Mystery Bites and Itches - Arthropod and Non-Arthropod Sources in Colorado

by Whitney Cranshaw
Colorado State University Extension Specialist

Everyone has experienced at some times various skin bumps, sores, or persistent itching that have no obvious source. Often these are suspected or self-diagnosed as being "bug bites" of some sort. Sometimes this is correct, as there are a few insects and arachnids found in Colorado that can bite humans. Unfortunately, diagnosed "bug bites" most often have a different origin.

This confusion is made worse as bites by insects or arachnids can rarely be diagnosed by symptoms alone. Similar symptoms can be, and almost always are, produced by many other causes. These include:

  • Environmental conditions (chemical or physical agents contacted or dispersed in air, abrupt changes in humidity)
  • Reaction to chemical agents (personal care products, cleaning agents, inks, etc.)
  • Underlying health conditions (bacterial infections, diabetes, drug reactions, etc.)

Properly diagnosing a "bug bite" is very difficult and often does not have a satisfactory end result. A suggested protocol for attempting to determine the cause follows (Table 1):

Table 1. Suggested protocol for county Extension offices in Colorado when handling queries regarding "invisible itches" or "arthropod bites" of unknown origin.

  1. Never attempt to diagnose the cause of a "bite" based on symptoms. Observation of the affected area by anyone other than appropriately trained medical personnel will not assist in the diagnosis. Similarly skin scrapings can not be evaluated except by medical personnel.

  3. Samples of arthropods or suspected arthropods can always be evaluated. If unable to identify them, county Extension offices can forward the sample to campus. Samples should be preserved as well as possible to prevent fragmentation. Samples collected on tape applied to the skin surface also can be examined for arthropods.

  5. Review possible sources of cryptic arthropod bites and itches that can occur in Colorado. These are discussed in the following section and consist of a fairly short list of options. If conditions exist that may be a source of bites/itches (e.g., mites moving off an infested pet, bat bugs migrating from bat roost in attic) then suggest to the client that they remove or treat these sources.

  7. If conditions of the bite/itch occur in the absence of conditions suggesting an arthropod or if they persist after treatment efforts have been made, the source of the bites is almost certainly of some other origin. Review recent changes that have occurred in one's habits. These might include recent travel or outdoor activity. Reaction to new products used in and about the home or work area may be a cause. Make changes that eliminate contact with these products. Review the list of some possible environmental causes that produce symptoms mimicking bug bites.

  9. If symptoms persist, an underlying medical condition is probable. Review the list of some possible medical conditions that can produce the sensation of bug bites or that can produce bumps and lesion mimicking bug bites. Resolution likely will require medical attention.

Biting Arthropods That Occur in Colorado

Several arthropods occur in the state that can bite humans (Table 2). These should be first considered as the possible cause of a "bug bite".

Table 2. Possible Sources of "Bug Bites" in Colorado that May be Due to "Bugs" (Insects and other Arthropods)

Other biting flies (biting midges, black flies, no-see-ums)
Mites originating from a bird nest
Mites originating from a pet
Mites originating from a grain storage
Bed bugs
Bat bugs
Swallow bugs
Masked hunter

Conditions that produce such biting often are very restricted. Some occur only outdoors and some occur only during certain times of the year. Many require that some animal host (household pet, nesting wildlife) be present as a source of the biting arthropod. The biting arthropods found in Colorado and their diagnoses are reviewed below.


Flea bites of humans are relatively rare in Colorado. Arid conditions in the state largely prevent development of the flea species that cause serious problems in the more humid areas of the US. Almost all cases involve fleas originating from wild animals, which then occur in homes either from transfer by pets or that disperse from dens under or in close proximity to the home. Fleas are covered in more detail in Fact Sheet 5.600.

Fleas, although small (ca 1/8-inch), are relatively easily detected, being active insects. Their appearance is also unique making them readily identifiable. Bites appear as small reddish spots that may itch and often are concentrated on the lower leg.

