Chronic Wasting Disease
by Beth Staben|
Scientists are constantly discovering previously unknown diseases in practically every species of animal. This is certainly true in the domestic livestock industry, and includes the so-called 'exotic' species. Chronic Wasting Disease (CWD) is a form of transmissible spongiform encephalopathy that, while still considered rare, is increasingly found among many species of both wild and domestic ungulates. First recognized by biologists in 1967, the disease is thought to be caused by a prion, a proteinaceous infectious material (Prusiner, 1995). The disease has become of particular concern due to its fatal nature, lack of known prevention or treatment, and its possible transmissibility to humans. Should Chronic Wasting Disease become prevalent, it has the potential to have serious negative impacts on the elk and deer farming industries in Alberta. Rather than ignoring the problem until it directly threatens the industry, the issue must be addressed and any possible preventive actions taken.
History of the Disease
Chronic Wasting Disease was first identified in captive mule deer (Odocoileus hemionus) and mule / white-tailed deer hybrids in 1967 in Fort Collins, Colorado. It has since also been recognized in many other species both in North America and Great Britain, including the greater kudu (Tragelaphus strepsiceros) (Kirkwood et al., 1993), Arabian oryx (Oryx leucoryx) (Kirkwood et al., 1990), white-tailed deer (Odocoileus virginianus) (Spraker et al., 1997), black-tailed deer (Odocoileus hemionus columbianus) (Williams and Young, 1980), and elk, (Cervus elaphus) (Williams and Young, 1982). Following the initial discovery of the disease at Fort Collins, most of the recorded cases have been from areas of Colorado and Wyoming. In cases that are not, the animal generally originates from these areas. Including wild animals, there have been well over one hundred cases identified in the United States, but the disease is relatively unknown in Canada. Only one official case has been reported thus far (Sullivan, 1998); this occurred in a farmed elk in Saskatchewan that originated from a game farm in South Dakota (Sullivan, 1998).
Chronic Wasting Disease affects both wild animals and those raised in captivity, and occurs in males, females, and castrates (Williams and Young, 1980). The age of onset varies, but is generally between 2.5 and 7 years (Spraker et al., 1997). The disease is first detected through its many clinical signs, which may first become obvious in any season (Williams and Young, 1980). The most common symptoms of the disease include: listlessness, weight loss, depression, ataxia, severe emaciation, marked weakness, dehydration, excessive salivation, a rough dull hair coat, and drooping of the head and ears (Kirkwood et al., 1994; Spraker et al., 1997 Spraker et al., 1997; Williams and Young, 1980; Williams and Young, 1982 ). Other evidence is also found through study of the affected animal's social behavior; a loss of fear of humans, decreased interactions with others in the group, hyperexcitability, and abnormal response to restraint are all characteristic signs of CWD (Williams and Young, 1980). While these signs may be detected in some animals, there is variation among individuals, and every symptom will not be found in every affected animal. The clinical course of CWD generally lasts from one to six months (Williams and Young, 1982). It is incorrect, though, to assume that these values are absolute, as the time will vary widely depending on the individual, and the disease is "often interrupted or terminated by secondary complications .... or by euthanasia" (Williams and Young, 1980).
Upon evaluation of blood samples, most hematologic values of animals infected with CWD are within a normal range (Williams and Young, 1980). If this is not the case, the values reflect the body condition of the individual. For example, non-regenerative anemia, a state in which red blood cells are unable to rebuild, may be found in infected animals due to emaciation, or there may be an increase in blood serum total protein that is indicative of dehydration (Schierman, pers. comm., 1998). Dehydration is also evident through urinalysis in a low urine specific gravity (Williams and Young, 1980).
The post mortem findings of animals with CWD are more numerous than is evidence discovered through blood and urine profiles. An excess of frothy fluid mixed with large amounts of sand or gravel in the rumen is found in affected deer (Williams and Young, 1980), and emaciation is often "accompanied by generalized absence or serous atrophy of subcutaneous and visceral adipose tissue and yellow gelatinous bone marrow" (Spraker et al., 1997). Abomosal and omasal ulcers are sometimes found, as are enlarged adrenal glands. On a microscopic level, the predominant lesions are found in the brain and spinal cord, and microcavitation of the gray matter by vacuoles occurs (Spraker et al., 1997). There is also neuronal degeneration and loss, the occurrence of amyloid plaques, and an absence of inflammatory response. Severe lesions in the olfactory tubercule and cortex, hypothalamus, and parasympathetic vagal nucleus are diagnostic of CWD (Williams and Young, 1993). Death of diseased animals is often not directly due to CWD, but to secondary infections that the body has been unable to fight due to its weakened condition. These include pneumonia, abscesses, enteritis, focal alopecia, and parasitism. In captive animals, death is also often due to euthanasia (Williams and Young, 1980).
