Colorado State University Extension
SafeFood Rapid Response Network


SAFEFOOD NEWS - Summer 1998 - Vol 2 / No. 4

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Are Cattle Susceptible to Chronic Wasting Disease (CWD)?

The Colorado Department of Agriculture State Veterinarian's Office has produced an informational packet on Transmissible Spongiform Encephalopathies (TSEs). Included in this packet is an update from the Colorado-Wyoming Interstate Forum on CWD that convened in October 1997. The group discussed issues related to chronic wasting disease and other transmissible spongiform encephalopathies of animals and man. The following information was assembled by the forum of scientists, animal and public health officials, and industry representatives, to provide a summary of the studies currently taking place and their possible implications.

The transmissible spongiform encephalopathies (TSEs)

The TSEs are a group of nervous system diseases that have a long incubation period, an invariably fatal course, and are caused by a newly recognized kind of transmissible disease agent called a prion. These diseases occur in human beings and animals. The more well know examples are Creutzfeldt-Jakob disease (CJD) of human beings, scrapie of sheep and bovine spongiform encephalopathy (BSE) of cattle (known informally as mad cow disease). In the region of northeast Colorado, southeast Wyoming and a few neighboring states, a TSE known as chronic wasting disease (CWD) occurs in free-ranging and captive deer and elk held for research purposes, as well as in a small number of privately-owned elk herds.

BSE represents a significant health problem for cattle and is a potentially transmissible disease to other animals and humans. Currently, it has been detected only in the United Kingdom (UK) and neighboring counties. It has not been detected in the United States. An important aspect of BSE is its possible transmissibility to humans. In the UK there is evidence that BSE may have been transmitted to humans via the food chain and is manifested as a new variant form of CJD (nvCJD). With the recognition of nvCJD, strict limitations on the movement of UK cattle and beef have been imposed by a variety of international regulatory organizations. The economic effects on the UK cattle industry have been negative and large.

Approaches to understanding the risk of TSE transmission

The issues described above underscore the importance of understanding the risk of transmission of TSEs between various species. Because TSE research is in such an early stage of development, the tools for investigating the important questions are limited. At this point, there is no good test to determine before death if animals have a TSE. TSEs can be diagnosed only by microscopic examination of brain tissue itself. Another limitation is that there is no simple means of determining how efficiently prions from one animal species can produce a TSE in another species. The classical experimental tool for studying TSE transmissibility is the injection of a suspension of brain tissue of TSE-affected animals into the brains of anesthetized, uninjected animals. The injected animals are then observed for TSE symptoms and when they die, or are euthanized, their brains are examined to determine if they developed TSE. Because of the long incubation periods, these experiments are usually quite lengthy, often close to the normal lifespan of the species studied.

From these types of studies, it has been possible to demonstrate that a species barrier may or may not exist for prion transmission between particular species. Studies that involve injection of prions into the brain represent only one step in understanding the risk of transmission between species. If the brain injection shows that a TSE can be transmitted, then other experiments are needed that focus on the ability of the prions to move from body entry sites to the brain. If this movement is inefficient, then development of the TSE is less likely. Other types of experiments can focus on the likelihood of prions arriving at body entry sites from the external environment in sufficient numbers to invade the body.

Understanding the risk of TSE transmission requires a number of different experiments, each focusing on one aspect of how the prion agent interacts with the new host species. By considering the results of all the experiments, the risk of transmission can be assessed.

Cattle susceptibility studies

Studies of cattle susceptibility to CWD using the classical tools of TSE research have been initiated. These studies, their potential outcomes, and the implications of these outcomes are described below:

Intracerebral Challenge of Cattle with CWD. The objective of this study is to determine if the CWD agent, when introduced directly into the brain, will cause neurologic disease in cattle and, if so, what type of disease is induced. Specifically, if disease is induced in cattle investigators will determine whether the disease resembles bovine spongiform encephalopathy (BSE). Test cattle have received intracerebral injections of brain material collected from CWD-infected deer. If disease occurs in these cattle, length of incubation period, nature of clinical signs, type and severity of lesions, and distribution of prion protein in the animal will be evaluated and compared to other TSEs (e.g., scrapie, transmissible mink encephalopathy, BSE) that have already been transmitted to cattle via intracerebral challenge. Intracerebral inoculation is recognized as a severe and unnatural route of exposure and is not indicative of susceptibility or resistance of cattle to natural transmission of the agent. Results are unpredictable, but his work will provide valuable data on the relative susceptibility or resistance of cattle to the CWD agent, as compared to other TSEs previously studied in cattle.

Lead agencies/institutions: USDA-Agricultural Research Services (Ames, IA), University of Wyoming; start date: 1997; duration of the study: 5 years; estimated budget: approximately $65,000 per year.

Oral and Contact Exposure of Cattle to CWD. In order to determine natural susceptibility of cattle to CWD under laboratory conditions, two approaches are being investigated. The first is to experimentally expose cattle to a large, single dose of CWD agent delivered orally. Cattle will be maintained in isolation for their lifetimes (or approximately 10 years) and continually monitored for development of disease. In a second set of studies, cattle are being housed with CWD-infected deer to determine if transmission occurs via contact between the species. A variety of diagnostic tests will be applied to the animals over the course of the study and at postmortem examination. Though oral challenge with a high dose of infectious agent and long-term contact with CWD infected deer both represent much greater degrees of exposure to cattle than expected on range, these studies will provide direct information on the potential for transmission to cattle.

Lead agencies/institutions: University of Wyoming, Colorado Division of Wildlife, and Wyoming Game & Fish Department; start date: 1997; duration of the study: 10 years; estimated budget: approximately $100,000 per year.

Source: Colorado State Veterinarian's Office, "Studies on the Susceptibility of Cattle to CWD," June 1998.

Updated Monday, August 29, 2011