What Is Chronic Wasting Disease?
CWD belongs to a group of diseases known as transmissible spongiform encephalopathies (TSEs). Within this family of diseases, there are several other variants that affect domestic animals: scrapie, which has been identified in domestic sheep and goats for more than 200 years, bovine spongiform encephalopathy (BSE) in cattle (also known as “mad cow disease”), and transmissible mink encephalopathy in farmed mink.
Several rare human diseases are also TSEs. Creutzfeldt-Jakob disease (CJD) occurs naturally in about one out of every one million people worldwide. Variant Creutzfeldt-Jakob disease (v-CJD) has been associated with the large-scale outbreak of BSE in cattle herds in Great Britain.
What Wildlife Species Are Affected By CWD?
Can Humans Get CWD?
The World Health Organization has reviewed available scientific information and concluded that currently there is no evidence that CWD can be transmitted to humans. During the period 1997-1998, three cases of sporadic Creutzfeldt-Jakob disease (CJD) occurred in the U.S. in young adults. These individuals had consumed venison. This led to speculation about possible transmission of CWD from deer or elk to humans. However, review of the clinical records and pathological studies of all three cases by the Centers for Disease Control and Prevention in Atlanta, Georgia, did not find a causal link to CWD.
Nonetheless, health and wildlife officials advise caution. Hunters are encouraged not to consume meat from animals known to be infected. In addition, hunters should take common sense precautions when field dressing and processing deer or elk taken in areas where CWD is found.
More information on the Centers for Disease Control and Prevention website.
Can Livestock Get CWD?
What Causes CWD?
The CWD infectious agent is smaller than most viral particles and does not evoke any detectable immune response or inflammatory reaction in the host animal. Based on experience with other TSE agents, the CWD infectious agent is assumed to be resistant to enzymes and chemicals that normally break down proteins, as well as resistant to heat and normal disinfecting procedures.
Where Does CWD Occur?
Where and How Did CWD Originate?
Scrapie, a TSE of domestic sheep, has been recognized in the United States since 1947, and it is possible that CWD was derived from scrapie. It is possible, though never proven, that deer came into contact with scrapie-infected sheep either on shared pastures or in captivity somewhere along the front range of the Rocky Mountains, where high levels of sheep grazing occurred in the early 1900s.
It may be possible that CWD is a spontaneous TSE that arose in deer in the wild or in captivity and has biological features promoting transmission to other deer and elk.
How Does CWD Spread?
Because CWD infectious agents are extremely resistant in the environment, transmission may be both direct and indirect. Concentrating deer and elk in captivity or by artificial feeding probably increases the likelihood of both direct and indirect transmission between individuals. Contaminated pastures appear to have served as sources of infection in some CWD epidemics. The apparent persistence of the infectious agents in contaminated environments represents a significant obstacle to eradication of CWD from either captive or free-ranging cervid populations.
The movement of live animals is one of the greatest risk factors in spreading the disease into new areas. Natural movements of wild deer and elk contribute to the spread of the disease, and human-aided transportation of both captive and wild animals greatly exacerbates this risk factor. The apparent spread of CWD between captive and wild cervids is a matter of hot debate. Although strong circumstantial evidence suggests that CWD has spread from positive captive elk to wild cervids in some instances, it may never be proven which group of animals represents the source of infection. It is likely that the disease has been passed in both directions (from captive to wild animals, and from wild to captive animals).
What Are the Symptoms of CWD?
The most obvious and consistent clinical sign is weight loss over time. CWD affected animals continue to eat but amounts of feed consumed are reduced, leading to gradual loss of body condition. Excessive drinking and urination are common in the terminal stages.
Behavioral changes also occur in the majority of cases, including decreased interactions with other animals, listlessness, lowering of the head, blank facial expression and repetitive walking in set patterns. In elk, behavioral changes may also include hyper-excitability and nervousness. Excessive salivation, drooling and grinding of the teeth also are observed.
