito control devices more effective?

Female mosquitoes use volatile chemicals, such as CO2, find their hosts. As they get close to the trap, they detect the CO2 and move towards the higher concentrations in the trap. More recently, traps that produce octenol, as well as CO2, have been shown to be even more attractive to female mosquitoes.

Are "bug zappers" effective in controlling mosquitoes or preventing mosquito bites?

No. "Bug zappers" rely on just ultraviolet light for attraction and are ineffective in reducing mosquito biting rates and kill many harmless or beneficial insects, as well. In one study, just 0.13% of the insects killed by one of these devices were biting flies, including female mosquitoes. Another study estimated that up to 350 billion beneficial insects are killed annually in the United States by these devices. No controlled scientific study has shown that these devices reduce mosquito biting rates outdoors.

Are mosquito traps that use CO2 and/or octenol effective in controlling mosquitoes or preventing mosquito bites?

Attract and kill devices become more effective at trapping mosquitoes as CO2 and octenol attractants are added. The most effective of these traps has been shown to be as effective as DEET in a contained environment (large outdoor tent) with a known number of mosquitoes. There is no scientific evidence to date that they are effective in reducing mosquito biting rates under natural conditions.

Where can I find more information on "attract and kill" mosquito control devices?

The American Mosquito Control Association has information on mosquito traps at http://www.mosquito.org/mosquito-information/traps.aspx. Colorado State University also has some information on these devices at http://www.ext.colostate.edu/westnile/mosquito_mgt.html.

Do yellow 'bug' lights repel mosquitoes?

No. Lighting that lacks certain wave lengths may not be attractive to mosquitoes, or other night flying insects. However, it has no repellent effect.

What are sonic mosquito repellers?

Sonic repellers were first marketed in the 1970s. They purported to imitate the flight sound of a male mosquito and worked on the assumption that a mated female mosquito would avoid further contact with males. Other devices were supposed imitate the flight sounds of dragonflies. Although testimonials as to their effectiveness abound, whenever these devices have been tested in the laboratory or field, they have shown no repellency.

Are sonic mosquito repellers effective in preventing mosquito bites?

There are many testimonials as to the effectiveness of sonic repellers. However, these devices when tested in the laboratory or field never have shown repellency.

What are ultrasonic mosquito repellers?

Devices have been developed, which are supposed to mimic the sounds made by bats. These have been tested in controlled experiments and were found to be ineffective in reducing mosquito biting rates. While some insects do avoid bats, mosquitoes do not.

Are ultrasonic mosquito repellers effective in preventing mosquito bites?

No. Ultrasonic mosquito repellers are supposed to mimic the sounds made by bats, which are known to be important mosquito predators. These devices have been tested in controlled experiments and were found to be ineffective in reducing mosquito biting rates.

Questions on Mosquito Control Programs

What can be done to prevent outbreaks of West Nile virus?

West Nile virus outbreaks are prevented through mosquito control programs. The components of a an effective mosquito control programs usually include (1) surveillance of mosquito and virus activity; (2) preventive control of mosquito larvae through reduction of standing water and treatment of water with larvicides; (3) personal protection through the use of mosquito repellents and proper clothing; and (4) control of adults with insecticides if other measures fail to keep the risk of West Nile virus transmission below acceptable levels.

Does the State control mosquitoes, or do the counties?

In Colorado, mosquito control programs operate at the county or city level. Contact your local health department to find out how mosquitoes are controlled in your area.

What is Colorado doing to control the mosquito population should West Nile virus return?

There is no statewide mosquito control program. However, the Colorado Department of Health and the Environment assist local mosquito control programs in determining local risks for West Nile virus and other mosquito borne diseases and the need for mosquito control. Consult with your local health department to find out what plans there are for local or community mosquito control programs.

What agency conducts mosquito control in my town?

Ask your local public health department about how mosquitoes are controlled in your community.

What is being done in my community to control mosquitoes?

Colorado communities vary in mosquito control from no control program to organized yearly control efforts. Contact your local Department of Health to find out what your community is doing to control mosquitoes.

How do mosquito control agencies know how, when, and where to treat?

Effective mosquito control programs monitor for mosquito and virus activity. They map out areas of mosquito and disease activity and increase their control efforts where mosquitoes and virus levels are increasing. As a last resort, they may spray for adults if the health risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide.

What agencies conduct mosquito and West Nile virus surveillance and provide mosquito control services?

