Chronic wasting disease is a rare disease that affects certain hoofed game animals such as deer and elk and can cause them to become thin, act strangely, and die. There is no evidence the disease can spread to humans from animals, but no firm evidence to rule out certain precautions.
When the disease infects and attacks the brains of deer, elk, or moose (called North American cervids, or hoofed animals, usually with males that have antlers), the animals begin to behave abnormally. Infected animals might lose coordination and become emaciated (thin and malnourished, or wasting), and the animal eventually dies.
CWD affects adult animals, mostly those between three and five years of age. It appears to affect animals of both sexes and to be transmitted during all seasons of the year. The incubation period from the time CWD is transmitted until symptoms appear can be at least 17 months; however, the maximum incubation time is not known. Once the disease takes hold, some animals have it for a few days, and others for up to a year. Most deer and elk are sick for a few weeks to several months before dying.
Researchers are not entirely sure how CWD is transmitted between animals. It likely happens during direct animal-to-animal contact or when animals share food and water supplies. Researchers are studying whether any group of humans might be at risk of CWD from eating the meat of infected game. The time between exposure and disease development is long, so research requires many years of follow-up.
It is likely that CWD has been present in deer populations since the late 1960s. The disease first was spotted in mule deer that were held captive in Colorado research facilities, but researchers did not identify the nature of the illness in the deer until the 1970s. Its origin before the 1960s is unclear, but CWD might have come from a prion disease in sheep called scrapie, which has been identified in the United States since 1947. It is also possible that CWD occurred spontaneously in deer. In 1979, CWD was identified in mule deer in a fish and game facility in Wyoming and in wild elk in Colorado in 1981. As of 2018, the disease was identified in mule deer, white-tailed deer, elk, and moose populations.
The geographical reach of CWD has widened in the twenty-first century. By 2018, the disease had spread to wild herds in at least 22 states, two Canadian provinces, in South Korea, and in some reindeer and moose in Norway. It was expected to continue spreading among cervids. Although it affects different types of deer and elk in particular and both sexes, some states have noted that CWD seems to affect certain populations more than others. On average, up to 10% of cervids can be affected where the disease is present in open ranges. Some geographic areas have more affected animals, such as parts of Wyoming and northeastern Colorado, where rates are much higher among mule deer. Captive deer have rates as high as 79%. CWD is less prevalent in elk.
It is most likely CWD proteins are spread between animals through their body fluids, including saliva, blood, urine, and feces. Research in 2015 revealed the prions can bind to soil, especially clay soil, and to grass and other plants. When the prion involved with CWD infects an animal, it causes abnormal proteins in the brain's cells to keep developing. An animal is infected with the prion through transmission from another infected animal. Deer and elk with CWD show progressive signs of weight loss. The animals' symptoms also can include strange behavior such as walking repeated courses, wider stances, lowered heads and ears, slight head tremors, or longer periods of sleep. The infected animals continue eating, but take in less food, and as they enter the late stages of the disease, they drink more water and urinate more often. They may begin to drool excessively and the signs of weight loss become extreme.
Though CWD can be inferred based on symptoms, it is not possible to accurately diagnose the disease without sample tissue from an animal, and the most accurate method is to obtain tissue from a specific portion of the animal's brain. This is done in a procedure after the animal dies. It is possible to look for the protein associated with CWD by testing a deer's lymph nodes and tonsil tissue early in the disease's incubation, but the test does not work well in elk. These tests can help diagnose CWD in an animal but are not considered food safety tests.
As of 2018, there was no reason to believe that CWD is a human public health issue, as no human cases have been found. However, in 2017, a study revealed that CWD can be transmitted to monkeys (macaques) that were fed muscle or brain tissue from infected deer or elk. The prions also were spread to monkeys that ate meat from deer or elk with no symptoms of CWD, but with the disease. Many states offer testing of animals for hunters, partly to assist in surveillance of CWD and partly to inform hunters whether a game animal they are hunting has the disease. Public health officials have issued cautions regarding shooting, handling, and consuming of animals that appear to be ill and about which parts of game animals to safely consume.