Clinical signs of CWD alone are not conclusive. There are other maladies that have symptoms that mimic those of CWD. Currently, the only conclusive diagnosis involves an examination of the brain, tonsils or lymph nodes performed after death.
How Is CWD Detected?
A definitive diagnosis is based on examination of the brain for the characteristic microscopic spongiform lesions and/or accumulation of the CWD associated prion protein in brain and lymphoid tissues using a technique called immunohistochemistry. Gross lesions seen at necropsy reflect the clinical signs of CWD, primarily emaciation. Aspiration pneumonia, which may be the actual cause of death, also is a common finding in animals affected with CWD.
Research is being conducted to develop live-animal diagnostic tests for CWD. Early results indicate that a new live-test utilizing tissues from an animal’s tonsils may be viable in deer, but so far has been ineffective in elk.
Why Are We Concerned About CWD?
- Ongoing surveillance programs are expensive and draw resources from other wildlife management needs.
- Impacts of CWD on population dynamics of deer and elk are presently unknown. Computer modeling suggests that CWD could substantially reduce infected cervid populations by lowering adult survival rates and destabilizing long-term population dynamics.
- Where it occurs, CWD may alter the management of wild deer and elk populations, and it has already begun to do so.
- Ultimately, public and agency concerns and perceptions about human health risks associated with all TSE’s may erode hunters confidence and their willingness to hunt in areas where CWD occurs.
What Is Being Done About CWD?
- Several state wildlife agencies are aggressively collecting and testing wild elk and deer for the presence of CWD, and have instituted surveillance programs to examine hunter-harvested deer and elk.
- Some state wildlife agencies are considering adopting or have adopted regulations regarding the transportation of hunter-harvested deer and elk carcasses out of known CWD areas. Colorado has implemented regulations that allow only boned meat, quarters (without spinal column or head) or processed meat from deer or elk to be transported out of certain CWD areas.
- One option for managing CWD in wild populations is to reduce the density of animals in the infected area to slow the transmission of the disease. This is done by selective culling of animals suspected to have been exposed to the disease. In Colorado, Nebraska, Wisconsin and Saskatchewan, efforts are underway to drastically reduce local wild cervid populations in an effort to eliminate CWD in areas where it recently was found.
- Jurisdiction over commercial captive cervid operations varies from state to state. In some states the regulatory authority resides with the State agricultural or animal health agency, in some with the State wildlife management agency, and in some the authority is shared between agricultural and wildlife management agencies. When CWD is detected in a captive cervid facility, generally that facility is quarantined and all captive cervids in that facility are killed.
- Several states have recently implemented a moratorium on the importation of live cervids. Some states have also halted intra-state movement of deer and elk, and banned supplemental feeding programs.
- CWD surveillance of captive cervid farming operations is not yet regulated by the federal government, but some states, in cooperation with the industry, conduct CWD surveillance and have captive herd certification programs. A cooperative surveillance program began in 1997 between some states and the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS). APHIS and the North American Elk Breeders Association have proposed a cooperative Federal-State-Private Sector program to eradicate chronic wasting disease (CWD) from captive elk herds in the United States.
- A National Chronic Wasting Disease Plan was released June 26 by the U.S. Department of Agriculture and U.S. Department of Interior.
- Federal legislation has been introduced to provide additional funding for CWD research and control efforts, upgrade diagnostic laboratories and create a National Chronic Wasting Disease Clearinghouse. The proposed legislation would also clarify the jurisdictional lines of responsibility for the Departments of Interior and Agriculture, which share federal responsibilities for CWD.
What Preventive Measures Should Hunters Take?
Practical CWD Video for hunters who want to learn and see more is available online.
Where Can I Learn More About CWD?
Is the Meat Safe to Eat?
In the absence of complete information on risk, and in light of similarities of animal and human TSEs, public health officials and wildlife management professionals recommend that hunters harvesting deer and elk in the endemic area, as well as meat processors and taxidermists handling cervid carcasses, should take some common sense measures to avoid exposure to the CWD agent and to other known zoonotic pathogens.