Some questions to ask to determine if fleas may be the source of a "bite":

Have there been skunks, foxes or squirrels nesting in or in/under the home?

  • Fleas developing on these hosts will scatter when the nests are abandoned. The human flea (Pulex irritans) is associated with skunks and fox and is the most common flea found biting humans in Colorado; an Orchopeas sp. of flea found in homes has been associated with squirrels.

Is there a dog in the home?

  • The cat flea, Ctenocephalides felis, is the common species associated with dogs in the US. It survives poorly in Colorado and rarely persists in a home, but infestations do occur. If this is the source of fleas, myriad treatments can be effectively used; these are covered in the Fact Sheet 5.600.

Does you dog go outdoors to sites where foxes or skunks are present?

  • In these sites fleas dispersing from denning animals may be picked up by the dog. These are then moved indoors causing temporary episodes of flea biting. These problems are self-limiting as the fleas do not reproduce in the home and ultimately die-out.

Cheyletiella Mites

Cheyletiella mites are small parasites of mammals that can infest the skin of dogs and, less commonly, cats. They may produce itching in the animal or irritated skin. However, often the animal may support a population of these mites but is asymptomatic. The mites can be detected by brushing the pet over paper or some other surface that can dislodge the mites.

Effective treatments exist that can control Cheyletiella mites on a pet, which will prevent further human biting. Treatment course is best handled in consultation with a veterinarian.

Cheyletiella mites may also incidentally bite humans that are in close contact with an infested pet. Usually bites occur if the pet often lies in the lap or similarly has prolonged close contact. In these cases bites are often concentrated in parts of the body where the animal rests.

Some questions to ask to determine if Cheyletiella mites may be the source of a "bite":

Do you own a dog or cat?

  • Bites from Cheyletiella mites only occur via incidental transfer from infested pets. They do not reproduce on humans and problems will quickly end in the absence of an infested pet

Do you let the dog/cat lie in your lap or have similar periods of prolonged close contact?

  • Infested pets that rest on one's body are most likely to allow transfer of mites to humans, which may then bite. A pattern of bites around the point of the body where the animal rests (e.g., lap) are most likely to show

Bird Mites

Mites that infest nesting birds (Ornithonyssus spp., Dermanyssus spp.) may sometimes bite humans. These situations arise only when a wild bird nest is attached to the home. Mite bites usually then occur after the nest is abandoned, at which time the starving mites disperse and will incidentally bite humans. However, they can not sustain themselves and such problems are of short duration, the mites dying out within a few weeks in the absence of the birds.

Such situations can be prevented in the future by taking measures that prevent birds nesting on the home. Insecticide treatments applied in the vicinity of the abandoned nest may kill some mites that otherwise may migrate into living areas.

Some questions to ask to determine if bird mites may be the source of a "bite":

Do you have a bird nest on the side of your house or behind the wall of your house?

  • Bird mites will only be found in a home if there has been a nesting bird attached to the building or behind the wall. They do not reproduce on humans and problems dissipate a few weeks after the birds have abandoned these nests.

Did the nesting birds recently abandon the nest?

  • Biting by bird mites usually takes place for a couple of weeks after nesting birds have abandoned the nest attached to the home.

Bird mites that are present on the skin may be able to be identified from samples on adhesive tape placed on an infested area of body.


Chiggers are the minute, first stage larvae of mites that occur outdoors and feed on the skin of various animals. They are rarely encountered in Colorado, but do occur in localized natural areas during the warm months. Chiggers are almost always found in areas of lush grass, usually near waterways.

Chiggers actually do not bite, but feed by digesting small areas of the upper skin through saliva. The "bite" that chiggers produce is a reaction to the proteins in the saliva. This reaction can develop within a day or so after exposure and often becomes very itchy. By the time the reaction occurs the chiggers often are no longer found on the body. Chiggers are very rarely noticed since they are not only extremely small but spend only a short period of time on the body and are removed by changing clothes and showering. Chiggers do not reproduce in the home and all bites will occur from accidentally acquiring them when moving through infested grasslands.