This will vary across the state. Ask your local health department how mosquitoes are managed in your community.

What are the various methods of surveillance used to trap mosquitoes?

A variety of methods have been developed over the years for monitoring mosquitoes. Larval mosquitoes typically are collected from standing water with a long handled white ladle (dipper). Adults can be collected in their resting areas with handheld vacuums or with a light trap. Light traps are usually a combination of a black light, sources of attractive volatile chemicals such as CO2 and octenol, and a receptacle to trap and hold the adult mosquitoes.

If there are public health advisories to avoid areas with high concentrations of mosquitoes, should I cancel planned outdoor activities in the evening?

It isn't necessary to limit any outdoor activities. You should take precautions to avoid mosquito bites if high mosquito populations are present. Follow recommendations to reduce mosquito activity around your home. Wear shoes, socks, long pants and a long-sleeved shirt made from tightly-woven, light-colored fabric if you need to be outdoors for an extended period of time, especially in the evening. Also, use an effective mosquito repellent according to the instructions on the label.

What is the sentinel chicken surveillance program and how are the chickens tested?

West Nile virus and related mosquito borne illnesses build up in bird populations before they move into humans, horses and other less favorable hosts. One way to monitor virus buildup is to cage chickens in high risk areas. Blood samples are taken periodically and tested for the presence and amount of virus.

What types of spraying for mosquitoes take place during the year and where?

Most control efforts are directed at controlling mosquito larvae. Typically, breeding areas are treated with granular or floating "briquet" insecticides. Adult control (fogging) occurs later in the season, usually in July and August, in limited areas where surveys indicate that preventive measures have failed to keep the risk of West Nile virus and other mosquito borne illnesses below acceptable levels.

Are storm water catch basins significant breeding areas for West Nile virus mosquito vectors?

Yes, any source of standing water can be an important source of mosquitoes. Mosquito control projects often treat these structures with larvicides.

Are residential storm water retention basins significant breeding areas for West Nile virus mosquito vectors?

Yes, any source of standing water can be an important source of mosquitoes. Mosquito control projects often treat these structures with larvicides.

Are storm water retention wetlands/ponds significant breeding areas for West Nile virus mosquito vectors?

Yes, any source of standing water can be an important source of mosquitoes. Mosquito control projects often treat these areas with larvicides.

What type of insecticides/pesticides are used to control mosquitoes?

Several types of insecticides are used to control mosquitoes. Mosquito larvae are controlled most commonly with a bacteria called Bti or an insect hormone mimic called methoprene. Another bacterial insecticide is based on Bacillus sphaericus. The surface of breeding sites are sometimes treated with oils or similar surfactants, and a conventional insecticide called temephos is used as well. The insecticides used to fog for adult mosquitoes include Anvil (sumithrin and piperonyl butoxide), malathion, and Scourge (resmethrin and piperonyl butoxide). Other insecticides, such as permethrin, are used to control adult mosquitoes around homes and out buildings.

How are adult mosquitoes controlled?

Adult mosquitoes are controlled by treating high risk areas with insecticides applied with ground equipment or aircraft. Adult control is usually done as a last resort when the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide. Adult control lasts a few days at best as the killed mosquitoes are replaced by others that have flown in from untreated areas or that have emerged from breeding sites after the treatment. Adult control may have to be repeated several times until the risk of disease transmission is sufficiently reduced.

Will my entire county, or the state, be sprayed either by trucks or aircraft?

No. West Nile virus activity tends to concentrate in certain high risk areas where infective mosquitoes and birds occur together. Effective mosquito control programs monitor mosquito and virus activity and spray for adults only in areas where the risk of West Nile virus or other mosquito borne illnesses is above acceptable levels.

Why is the local government fogging for mosquitoes in my community?

If local health officials decide to spray, this means that they have decided that the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide.

Will local officials notify me before fogging for adult mosquitoes?

Yes, public notice is required before fogging for mosquitoes.

Why do they fog for mosquitoes when I am out taking my evening walk?

The mosquitoes that are most important in West Nile virus transmission are active during the evening and early morning hours. Fogging is more effective if the adult mosquitoes are active, so the evening is one of the best times for this control activity.

How long does the fog kill mosquitoes?

The killing effects of the fog only last for an hour or two. Mosquitoes that are killed are replaced by others that have flown in from untreated areas or that have emerged from breeding sites after the treatment.

