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What is Chronic Wasting Disease?
Chronic wasting disease (CWD) is an infectious, neurologic disease of cervids, which includes North American deer, elk, and moose. A member of the family of diseases known as transmissible spongiform encephalopathies (TSEs), CWD has been diagnosed in free-ranging and captive cervids. Other TSEs include bovine spongiform encephalopathy (BSE, sometimes called "mad cow disease"), scrapie in sheep, and Creutzfeldt-Jakob disease (CJD) in humans.
What causes CWD?
Chronic wasting disease is believed to be caused by an abnormal form of a normal protein known as prion protein. Most commonly found in the central nervous system, it is capable of spreading to the peripheral nervous system, thus infecting meat (or muscle) of deer and elk. The abnormal prion is resistant to conventional disinfectants, high temperatures, and enzymes that break down normal proteins. The abnormal prion is a self-replicating (self-duplicating) protein that has the capacity to transmit an infectious disease. CWD infection does not cause a detectable immune response or inflammatory reaction like those found with typical bacterial, viral, or parasitic infections.
How is CWD transmitted?
The exact mechanism of transmission is still unclear. However, it is known that CWD is highly transmissible within deer and elk populations. Researchers believe that CWD is spread through direct animal to animal contact. Evidence exists that the disease can be transmitted from mother to offspring but this method of transmission appears to be relatively unimportant. Environmental transmission through feed, such as grass, and water sources is also possible as a result of contamination with abnormal prions. Methods of feeding that concentrate herds may increase the risk of animal-to-animal transmission.
What clinical signs are associated with CWD?
Chronic wasting disease causes progressive degeneration of the central nervous system and most often affects adult animals. The most obvious and consistent clinical signs are progressive weight loss and dehydration. Other signs include changes in temperament (loss of fear of humans, nervousness, hyperexcitability), changes in behavior (teeth grinding, walking in repetitive patterns in pens), incoordination, excessive thirst and urination, drooping of the head and ears, and excessive salivation. The incubation period is usually 18 to 24 months, but there has been evidence of an incubation period of more than five years. The health of affected animals typically deteriorates over a period of 12 months. There is no treatment for CWD and infection typically leads to death.
How is CWD diagnosed?
Chronic wasting disease is usually diagnosed by microscopic examination of the brain stem, lymph nodes, and tonsils after an animal's death. The presence of the abnormal prion protein in brain tissue is a positive indication of infection; however, the protein is not detectable until at least 3 months after infection, therefore negative results do not confirm the absence of infection.
Early results indicate that a live animal test has been developed that may be used to successfully detect CWD in deer, but has been unsuccessful in elk.
How is CWD prevented?
Because transmission mechanisms are still not well understood, it is difficult to recommend specific preventive strategies. Testing, surveillance, and culling are performed in an attempt to identify and contain the disease. Unfortunately, no vaccine or preventatives exist, and there is no evidence that immunity to CWD develops.
The United States Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) provides assistance to state officials in diagnosing CWD and in monitoring international and interstate movements of animals to help prevent its spread. In an effort to contain and eventually eradicate CWD, state and federal agencies are regulating and/or banning interstate movement of captive cervids, enforcing more stringent hunting policies, creating joint task forces, and developing state-specific guidelines. USDA also supports surveillance, diagnostics, and research of CWD in wild, free ranging deer and elk to ensure a coordinated and collaborative approach to control the spread of the disease.
How does CWD affect farmers and hunters?
Members of the general public, hunters, and owners of captive cervid herds must take precautions necessary to reduce transmission. Wild animals that appear ill should be reported to wildlife officials. Feeding and baiting wild deer should be limited, as this is an activity that brings deer into close contact with one another. Double fencing of captive herds lessens contact with wild animals, and vigilant surveillance and testing of these herds is recommended. Hunters should:
- avoid harvesting deer or elk that appear sick
- wear rubber gloves while field dressing animals and wash hands thoroughly afterwards
- remove all bone and fatty tissue from the meat of animals
- not saw through brain or spinal cord
- minimize handling of the brain, spinal cord, spleen, tonsils, lymph nodes, and eyes
- avoid consuming any animal with positive test results for CWD
- not remove anything but pure meat (muscle) from areas where CWD is known to exist
Can CWD be transmitted to other animal species?
There is currently no evidence that CWD can be transmitted to other ruminants such as sheep, cattle, or goats under natural conditions. Livestock housed with infected deer or elk, and those that have ingested the brain tissue of animals infected with CWD have not developed the disease.
Can humans contract CWD?
No evidence exists that CWD is naturally transmitted to humans, either through contact with or by eating meat from infected animals. However, routine precautions should be taken when handling carcasses of animals to prevent transmission of other diseases. Public health officials continue to evaluate any potential health risks associated with CWD.
According to the Centers for Disease Control and Prevention, "It is generally prudent to avoid consuming food derived from any animal with evidence of a transmissible spongiform encephalopathy. To date, there is no evidence that CWD has been transmitted or can be transmitted to humans under natural conditions. However, there is not yet strong evidence that such transmissions could not occur."
For the most recent information on CWD, please consult the following Web sites:
http://www.cdc.gov/ncidod/dvrd/cwd/
http://www.aphis.usda.gov/animal_health/animal_diseases/cwd/
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