Chigger "bites" can largely be prevented by use of insect repellents containing DEET. Salves and, sometimes, antihistamines are used to reduce itching from chigger "bites".

Some questions to ask to determine if chiggers may be the source of a "bite":

Have you recently (within the past 3 days) been walking through a grassy area, particularly near a waterway?

  • Such natural sites are the only place where one would acquire chiggers, as they do not reproduce within homes and rarely, if ever, occur in yards in Colorado. Reaction to the "bite" usually takes a couple of days to develop.

Are the bites concentrated along the sock area or at the waist?

  • Biting by chiggers is typically concentrated around the ankles, less commonly at the waistline where clothing is constricted.


Scabies mites (Sarcoptes scabei) develop in tiny burrows made under the skin. This typically produces a pimple-like rash that itches, particularly at night. Scabies occurs worldwide but is particularly common in regions dense crowding and where optimal sanitation is not present. Scabies is rarely acquired in Colorado and essentially all cases originate during travel to areas of the world where these parasites are more common.

Related species of mites cause mange in livestock. Sometimes these mites can transfer to humans if they are in very close contact with the animal. A transitory irritation may be produced by these mites. However, the mange producing mites can not survive on humans and die quickly.

Diagnosis of scabies is done with the assistance of a physician using skin scrapings. These tests can not be done by Extension personnel. Effective medical treatments for scabies exist that can eliminate the mites.

Some questions to ask to determine if scabies may be the source of a "bite":

Have you traveled abroad in the last year to an area of the world where scabies is endemic?

  • Scabies does occur worldwide but is most common in areas of heavy human crowding and relatively poor sanitation.

Have you had close physical contact with someone who has scabies?

  • Scabies is not transmitted readily by casual contact, such as hand shaking. Closer, prolonged contact with a person harboring scabies can allow transmission.

Mites Associated with Stored Products

Stored foods are sometimes infested with various species of mites. This almost always occurs when storage conditions are sufficiently humid to allow growth of molds. Many of the mites that are associated with food storages develop only in moldy conditions.

Some of these mites that feed on stored foods can cause irritation. Furthermore, large numbers of insects developing in stored foods may allow development of a predatory species, the straw itch mite (Pyemotes tritici). Straw itch mite can bite humans and produce a reddish spots of irritation.

Some questions to ask to determine if mites associated with stored foods may be the source of a "bite":

Do you have a substantial storage of stored grain, seeds, or cereal products in your home?

  • Mites associated with stored foods, and the straw itch mite, develop in such sites.

Is the grain stored in an area that is humid and allows growth of mold?

  • High humidity conditions are required for development of most stored grain mites, which often feed on associated molds. Stored products maintained at the normal humidities common to Colorado will almost never be infested by stored grain mites.


No source of "bites" is nearly so over-diagnosed as are spider bites. Unfortunately this situation is often aggravated not only by self-diagnosis but also by mis-diagnosis by medical personnel.

Spider bites, when they do occur, are of defensive nature. Biting occurs when the spider is confined or threatened; they do not attack humans. Most often spider bites occur when a web is gently disturbed, when a spider has retreated for shelter in shoes/clothing, or during other accidental contacts.

Spider bites often are immediately felt as a point of sharp pain. However, this is not always the case, and sometimes bite reactions are so mild that they are not noticed. Regardless, a single defensive bite is typical of spiders. The occurrence of multiple "bites" indicate some other cause.

The venom of spiders is the primary medical concern. Fortunately very few spiders have venom that produce anything more than a transitory, mild irritation to humans. Those that most often are discussed as possibly of medical importance include widow spiders, brown recluse, the 'hobo spider', and yellow sac spiders.