If the city has been fogged for mosquitoes, are all mosquitoes in my area eliminated?

No, not all the mosquitoes in the treated area will be killed. A 50% reduction in adult mosquito after fogging often is considered to be a successful treatment. Also, those that are killed will be quickly replaced by others that have flown in from untreated areas or that have emerged from breeding sites after the treatment. The objective of fogging for mosquitoes is to kill enough adult female mosquitoes so the disease transmission cycle is broken and the risk of West Nile virus and other mosquito borne illnesses is reduced to an acceptable level.

Will the fogging for mosquitoes kill birds or other large animals?

No, mosquito fogging products are selected for their relative safety. Also, they are applied in small amounts and break down quickly in the environment.

Are individuals who do mosquito fogging required to be licensed?

It depends on whether they are public employees as well as on what product they are applying. Anyone who applies a Restricted Use Product must be certified by the Colorado Department of Agriculture (CDA). A person hired to fog for mosquitoes, who is not a public employee, needs to have a CDA commercial applicator license. A public employee applying a Restricted Use Product to control mosquitoes must be licensed by CDA in public health pest control. However, a public employee spraying for mosquitoes with a nonrestricted insecticide product does not need to be certified or licensed.

What can I do if there is spraying in my community?

The insecticides used to control adult mosquitoes are among the safest available chemicals, but they do carry some health risks. If local health officials decide to spray, this means that they have decided that the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide. There are precautions you can take to minimize your exposure during the application, which include:

  • Keep in mind that these insecticides are short lived and most of the risk occurs during the application and for an hour or so afterwards.
  • Remain inside during the spraying.
  • Keep children inside during spraying and for about one hour after spraying.
  • Close your windows and doors. If you use an air conditioner turn it off or set it to recirculate indoor air.
  • Avoid eye contact with the spray if you are outside during spraying, and rinse your eyes with water or eye drops if they do come in contact with the spray.
  • Wash skin surfaces that come into contact with the insecticide.
  • Rinse fruits and vegetables from your garden thoroughly with water before cooking or eating.
  • Cover outdoor furniture and play equipment before the application, or wash them with soap and water afterwards.
  • Bring in laundry and toys before spraying or wash them afterwards.
  • Bring pets inside and cover ornamental fishponds during the application.
  • Consult your doctor if you think your health was affected by the spraying.

Can communities use other methods to control mosquitoes besides fogging?

Yes. The components of a an effective mosquito control programs usually include (1) surveillance of mosquito and virus activity; (2) preventive control of mosquito larvae through reduction of standing water and treatment of water with larvicides; (3) personal protection through the use of mosquito repellents and proper clothing; and (4) control of adults with insecticides if other measures fail to keep the risk of West Nile virus or other mosquito borne illnesses below acceptable levels.

What insecticides are used to fog for mosquitoes?

The insecticides used to fog for adult mosquitoes include Anvil (sumithrin and piperonyl butoxide), malathion, and Scourge (resmethrin and piperonyl butoxide). Other insecticides are used to control mosquito larvae in water or to control mosquitoes around the home.

Are the insecticides/pesticides used for mosquitoes safe?

Pesticides are tested extensively for human and environmental safety before their use is allowed. Because of the way they are applied, mosquito treatments use some of the safest available insecticides. Certain mosquito larvicides need to be safe enough to use in sources of drinking water and to avoid harming other aquatic life. However, no insecticide is completely safe so adult mosquito sprays used in inhabited areas are applied only when public health officials decide that the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide.

Are the insecticides used for fogging for mosquitoes safe?

Pesticides are tested extensively for human and environmental safety before their use is allowed. Because of the way they are applied, mosquito sprays use some of the safest available insecticides. However, no insecticide is completely safe so mosquito fogging is done only when public health officials decide that the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide.

What health risks are posed to people and pets from pesticides for adult mosquitoes?

The insecticides used to control adult mosquitoes are among the safest available chemicals, but they do carry some health risks. The risks are affected by the type of product, the application method, the amount used, the length of exposure, and many individual characteristics such as age, gender, general health or lifestyle. Consult your local health department for information on the mosquito control products they are using. The Environmental Protection Agency has a website with toxicology information on the pesticides used to control mosquitoes, http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm.

How do pesticide regulations protect me (and the environment) during mosquito control programs?