The cause of encephalopathies is uncertain, but is thought to be related to other transmissible spongiform encephalopathy diseases such as scrapie and Bovine Spongiform Encephalopathy (BSE). There is strong evidence to suggest that the BSE outbreak in Great Britain was via consumption of meat and bonemeal supplements in feed sources contaminated with a scrapie-like agent (Kirkwood et al. 1990). BSE and CWD are, in all likelihood, closely related, and it follows that cervids that consume infected meat are also at risk of contracting CWD.
As there remains some speculation regarding the cause of CWD, the length of time required for visible signs of the disease to become apparent is also uncertain. Mark Zornes, a wildlife biologist in Wyoming, states that it takes at least 2 years to show symptoms in mule deer, white-tailed deer, and elk (Anonymous (1), 1998). In one case of an infected elk in Saskatchewan, the animal was imported from a game farm in South Dakota in 1989, and was diagnosed with CWD in 1996 (Sullivan, 1998). The animal presumably contracted the disease in South Dakota, thus it is possible that at least 7 years passed before clinical signs were apparent.
Chronic Wasting Disease, once thought to be caused by a 'slow virus' (Williams and Young, 1980), is now believed to be caused by a prion, or protein infectious particle (Prusiner, 1995). It closely resembles transmissible spongiform encephalopathies in other species, such as scrapie in sheep and transmissible mink encephalopathy (Williams and Young, 1982). Prusiner (1995) believes that the reason for the spongy appearance of the brain of animals infected with transmissible spongiform encephalopathies (TSE) to be as follows: mutations in the prion proteins allow the proteins to convert from their normal alpha-helical form to the mutated beta-sheet formation. This occurs inside neurons, after which the mutated form accumulates in lysosomes. In the brain, the filled lysosomes burst and damage cells. As the affected cells die, holes are created in the brain, and the prions that had been contained within the cells are released and go on to attack more cells. The holes created by this process produce the 'spongy' appearance of the brain.
CWD presently seems to be an extremely localized disease in North America. The vast majority of the cases are found in Colorado and Wyoming, although it is not understood why this is occurs (Williams, pers. comm., 1998). A small number of cases have been reported elsewhere, but their origin is either unknown, or can be traced to this area of the United States.
Like the effect of geographic location, modes of transmission are also uncertain. As discussed previously, the disease may be due to animal proteins in feed that contain prions. In free-ranging cervids, there is speculation that CWD is being contracted from domestic livestock infected with other transmissible spongiform encephalopathies (Anonymous (2), 1998).
While the most likely cause of the disease seems to be prion proteins in feed sources, research also indicates that there may be both horizontal and vertical transmission of CWD that promotes its spread in populations, although this has not been proven (Kirkwood et al., 1993; Kirkwood et al., 1994; Spraker et al., 1997). In the case of a kudu in Regent's Park, the animal was very unlikely to have acquired CWD from feed, as it was born after the ban of ruminant-derived proteins in ruminant feeds. The animal's dam, however, had been exposed to such feeds, and "it is possible that the agent [was] transmitted maternally in [the] kudu" (Kirkwood et al., 1994). The kudu could also have contracted the disease following its arrival at Regent's Park, providing evidence of horizontal transmission (Kirkwood et al., 1994). In another study by Spraker et al.(1997), it was stated that "both circumstantial and experimental evidence implicate animal-to-animal transmission of CWD in captive cervids." The study also notes that horizontal transmission has been found to occur among sheep, from sheep to goats, and among goats.
Implications of Chronic Wasting Disease
The likelihood of widespread incidence of CWD in Canada seems remote, but since little is known about the disease, the possibility must not be ignored. When BSE was diagnosed in Britain and its effects discovered, widespread panic ensued, practically decimating the beef market (McVicar, 1998). Public perception will also largely determine the fate of the markets in Canada should CWD become even somewhat prevalent (Williams pers. comm., 1998).