Widow spiders, specifically the western widow (Latrodectus hesperus), are quite common in many areas of the state. However, bites are very rare. The venom that widow spiders use has neurotoxic effects which include pain in various parts of the body, a general sense of malaise, and other distinct symptoms. Widow bites do not produce persistent, noticeable irritation at the bite site nor any secondary lesions. A new Extension fact sheet on the western widow (5.605) was made available in August 2007.

Brown recluse (Loxosceles reclusa) is a common household spider of some midwestern and south central states, but is extremely rare in Colorado. A very few (only one on record at the Denver Museum of Nature and Science) confirmed cases of brown recluse have ever been recorded in the state, apparently involving recent transfer from chronically infested areas (e.g., eastern Oklahoma, Missouri). The closely related Mediterranean recluse, Loxosceles rufescens, has been collected somewhat more frequently in Colorado (4 cases on record); the medical importance of this latter species is considered to be less than for the brown recluse.

The venom of brown recluse spiders can cause cell damage at the bite site. In a small fraction of individuals this can progress, producing substantial tissue death and a slow healing wound. However, no cases of confirmed brown recluse bites have ever been reported in Colorado. Unfortunately a tremendous number of >brown recluse spider bites= have been diagnosed incorrectly. In the absence of the spider, all of these diagnoses must be considered highly suspect and are due to other causes. A list of medical conditions that mimic the slow healing lesions sometimes associated with brown recluse bites is listed below (Table 3). Probably the largest single cause is infection of a wound with methicillin-resistant Staphlococcus aureus (MSRA), a bacterial infection sometimes known as "false spider bite diagnosis".

The 'hobo spider' (Tegenaria agrestis) is a type of funnel weaver spider, closely related to many of the harmless funnel weaver spiders that commonly invade homes. It is a European species that recently found its way into Colorado and in Europe it is found everywhere and is not associated with medically important spider bites. Unfortunately, a US report shortly after the spider was found in Washington state suggested that bites of the hobo spider have serious potential to cause a slow healing, necrotic wound similar to that from brown recluse. There have been no subsequent confirmed reports of the hobo spider causing serious wounds following a bite on humans and the original report is now largely disregarded.

Yellow sac spiders in the genus Cheiracanthium are also among the spiders where visible wounds with secondary complications have sometimes been reported; credible medical reports on this issue do not appear to support that this spider is a source of serious medical concern. These spiders are common in homes in Colorado. At the point where bites occur there is usually an immediate stinging sensation and redness develops. Two puncture wounds, produced by the two fangs, may be visible. Sometimes sac spider bites can result in a small blister that, when broken, may produce a small sore. However, this normally will dry and heal quickly.

With the probable exception of the widow spiders, which have a neurotoxin that produces characteristic effects, no spider bite can be diagnosed based on symptoms. Whenever possible a spider that has bitten should be collected. These spiders can usually be identified using Fact Sheet 5.512 (Spiders in the Home) that discusses the common species found in the state. Spiders can be sent from Extension offices to campus for identification, although often only family-level identifications are possible.

Table 3. Medical Conditions that Produce Symptoms that may Mimic Insect or Spider Bites (from Vetter 2004)

Methicillin-resistant Staphylococcus aureus (MRSA or false spider bite diagnosis)
Other Staphylococcus infections
Streptococcus infection
Gonococcal arthritis dermatitis
Cutaneous anthrax

Reaction to drugs
Warfarin poisoning

Infected herpes simplex
Chronic herpes simplex
Varicella zoster (shingles)

Lyme disease
Rocky Mountain spotted fever
Ornithodoros coriaceus bite (soft tick)
Insect bites (flea, mite, biting fly)

Keratin cell mediated response to fungus

Poison ivy/poison oak
Chemical burn

Lymphoproliferative disorders
Lymphoma Lymphomatoid papulosis

Underlying disease states
Diabetic ulcer

Vascular disorders
Focal vasculitis
Purpura fulminans
Thromboembolic phenomena
Polyarteritis nodosa

