Chronic wasting disease is a fatal neurological disease effecting deer and elk in North America. It was first identified in captive free-ranging deer at a research facility in Colorado in 1967 and has since become a major threat to cervid (deer and elk) populations in large areas of Colorado and Wyoming and parts of Nebraska, Wisconsin and South Dakota. The disease has no cure or preventative vaccine and threatens to cause serious long term problems for wild and farmed cervid population levels (South Dakota; Williams 1, 5). This threat not only effects cervid populations but also poses indirect problems for related species as wells as the economic effect of reduced hunting and recreational income. CWD will require intensive management, conservation and regulatory cooperation between state and federal wildlife agencies, if we are to eradicate or suppress the spread of the disease (Ament).
After its initial discovery in 1967, CWD was identified as a Transmittable Spongiform Encephalopathy (TSE) in 1978 and was recognized in elk shortly thereafter (Williams, 1). Because the animals that CWD was first identified in were frequently interchanged with other farmed cervids and transported to and from facilities in Colorado and Wyoming, it is impossible to determine if the disease originated from farmed or wild deer or elk. The first case of CWD found in free-ranging animals was in a mule deer in northeastern Colorado around 1981 (South Dakota). It is also estimated that CWD has also been present in farmed cervids since around that time.
In the early stages of infection, just after the disease leaves the incubation period, the symptoms are so slight that a casual observance of either farmed of free-ranging animals may give no indication that there is an infection present (Miller,3). This may contribute to failed identification of the disease by caretakers and wildlife professionals and is something that needs to be addressed in any CWD management plan. Towards the later stages of infection, symptoms include drooling, staggering, weakness; the animal will display aggressive or abnormal behavior and will become very emaciated (CO Division). Infected animals may develop aspiration pneumonia as a result of the increased drooling and may die prematurely due to this secondary infection. This pneumonia may also be a cause of misdiagnosis by caretakers. Administration of antibiotics will help treat the infection but is an unnecessary expense as the animal may as well already be dead (Miller 3). The end result of a CWD infection is always death.
The mode of infection in CWD has still not been proven, however it is not caused by a virus or bacteria. While the disease is most likely caused by an abnormal protein called a prion which infects and changes the molecular structure of other proteins it comes into contact with. This hypothesis is in line with the fact that the disease is a form of TSE like Bovine Spongiform Encephalopathy or Mad Cow Disease, which is also caused by a prion (CWD Alliance “What Causes”). Other types of TSEs include Scrapie, in sheep and Crutzfeld Jakob’s Disease in humans. In all of these diseases, prions attack the central nervous system and brain of the subject and also manifest in the tonsils and lymph system. Because prions are proteins and do not have DNA, they are extremely resistant to the environment, disinfectants, and enzymes that normally break down other types of proteins. The lack of DNA in a prion also explains why infected animals do not provide an immune response to a prion infection (CWD Alliance “What Causes”; Miller 3). Other infectious agents might include unconventional viruses, and/or a virino or an incomplete virus which only contains some of its DNA. More extensive research will be required before the agent of infection can be proven.
In addition to uncertainty about the causative agent of CWD there is also uncertainty about the way the disease is transmitted. Unlike other TSEs, transmission cannot be linked to food born sources as in Bovine Spongiform or Crutzfeld Jakob’s Disease. It is thought that transmission most likely occurs when residue from saliva, urine of feces is ingested by healthy animals. Infected animals may contaminate pasture lands just by eating or relieving themselves and because of the resistant nature of the infections agent; those lands may stay infectious for years. This is extremely significant for commercial cervid ranches where animals all share the same pastures, no matter how large (CDW Alliance; SE Cooperative 1). This makes CWD extremely dangerous to these operations and as in elk, can become the number one cause of adult mortality (SE Cooperative 1). It is not known when an infected animal becomes contagious but initial research suggests that shedding of the agent occurs sometime just before the disease comes out of the incubation period which can be as long a 16 months in some animals (SE Cooperative; Miller 4).
It is clear that CWD must be intensively managed by wildlife agencies, and regulations on the commercial cervid industry must be legislated if we are to curb the spread of the disease in the future. States that have infected populations as well as those that do not are constructing management plans to help deal with the disease. These plans need to address the free-ranging and captive populations of cervids on an individual and pro active basis (Ament). Colorado has an extensive plan for managing the disease in free-ranging cervids which includes aggressive testing of animals taken by hunters, road kill and animals euthanized by state officials. Suspect animals will be disposed of and healthy animals will be removed from infected herds. Colorado is also increasing funding for joint research projects with other state agencies in an effort to increase knowledge of CWD (Ament; Colorado Division). Management of CWD is easier if approached from a preventive standpoint. CWD is easier to manage on game ranches and commercial operations than in the wild (Texas Parks). As a result, any management strategy must contain a provision for aggressive testing of animals. A mandatory test should be done on all animals harvested from areas known to contain the disease and special attention should be paid to areas where free-ranging cervids may come into contact with commercial operations as well as state border areas (South Dakota). Testing gives a good indicator of the extent and spread of the disease which can aide in making further management decisions.
Management of the disease in commercial operations will require increased regulations regarding transportation of animal within and across state lines as this is the easiest way to unknowingly spread the disease (Ament; Miller 4). The only ways an operation can protect against CWD it to prevent its introduction by quarantine. Unfortunately, commercial operations where CWD is identified are required to dispose of every animal there. While they are compensated for their animals, there is still an economic loss to them and an additional expense to the state. This is why it is necessary to prevent the introduction of the disease into an operation through surveillance and a ban on transportation of live animals (Ament).
CWD is an extremely problematic disease to not only commercial cervid operations, but also to the environment, and to hunting recreation. States where the disease is not yet endemic need to aggressively eradicate CWD by disposing of infected herds and implementing surveillance programs. In states that already have endemic conditions, like Colorado, need to focus all their efforts on controlling the spread of the disease. The USDA should work on instituting a standard management program and allocate funds to help states with the increased cost of CWD management (Ament).