The Story of Chronic Wasting Disease
Chronic wasting disease (CWD) is a prion disease that affects deer, elk, reindeer, sika deer and moose. It has been found in some areas of North America, including Canada and the United States, Norway and South Korea. It may take over a year before an infected animal develops symptoms, which can include drastic weight loss (wasting), stumbling, listlessness and other neurologic symptoms. CWD can affect animals of all ages and some infected animals may die without ever developing the disease. CWD is fatal to animals and there are no treatments or vaccines.
To date, there have been no reported cases of CWD infection in people. However, some animal studies suggest CWD poses a risk to certain types of non-human primates, like monkeys, that eat meat from CWD-infected animals or come in contact with brain or body fluids from infected deer or elk. These studies raise concerns that there may also be a risk to people. Since 1997, the World Health Organization has recommended that it is important to keep the agents of all known prion diseases from entering the human food chain.
As of November 2019, CWD in free-ranging deer, elk and/or moose has been reported in at least 24 states in the continental United States, as well as two provinces in Canada. In addition, CWD has been reported in reindeer and/or moose in Norway, Finland and Sweden, and a small number of imported cases have been reported in South Korea. The disease has also been found in farmed deer and elk.
CWD was first identified in captive deer in a Colorado research facility in the late 1960s, and in wild deer in 1981. By the 1990s, it had been reported in surrounding areas in northern Colorado and southern Wyoming. Since 2000, the area known to be affected by CWD in free-ranging animals has increased to at least 24 states, including states in the Midwest, Southwest, and limited areas on the East Coast. It is possible that CWD may also occur in other states without strong animal surveillance systems, but that cases haven’t been detected yet. Once CWD is established in an area, the risk can remain for a long time in the environment. The affected areas are likely to continue to expand.
Nationwide, the overall occurrence of CWD in free-ranging deer and elk is relatively low. However, in several locations where the disease is established, infection rates may exceed 10 percent (1 in 10), and localized infection rates of more than 25 percent (1 in 4) have been reported. The infection rates among some captive deer can be much higher, with a rate of 79% (nearly 4 in 5) reported from at least one captive herd.
CWD in Animals
CWD is a progressive, fatal disease that affects the brain, spinal cord, and many other tissues of farmed and free-ranging deer, elk, and moose.
CWD belongs to a family of diseases called prion diseases or transmissible spongiform encephalopathies (TSEs). Other TSEs include bovine spongiform encephalopathy in cattle, scrapie in sheep and goats, and Creutzfeldt-Jacob disease and other human prion diseases in people, including variant Creutzfeldt-Jacob disease (the human form of “mad cow disease”) in people. The infection is believed to be caused by abnormal proteins called prions, which are thought to cause damage to other normal prion proteins that can be found in tissues throughout the body but most often in the brain and spinal cord, leading to brain damage and development of prion diseases.
Some of the known carriers of CWD are mule deer, white-tailed deer, elk, and moose.
Scientists think CWD spreads between animals through contact with contaminated body fluids and tissue or indirectly through exposure to CWD in the environment, such as in drinking water or food. CWD affects many different species of hoofed animals including North American elk or Wapiti, red deer, mule deer, black-tailed deer, white-tailed deer, Sika deer, reindeer, and moose.
Like other prion diseases, CWD may have an incubation period of over a year and clear neurological signs may develop slowly. Deer, elk, reindeer, sika, and moose with CWD may not show any signs of the disease for years after they become infected. As CWD progresses, infected animals may have a variety of changes in behavior and appearance. These may include:
• drastic weight loss (wasting)
• lack of coordination
• excessive thirst or urination
• drooping ears
• lack of fear of people
It is often difficult to diagnose a deer, elk, or moose with CWD based on these symptoms alone because many of CWD symptoms also occur with other diseases and malnutrition. CWD is always fatal.
CWD does not appear to naturally infect cattle or other domesticated animals.