Miscellaneous/Multiple causative agents
Pyoderma gangrenosum
Pressure ulcers
Stevens-Johnson syndrome
Erythema multiforme
Erythema nodosum
Toxic epidermal necrolysis
Lyell syndrome


Mosquitoes are present during the warm months if local conditions allow their breeding. All mosquitoes develop in water but the different types of Colorado mosquitoes require different conditions. The ones that most commonly bite are floodwater types (Aedes spp., Ocholeratus spp.) that lay their eggs at the edge of temporary ponds. Eggs hatch when a subsequent flooding event covers the eggs. Other mosquitoes, including Culex spp. that can vector West Nile virus, lay their eggs on the surface still water and successfully breed in pools where fish are not present.

Almost all mosquito bites occur outdoors. The floodwater types bite during the day, particularly at dusk. Culex mosquitoes may feed at night and, if they manage to enter homes, may bite humans as they sleep. Individual response to bites varies widely. Itching and irritation rarely last more than a few days, although bites can become infected, particularly if they are scratched.

Mosquito bites are easily diagnosed if mosquitoes are observed to feed. However, since the bite itself is often not felt and reaction to bites only begins to occur hours later, they sometimes are not suspected.

Some questions to ask to determine if mosquitoes may be the source of a "bite":

Is it the time of year when mosquitoes bite?

  • Mosquitoes bite during warm periods from late May through late September. Adults of some species survive winter in the adult stage but remain dormant and do not bite during the cool months.

Have you been outdoors in the past few days, particularly at dusk?

  • Almost all mosquito bites occur outdoors and dusk is the peak period of biting of the most common species that bite humans.

Are the bites on parts of the body that mosquitoes could reach to bite?

  • Mosquitoes will feed on parts of the body that are exposed and should not occur on areas where protective clothing has covered during periods of mosquito biting

Other Biting Flies

Other biting flies occur in Colorado. Black flies are sometimes a problem in the vicinity of rivers, in particular during years of high run-off. Biting midges in the genus Leptoconops occur in areas of western Colorado, developing in damp areas of ravines and other sheltered sites. No-see-ums (Culicoides spp.) occasionally occur near areas of running water and deer flies may develop in muddy areas near ponds. These biting flies are discussed in more detail in Fact Sheet 5.582.

None of these flies naturally occur in homes and die-out rapidly if they accidentally enter a home. All biting occurs outdoors and none have any stages that develop within homes. These flies are all visible and produce painful bites so their identity as a source of bites is always obvious.

Bed Bugs

Bed bugs are one of the few biting insects found in Colorado that can develop entirely on a human host. Although not abundantly found in homes, in recent years they are more commonly encountered and there has also been great increase in awareness about bed bugs in Colorado.

Bed bugs are active at night and usually hide during the day in the close vicinity of where people sleep. Bites are painless, but may produce an itchy spot; individual reaction to bites ranges very widely. Bed bug bites will primarily occur on parts of the body that are exposed during sleep, rather than covered by bedding.

Diagnosis of bed bugs requires a search of possible hiding sites in the vicinity of the sleeping area. The insects are readily visible, with a distinct shape. (This is shared by the closely related bat bugs and swallow bugs.) Excreted spotting following a blood meal is also a symptom used to identify areas where bed bugs hide during the day. Information on bed bugs is covered in Fact Sheet 5.574, and in similar Extension publications produced by many states.

The primary question to ask to determine if bed bugs may be the source of a "bite" is "Have you examined the area where you sleep for the presence of bed bugs?"

Bat Bugs

Bat bugs are usually the most common of the biting insects of the "bed bug" family that occur in Colorado. These are parasites of bats and develop within bat colonies which often develop in attic areas or in cavities of the building. Bites of humans are incidental and occur as the bat bugs disperse from the nesting bats. Control of bat bugs involves elimination of nesting bats on or in a residence. Along with this are methods to exclude (e.g., sealing entrances from nesting area) or kill (e.g., insecticides) the residual bat bugs. In the absence of the bat hosts, these insects ultimately die-out. Bat bugs and their control are discussed further in Fact Sheet 5.574.