Pesticides are tested extensively for human and environmental safety before their use is allowed. Because of the way they are applied, mosquito sprays use some of the safest available insecticides. However, no insecticide is completely safe so adult mosquito sprays are made only when public health officials decide that the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide.

Where can I get information about the pesticide sprays that are being used for mosquito control?

Consult your local health department for information on the mosquito control products they are using. The Environmental Protection Agency has a website with toxicology information on the pesticides used to control mosquitoes, http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm.

Do I need to wash home-grown fruits and vegetables after the mosquito fogging?

Rinse fruits and vegetables from your garden thoroughly with water before cooking or eating.

What should I do if I have medical questions about the insecticides used to control mosquitoes?

Consult your local health department for information on the mosquito control products they are using. Your physician can provide you information on potential health risks from a given insecticide. The Environmental Protection Agency has a website with toxicology information on the pesticides used to control mosquitoes, http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm.

Questions on MosquitoLarvicides

What are larvicides?

Larvicides are insecticides applied to control mosquito larvae.

In what form are mosquito larvicides used?

Typically, breeding areas are treated with granular or floating "briquet" insecticides. The most commonly used active ingredients are two kinds of bacteria (Bti and Bacillus sphaericus) and an insect hormone mimic called methoprene. A conventional insecticide called temephos also is used under certain conditions where Bti and methoprene have limited effectiveness. The surface of breeding sites is sometimes treated with oils or other surfactants.

How do oils and surfactants kill mosquito larvae?

Oils and surfactants spread across the water surface in a very thin layer. When mosquito larvae come to the surface to breathe, the oils or surfactants penetrate their respiratory system and suffocate them.

Where are larvicides used to kill mosquitoes?

Larvicides are applied to standing water, which is where mosquito larvae are found.

Are larvicides any better at killing mosquitoes than pesticides that attack adult mosquitoes (adulticides)?

The use of mosquito larvicides generally is considered preferable to the use of adulticides for several reasons. (1) Use of mosquito larvicides prevents the appearance of the blood feeding adults. (2) Mosquito larvicides can provide up to a month of control, rather than the few hours provided by fogging with adulticides. (3) The commonly used mosquito larvicides are less toxic than the adulticides and are applied in such a way that there is much less human exposure. (4) Mosquito larvicides generally are applied to smaller areas than are adulticides.

When will larvicides be used for mosquito control?

Larvicides are used preventively, so applications start early in the mosquito season and may be repeated several times. The mosquito season will vary throughout the state, typically May - August at lower elevations with progressively shorter periods as you move to higher elevations.

If we use larvicides for mosquito control now, does that mean the use of pesticides for adult mosquitoes won't be needed later on?

The use of mosquito larvicides minimizes the need for adult control, but may not eliminate it altogether. Adult mosquitoes are strong fliers and may travel several miles to feed. Also, certain breeding areas may be inaccessible or may be untreatable for environmental or economic reasons.

Are mosquito larvicides available to the public for home use?

The best way to eliminate mosquito larvae around the home is to eliminate standing water (clogged gutters, buckets, etc) or change it frequently (birdbaths, wading pools). However, Bti and methoprene based larvicides are available to the homeowner for treating permanent standing water such as ornamental fish ponds or stock tanks.

Am I likely to be exposed to mosquito larvicides?

If you apply homeowner mosquito larvicides according to the label your exposure will be within acceptable limits. Label precautions for larvicides applied by public mosquito control programs also are designed to keep public exposure within acceptable limits. In addition, the bacterial larvicides are toxic only to mosquito larvae and a few closely related insects. Methoprene is toxic only to certain immature insects.

What problems could I have if I am exposed to mosquito larvicides?

There are no known serious health threats associated with commonly used mosquito larvicides.

The Environmental Protection Agency has a website with toxicology information on the pesticides used to control mosquitoes, http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm.

What should I do if I am exposed to mosquito larvicides?

If you are concerned that you are suffering adverse health effects from exposure to mosquito larvicides, contact your local health department to find out what products were used. After you know what you were exposed to you may consult with your doctor or contact the Rocky Mountain Poison and Drug Center (1-800-332-3073) for further information.

What is Bti?

Bti are the initials of a bacteria, Bacillus thuringiensis israelensis. This bacteria produces a protein that is toxic to mosquito larvae and other closely related insects. It is considered nontoxic to humans and other animals. Because of its effectiveness against mosquito larvae and its relative safety it is a very important larvicide. It is available in several formulations under several brand names.