Improved surveillance to monitor and manage CWD prevalence is a priority for national, state, and local public health agencies. Because little still is understood about exactly how CWD transmits from one cervid to another, professionals must be vigilant in tracking the disease to help protect wildlife populations and prevent public health problems among communities.
The Centers for Disease Control and Prevention has stated that nearly all U.S. states and Canadian provinces have surveillance programs in place and that nationwide surveillance has increased. More than one million cervids that roam freely have been tested. Surveillance focuses on free-ranging animals, but the disease also occurs in herds that have been farmed or captured. A 2002 report from the National Wildlife Health Center emphasized the need to use surveillance data to help manage CWD. Various state agencies can work together to determine risk factors for the disease and how best to assess and monitor CWD when identified in a cervid population.
Managing CWD entails several approaches, including banning of translocating practices to limit disease transmission. In Colorado, attempts have been made to cull animals suspected of having CWD to help prevent further spread, but the effectiveness of this practice is unclear. It is extremely difficult to openly test wildlife in ranges to determine prevalence of CWD by obtaining tonsil tissue samples. Still, several state wildlife agencies are considering or have put in place regulations to control transportation of deer and elk carcasses obtained in areas known to have CWD. States also have taken steps such as banning import of live cervids or controlling movement of deer and elk within states.
Sound management of wildlife and captive herds can help control CWD, at least until more information is available on transmission or prevention. Reducing populations of cervids, and particularly culling animals selectively, appears to be the most effective practice for preventing further spread of CWD. Residents who live near affected areas often are discouraged from winter feeding of herds, which can increase the spread of CWD.
Though aggressive public health prevention does not appear to be necessary, precaution is advised when handling deer, elk, or moose carcasses or eating meat from these animals. Hunters and taxidermists who might come in contact with cervid tissue can take precautions such as avoiding shooting, handling, or consuming animals that appear to be sick. They should wear latex gloves when field dressing or processing deer, elk, or moose, and thoroughly disinfect all utensils, saws, blades, and tables used with a 50% solution of household unscented bleach. The bleach should be thoroughly rinsed from the utensils. People should minimize handling of brain and spinal tissues and immediately wash their hands or instruments after handling these tissues. People should not consume brains, spinal cords, eyes, spleens, tonsils, lymph nodes, or the pancreas tissue of deer, elk, or moose.
See also Zoonoses .
Dorak, Sheena J., et al. “Clay Content and pH: Soil Characteristic Associations with the Persistent Presence of Chronic Wasting Disease in Northern Illinois.” Scientific Reports 7 (2017). Published electronically December 22, 2017. doi: http://doi.org/10.1038/s41598-01718321-x .
Aimberg, E. S., et al. “Recommendations for Adaptive Management of Chronic Wasting Disease in the West.” Chronic Wasting Disease Alliance. http://www.wafwa.org/Documents%20and%20Settings/37/Site%20Documents/Committees/Wildlife%20Health/docs/CWDAdaptiveManagementRecommendations_WAFWAfinal_approved010618.pdf (accessed March 27, 2018).
Beecher, Cookson. “Suprising Discovery Made about Chronic Wasting Disease.” Food Safety News. http://www.foodsafetynews.com/2015/06/researchers-makesurprising-discovery-about-spread-of-chronic-wastingdisease/#.Wrp5bojwaM8 (accessed March 27, 2018).
Centers for Disease Control and Prevention. “Chronic Wasting Disease (CWD).” https://www.cdc.gov/prions/cwd/index.html (accessed March 27, 2018).
Wildlife Management Institute, 1440 Upper Bermudian Rd., Gardners, PA, 17324, (717) 677-4480, https://wildlifemanagement.institute .
Teresa G. Odle