Bat bugs are very similar in shape and size to bed bugs and with careful search often may be discovered in the vicinity where biting has occurred.

The primary question to ask to determine if bat bugs may be the source of a "bite":

Are bats nesting within the structure?

  • Bat bugs only breed and develop on bats, although they will incidentally bite humans if they disperse from bat roosts.

Swallow Bugs and Other Wild Bird Bugs

Swallow bugs (Oeciacus vicarius) are a member of the "bed bug" family that develop on swallows. Incidentally they may bite humans when swallows nest on the sides of buildings. Swallow bug biting occurs most frequently in spring, shortly before the return of nesting swallows. Biting may also occur shortly after swallows leave nests, as the bugs disperse for shelters where they remain largely dormant until the following spring. Control of swallow bugs involves elimination of nesting swallows on or in a residence. Along with this are methods to exclude (e.g., sealing entrances from nesting area) or kill (e.g., insecticides) the residual swallow bugs. In the absence of the swallow hosts, these insects ultimately die-out. Swallow bugs and their control are discussed further in Fact Sheet 5.574.

At least two other species in the "bed bug" family that also feed on wild birds occasionally occur in homes. In western Colorado Hesperocimex coloradensis is associated with woodpecker nests and may wander into homes and bite human if there are nesting woodpeckers behind walls. The poultry bug, Haematosiphon inodorus, is associated with poultry and owls.

Swallow bugs are very similar in shape and size to bed bugs and with careful search often may be discovered in the vicinity where biting has occurred.

Some questions to ask to determine if swallow bugs may be the source of a "bite":

Are there swallow nests attached to the building now or the previous season?

  • Swallow bugs only breed and develop on swallows, although they will incidentally bite humans if they disperse from bat roosts.

Is biting most common in late winter/early spring - coincident with the return of nesting migratory swallows?

  • Peak period of swallow bug biting is around the time when swallows return to nests, as the bugs resume activity following a period of winter dormancy.

(For other species) Are woodpeckers or owls nesting in the building?

  • Two species in the "bed bug" family known from Colorado are found associated with nests of these bird species.

Masked Hunter

The masked hunter (Reduvius personatus) is a species of assassin bug that is sometimes found in homes. Adult stages are dark brown or black and about 3/4-inch long. However, immature stages are smaller and cover themselves with lint or other debris. Sometimes they may appear as moving balls of dust.

Never common in homes, masked hunter is a predator of other insects. It is sometimes called the "bed bug hunter" but will likely feed on any insect and probably some spiders that it can over power. The masked hunter paralyzes its prey with saliva it injects from its piercing-sucking mouthparts.

If handled, this insect may also bite humans. However, the bite is immediately painful and this bite source readily recognized.


Two species of lice are sometimes associated with humans in Colorado. By far the most common is the head louse (Pediculus humanus capitatis), which sometimes becomes epidemic in schools, day care and other areas where people are crowded together. As the name applies the head louse is almost exclusively found among the hairs of the scalp. Biting can cause itching but the cause is easily identified by close inspection of the head. In addition to the active adult and nymph stages, white eggs are glued tightly to hairs. Numerous publications are available that can be used to identify head lice and offer treatment. One good on-line source is the Harvard University School of Public Health site for head lice -

Far less commonly encountered is the crab louse (Phthirus pubis). This is sometimes known as the "crab" and is a species almost entirely transmitted via very close, usually sexual, contact. It lives among the coarse hairs of the pubic region, occasionally occurring on facial hair. Feeding by this species can cause a local irritation. However, if present it can usually be easily found upon inspection.