What is methoprene?

Methoprene is a type of insecticide known as an insect growth regulator. Insects, including mosquitoes, control their growth and development through the balance of two hormones. Methoprene disrupts this balance by mimicking one of the hormones, thus preventing normal growth and development.

What is ALTOSID?

Altosid is the commercial name for several mosquito larvicides that contain methoprene. The methoprene larvicide available to homeowners is Zodiac Preventive Mosquito Control.

What is VECTOLEX?

Vectolex is the commercial name for several mosquito larvicides that contain Bacillus sphaericus.

What is VECTOBAC?

Vectobac is the commercial name for several mosquito larvicides that contain Bacillus thuringiensis israelensis (Bti). Bti larvicides also are sold under the names Aquabac, Bactimos, Bti Briquets, Mosquito Dunks, Mosquito Bits, and Teknar. Several of these products are available to homeowners.

What are mosquitofish?

Mosquitofish, also known as Gambusia, are native to the southern United States and are know to be effective biological controls for mosquito larvae.

Are mosquitofish (Gambusia) the only fish I can use to control mosquito larvae?

No, many kinds of fish eat mosquito larvae. Alternatives include fathead minnows, fingerling trout and fingerling bluegill.

What kind of fish should I use to control mosquito larvae in eastern Colorado?

Stocking of fish is controlled by the Division of Wildlife (DOW). DOW recommends against stocking Gambusia in eastern Colorado waters that contain native fish species or that are connected to waters containing native fish species because the Gambusia can displace the natives. Consider the alternatives such as fathead minnows, fingerling trout, and fingerling bluegill. Fingerling bluegills are recommended where sport fisheries exist.

What kind of fish should I use to control mosquito larvae on Colorado's Western Slope?

Stocking of fish is controlled by the Division of Wildlife (DOW). DOW recommends only fingerling trout for mosquito control on the Western Slope. The use of Gambusia is prohibited in most situations.

Where can I obtain fish for mosquito control?

Producers of fathead minnows, fingerling trout and other alternatives to Gambusia can be found through the Colorado Aquaculture Association at www.colaqua.org/. Keep in mind that importation licenses are required for any fish purchased through out-of-state sources.

Questions on Mosquito Adulticides

What are mosquito adulticides?

Adulticides are insecticides applied to kill adults mosquitoes. The most commonly used adulticides include Anvil (sumithrin and piperonyl butoxide), malathion, and Scourge (resmethrin and piperonyl butoxide). Other insecticides, such as permethrin, are used to control adult mosquitoes around homes and out buildings.

What is adulticiding?

Adulticiding is the application of insecticides to control mosquito adults. This is usually the least efficient mosquito control technique, however, it is an important part of an effective mosquito management program if it is based on mosquito surveillance information.

How does mosquito adulticiding work?

The idea behind mosquito adulticiding to release the insecticide in as many very fine droplets as possible. Adult mosquitoes are killed when they come into contact with one of these droplets as they are flying. Very fine droplets are used because the more droplets there are and the longer the droplets stay in the air, the better the chances are that a droplet will come in contact with a mosquito.

In what form are mosquito adulticides used?

Adulticides are most commonly applied as very fine droplets to quickly kill adult mosquitoes present in a given area. Longer-lasting adulticides also may be applied to known adult resting areas to provide longer term control. Such treatments are referred to as barrier treatments.

Where are adulticides used to kill mosquitoes?

Mosquito adulticides usually are applied as a last resort in areas where public health officials have decided that the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide.

When will adulticides be used for mosquito control?

Mosquito adulticides are usually applied as a last resort when public health officials have decided that the healths risks from West Nile virus or other mosquito borne illness outweigh the risk from exposure to the insecticide. The public is notified when this decision is made.

How are mosquito adulticides applied?

Mosquito adulticides are most often applied as very fine droplets that are intended to come into contact and kill flying mosquitoes. This is often called fogging. Longer-lasting adulticides also may be applied to known adult resting areas to provide longer term control. Such treatments are referred to as barrier treatments.

What are barrier treatments for control of mosquito adults?

Barrier treatments are intended to make a chemical barrier between a mosquito breeding site and an inhabited area. Typically, these are applied by homeowners with handheld equipment to vegetation and other mosquito resting areas around the edges of their property. The insecticides used for barrier treatments last much longer in the environment that those applied by fogging. Public or organized mosquito control program generally do not use barrier treatments as one of their main control strategies.