Preventing Chronic Wasting Disease
If CWD could spread to people, it would most likely be through eating of infected deer and elk. In a 2006-2007 CDC survey of U.S. residents, nearly 20 percent of those surveyed said they had hunted deer or elk and more than two-thirds said they had eaten venison or elk meat. However, to date, there is no strong evidence for the occurrence of CWD in people, and it is not known if people can get infected with CWD prions.
Hunters must consider many factors when determining whether to eat meat from deer and elk harvested from areas with CWD, including the level of risk they are willing to accept. Hunters harvesting wild deer and elk from areas with reported CWD should check state wildlife and public health guidance to see whether testing of animals is recommended or required in a given state or region. In areas where CWD is known to be present, CDC recommends that hunters strongly consider having those animals tested before eating the meat.
Tests for CWD are monitoring tools that some state wildlife officials use to look at the rates of CWD in certain animal populations. Testing may not be available in every state, and states may use these tests in different ways. A negative test result does not guarantee that an individual animal is not infected with CWD, but it does make it considerably less likely and may reduce your risk of exposure to CWD.
To be as safe as possible and decrease their potential risk of exposure to CWD, hunters should take the following steps when hunting in areas with CWD:
• Do not shoot, handle or eat meat from deer and elk that look sick or are acting strangely or are found dead (road-kill).
• When field-dressing a deer:
o Wear latex or rubber gloves when dressing the animal or handling the meat.
o Minimize how much you handle the organs of the animal, particularly the brain or spinal cord tissues.
o Do not use household knives or other kitchen utensils for field dressing.
• Check state wildlife and public health guidance to see whether testing of animals is recommended or required. Recommendations vary by state, but information about testing is available from many state wildlife agencies.
• Strongly consider having the deer or elk tested for CWD before you eat the meat.
• If you have your deer or elk commercially processed, consider asking that your animal be processed individually to avoid mixing meat from multiple animals.
• If your animal tests positive for CWD, do not eat meat from that animal.
The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service regulates commercially farmed deer and elk. The agency operates a national CWD herd certification program. As part of the voluntary program, states and individual herd owners agree to meet requirements meant to decrease the risk of CWD in their herds. Privately owned herds that do not participate in the herd certification program may be at increased risk for CWD.
Transmission of Chronic Wasting Disease
Scientists believe CWD proteins (prions) likely spread between animals through body fluids like feces, saliva, blood, or urine, either through direct contact or indirectly through environmental contamination of soil, food or water. Once introduced into an area or farm, the CWD protein is contagious within deer and elk populations and can spread quickly. Experts believe CWD prions can remain in the environment for a long time, so other animals can contract CWD from the environment even after an infected deer or elk has died.
The CWD prion has been shown to experimentally infect squirrel monkeys, and also laboratory mice that carry some human genes. An additional study begun in 2009 by Canadian and German scientists, which has not yet been published in the scientific literature, is evaluating whether CWD can be transmitted to macaques—a type of monkey that is genetically closer to people than any other animal that has been infected with CWD previously. On July 10, 2017, the scientists presented a summary of the study’s progress (access the recorded presentation and slides ), in which they showed that CWD was transmitted to monkeys that were fed infected meat (muscle tissue) or brain tissue from CWD-infected deer and elk. Some of the meat came from asymptomatic deer that had CWD (i.e., deer that appeared healthy and had not begun to show signs of the illness yet). Meat from these asymptomatic deer was also able to infect the monkeys with CWD. CWD was also able to spread to macaques that had the infectious material placed directly into their brains.
This study showed different results than a previous study published in the Journal of Virology in 2018, which had not shown successful transmission of CWD to macaques. The reasons for the different experimental results are unknown. To date, there is no strong evidence for the occurrence of CWD in people, and it is not known if people can get infected with CWD prions. Nevertheless, these experimental studies raise the concern that CWD may pose a risk to people and suggest that it is important to prevent human exposures to CWD.
Additional studies are under way to identify if any prion diseases could be occurring at a higher rate in people who are at increased risk for contact with potentially CWD-infected deer or elk meat. Because of the long time it takes before any symptoms of disease appear, scientists expect the study to take many years before they will determine what the risk, if any, of CWD is to people.