Non-Arthropod Causes for Itching/Irritation Symptoms
that Mimic Arthropod Infestation

There are a great many things that can produce persisting itching or irritation that may resemble infestation with insect or other arthropods. This subject is well covered in the University of Kentucky Extension publication "Invisible Itches: Insect and Non-Insect Causes" authored by M. F. Potter (1997). Comments involving arthropod causes are deleted since they are covered above. Everything that follows, except for comments in parentheses ( ) are from this article.

It is important to recognize that there are many potential causes of itching and irritation other than pests. Allergies, cosmetics, medications, and environmental contaminants all can produce reactions similar to insect bites. While this makes the experience no less real or unpleasant for the affected individual, it underscores the importance of keeping an open mind to the possibility of non-insect causes of such reactions. Much like a detective, one should attempt to rule out all potential sources of irritation through the process of elimination...

Household Products

There are literally hundreds of non-insect agents capable of causing itching and irritation. Household products are involved far more often than are pests and may cause skin reactions similar to insect bites. Products most often implicated include phosphate detergents, soaps, cosmetics, ammonia-based cleaning agents, hair products, medications, printing inks (especially from multi-form carbonless carbon paper), and certain types of clothing, particularly those which contain fire retardants. If a connection can be made between irritation and exposure to one of these potential irritants, avoiding further exposure may correct the problem. A dermatologist can usually confirm that a product, rather than a pest, is causing the irritation.

Environmental Factors

When two or more individuals experience irritation in the absence of pests, the cause is likely to be environmental conditions or contaminants dispersed in the air. The irritant(s) may be either physical or chemical in nature.

Physical Irritants. The most common physical irritants are tiny fragments of paper, fabric, or insulation. When these fibers contact the skin, they can produce symptoms ranging from a "crawling sensation" to intense itching accompanied by a rash, welts, or open sores. If fibers or fragments are involved, the irritation usually occurs over exposed areas of the body such as arms, legs, neck, and head.

Irritation produced by paper fragments is especially common in offices where large quantities of paper are processed daily. Continuous-feed paper from computers and multi-page forms generate large amounts of fragments, resulting in accumulations on desktops and other surfaces. Newly installed or badly worn synthetic carpet, drapes or upholstery also shed fibers which can irritate skin.

Other potential sources of irritation are insulation fibers released into the air by heating/cooling systems in need of repair and sound-deadening fibers embedded into drop-ceiling tiles. These latter sources are especially suspect if there have been problems with the air-handling system or recent repair work on the ceiling.

Irritation is aggravated by static electricity which increases the attraction of the tiny charged fibers to exposed skin. Low humidity, electronic equipment, and nylon (e.g., from carpeting, upholstery, or women's stockings) all increase levels of static electricity and the potential for problems from fragments or fibers. Static electricity may also cause body hair to move, giving the impression of insects crawling over the skin.

If fibers or fragments are suspected of causing the reactions, floors, rugs, work surfaces, and furniture should be thoroughly and routinely vacuumed, and desktops and tables wiped down with a damp cloth. Static-reducing measures should also be considered such as raising the humidity level of the air and installing static-resistant mats and pads under chairs and electronic equipment in offices. Anti-static sprays can be used to treat seat cushions and nylon stockings.

Dry air alone can cause irritation, producing a condition known as "winter itch." As skin loses moisture, itching results. A similar reaction can occur from changes in temperature; these tend to make skin more sensitive. A skin moisturizer is often helpful in these situations.

(In Colorado, dry air is very likely a primary contributor to many cases of invisible itches. Humidity is very low at any time of the year and drops sharply indoors with cold weather.)

Airborne Chemical Irritants. Indoor air pollution can be a serious problem in modern office buildings and other energy-efficient structures where air is recirculated over and over. Indoor air pollution can also be a problem in homes. As the concentration of chemical contaminants in the air increases, people may experience dizziness, headaches, and eye, nose, or throat irritation. Certain air-borne contaminants can also produce rashes and skin irritation similar to insect bites. Chemical contaminants most often responsible for these reactions include ammonia-based cleaning agents, formaldehyde emitted from wall and floor coverings, tobacco smoke, and solvents and resins contained in paints, glues, adhesives, and pesticides repeatedly applied for control of suspected pest infestations.