Why is adulticiding often called fogging?

Adulticides used to be applied with thermal applicators that released a smoke-insecticide mixture that looked like fog. While these applicators are still in limited use today, most adulticides are applied with truck or aircraft mounted equipment, using small amounts of insecticides mixed in a small amount of water or other carrier. The correct name for these applications is ultra low volume (ULV), but the term fogging is still commonly used.

Are adulticides any better at killing mosquitoes than pesticides that attack larval mosquitoes (larvicides)?

The use of mosquito larvicides generally is considered preferable to the use of adulticides for several reasons. (1) Use of mosquito larvicides prevents the appearance of the blood feeding adults. (2) Mosquito larvicides can provide up to a month of control, rather than the few hours provided by fogging with adulticides. (3) The commonly used mosquito larvicides are less toxic than the adulticides and are applied in such a way that there is much less human exposure. (4) Mosquito larvicides generally are applied to smaller areas than are adulticides.

If we use adulticides for mosquito control now, does that mean we do not need to use mosquito larvicides?

Effective mosquito control programs emphasis larval control. Mosquito larvicides are used first and adulticides are used only as a last resort. The use of larvicides minimizes the need for adult control, but may not eliminate it altogether. Adult mosquitoes are strong fliers and may move several miles to feed. Also, certain breeding areas may be inaccessible or may be untreatable for environmental or economic reasons.

Am I likely to be exposed to mosquito adulticides?

Mosquito adulticides are used only in certain high risk areas. If adulticides have been used in your community in the past it is likely that they will be used again. Additional communities may be treated during a bad mosquito year or if West Nile virus or other mosquito borne illnesses are active. Precautions to minimize your exposure to mosquito adulticides if it becomes necessary to treat include:

  • Keep in mind that these insecticides are short lived and most of the risk occurs during the application and for an hour or so afterwards.
  • Remain inside during the spraying.
  • Keep children inside during spraying and for about one hour after spraying.
  • Close your windows and doors. If you use an air conditioner turn it off or set it to recirculate indoor air.
  • Avoid eye contact with the spray if you are outside during spraying, and rinse your eyes with water or eye drops if they do come in contact with the spray.
  • Wash skin surfaces that come into contact with the insecticide.
  • Rinse fruits and vegetables from your garden thoroughly with water before cooking or eating.
  • Cover outdoor furniture and play equipment before the application, or wash them with soap and water afterwards.
  • Bring in laundry and toys before spraying or wash them afterwards.
  • Bring pets inside and cover ornamental fish ponds during the application.
  • Consult your doctor if you think your health was affected by the spraying.

What problems could I have if I am exposed to mosquito adulticides?

The known health threats associated with commonly used mosquito adulticides are considered to be less serious than those posed by West Nile virus and other mosquito borne illnesses. The Environmental Protection Agency has a website with toxicology information on the pesticides used to control mosquitoes, http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm.

What should I do if I am exposed to mosquito adulticides?

If you are concerned that you are suffering adverse health effects from exposure to mosquito adulticides, contact your local health department to find out what products were used. After you know what you were exposed to you may consult with your doctor or contact the Rocky Mountain Poison and Drug Center (1-800-332-3073) for further information.

Are mosquito adulticides available to the public for home use?

Yes, a number of insecticides available to the public are labeled for mosquito control. Be sure to read, understand and follow all label instructions and precautions.

What is Anvil insecticide?

Anvil insecticide is used to fog for adult mosquitoes. It contains sumithrin and piperonyl butoxide, which is a synergist (makes sumithrin more toxic to the mosquitoes). The Environmental Protection Agency has a website with toxicology information on the pesticides used to control mosquitoes, http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm.

What is malathion?

Malathion is an insecticide that has been used for many years to fog for mosquito adults, as well as to control a number of other pest insects. The Environmental Protection Agency has a website with toxicology information on the pesticides used to control mosquitoes, http://www.epa.gov/pesticides/health/mosquitoes/mosquito.htm.

Can malathion be used as a larvicide?

Under certain limited conditions some formulations of malathion may be used as a larvicide. It may be applied to intermittently flooded areas, stagnant water, and temporary rain pools. Malathion may not be applied to any fish-bearing waters.

Updated Tuesday, July 22, 2014