Reactions to airborne chemicals most often occur in buildings with inadequate ventilation, especially those that are new or have been refurbished with new paint or wall or floor coverings. If indoor air pollutants rather than insects are suspected, you may wish to consult an industrial hygienist who is equipped to monitor ventilation levels and the presence of allergy-producing contaminants. Companies specializing in environmental health monitoring have listings in the telephone directories of most metropolitan areas.

Health-Related Conditions

Health-related conditions may be responsible for irritation mistakenly attributed to insects. Itching and skin irritation are common during pregnancy (especially during the last trimester) and may also occur in conjunction with diabetes, liver, kidney, and thyroid disease, and shingles. Food allergies are another common cause of itching and irritation.

A person's emotional state can also induce skin reactions that can be mistaken for insect bites. Stress and conflict at work or home can produce itching and irritation. The itching response can be induced in other individuals simply by the "power of suggestion;" i.e., when one person in a group feels an itch or bite and begins to talk about it, others also feel the urge to scratch as well (a condition known as Bell's syndrome).

Delusory parasitosis is a more serious emotional disorder characterized by an irrational fear that living organisms are infesting a person's body. Cases of delusory parasitosis often have similar symptoms and patterns of behavior. Patients typically report "bugs" invading their ears, nose, eyes, and other areas of their body. The "creatures" frequently disappear and reappear and change colors while being observed. Specimens brought in for identification usually consist of bits of dead skin, hair, lint, and miscellaneous debris. The skin of the individual is often severely irritated from desperate scratching, excessive bathing, and application of ointments, bleaches, gasoline and other solvents. While these occurrences may seem bizarre to persons who are not affected, they are frighteningly real to the patient. Delusory parasitosis as well as other suspected emotional or medical conditions should be brought to the attention of a dermatologist or other physician. From University of Kentucky publication Invisible Itches: Insects and non Insect Causes" (1997) M.F. Potter.

(Use of certain drugs, notably methamphetamine, is another source that may produce sensations of insects crawling on the body, sometimes described as "crank bug bites". Metabolism of the drug near the skin can produce small, hot bumps. Scratching of these then may produce sores which in turn become infected. This reinforces a cycle of bug bite sensation.)

(Table 4. Household Products that May Produce Irritation that Mimics Infestation with Arthropods)

Detergents (especially phosphate-based)
Cosmetics/hair products
Ammonia-based cleansers
Printing inks (e.g., carbonless)
Clothing (especially fire retardant)

(Table 5. Environmental Factors that May Produce Irritation that Mimics Infestation with Arthropods)

Physical irritants

Paper, fabric, or insulation fibers
Low humidity
Seasonal changes in temperature
Static electricity

Chemical irritants

Formaldehyde (e.g., from particle board, wall and floor coverings)
Solvents/resins associated with paints and adhesives
Volatiles from asphalt and tar installation

(Table 6. Health-related Conditions that May Produce Irritation that Mimics Infestation with Arthropods)

Communicable diseases (e.g., chicken pox, measles)
Diabetes, liver, or kidney disorders
Food allergies
Insect phobias
[Methamphetamine use ("crank bug bites")]

References Used Most Heavily in This Publication

Benoit, R. And J.R. Suchard. 2006. Necrotic Skin Lesions: Spider Bite-or Something Else? Consultant. Volume 46 (12) (October 1, 2006)

Potter, M.F. 1992. Invisible Itches: Insect and Non-Insect Causes. University of Kentucky Extension Pub. ENT-58. 4 pp.

Vetter, R. 1999. Identifying and Misidentifying the Brown Recluse Spider. Dermatology Online Journal 5(2): 7

Vetter, R. 2004. Causes of necrotic wounds other than brown recluse spider bite. (updated October 2004)

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Updated Tuesday, August 05, 2014

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