Prusiner (1995) discusses a species barrier that prevents the spread of disease between different species of animals. This 'rule' seems to have been broken among ruminants; prion diseases appear to be transmissible among some species (Prusiner, 1995). If this may occur between animals, the next logical consideration is whether or not humans are able to contract TSE's. The discovery that Creutzfeldt-Jakob disease (CJD) is caused by a prion and is a type of TSE has been a major source of concern. Evidence suggests that prions that cause, for example, scrapie and CWD, may be transferred to humans; development of CJD results (McVicar, 1998). With this knowledge, should the public ever be faced with the presence of CWD on Alberta game farms, the strong possibility exists that breeding stock, velvet, and venison markets will be destroyed. This would occur as a result of consumer fear that the product will cause disease both in themselves, and in domestic livestock.
Treatment and Prevention
As with other TSE's, there is currently no vaccine, nor any known treatment for CWD (Martin, 1997; Spraker et al., 1997; Williams and Young, 1980; Williams and Young, 1982). However, preventive measures may be taken in an attempt to stop the spread of the disease in farmed animals. Perhaps the most obvious method is to restrict feeds to those that do not contain animal proteins. It is also important to closely monitor the herd for illness and to isolate sick individuals as quickly as possible. It should be recognized, though, that due to the long incubation period of CWD, this alone will not prevent spread of the disease.
When purchasing new stock for the herd, the history and origin of the animal should be investigated. American producers that are not within the area affected by CWD should not purchase animals from the region, as this is not a necessity and thus should be considered a risk that is not worth taking. Border closures prohibit the movement of cervids into Canada from the United States, but this does not negate the need of Canadian producers to closely examine the history of new stock. This has been demonstrated in the case of the elk in Saskatchewan that was found to have CWD. The animal was purchased from a South Dakota herd one year prior to border closures, and was found to have CWD 7 years later (Sullivan, 1998). Care must be taken to select animals that have an extremely low chance of ever having come into contact with the disease.
Another common sense method of preventing CWD in a domestic herd is to isolate the animals completely from wild animals that may spread the disease. This generally involves the use of fences. Double fencing ensures that direct contact is not possible between wild and domestic animals. Also, fence height must be tall enough to prevent wild animals from jumping into the farm, and must also stop animals from entering under the fence.
Research and Surveillance
Currently, research is being conducted on both prions and CWD, as well as numerous other TSE's. While attempts may be made, it is impossible to effectively deal with a disease when so much remains to be discovered. In Iowa, "researchers are injecting brain material of infected wildlife into the brains of cattle. At the University of Wyoming, infected brain material is being given orally" (Bellisle (1), 1998). This will help determine if the disease can be contracted through consumption of infected feed, and if the species barrier may be broken. The latter is also being studied by allowing cattle to live with deer herds that have contained infected animals (Bellisle (2), 1998).
Ongoing research is also being carried out through surveillance programs in Colorado (Martin, 1997). Hunters voluntarily submit heads of animals to the Wildlife Division and these undergo rigorous testing to determine if a TSE is present. This allows officials to discover the prevalence of the disease in wild populations, and thus to gain insight on how this may affect domestic herds. Similar programs are established in both Alberta and Saskatchewan, and a program is being developed in British Columbia (Anonymous (3), 1998). Should the disease become more prevalent, it is likely that the industry will develop procedures to establish certified-free herds; this may be accomplished through surveillance for several years that will include necropsy and examination of the brains of adult animals that die for any reason (Williams pers. comm., 1998).
One major obstacle in solving the problem of CWD may be in producers' reluctance to alert authorities if their animals show symptoms of the disease, as this would likely result in the destruction of the herd and the potential loss of thousands of dollars (Anonymous (2), 1998). It is understandable that farmers will prevent this if possible. However, without cooperation between biologists, hunters, and producers, it will be significantly more difficult, perhaps impossible, to effectively deal with CWD. To prevent the problem, government must establish a policy that will at least partially compensate farmers for losses, thereby providing some incentive to alert authorities if CWD is suspected.
Chronic Wasting Disease, caused by a prion, is becoming more prevalent in both wild and domestic cervids. While the disease is not thoroughly understood, it is thought to be contracted through the consumption of feed that contains infected animal-derived proteins, and possibly by horizontal and vertical transmission among animals. Due to the fatal nature of the disease, a lack of treatment, and the possibility of transmission to humans, CWD has the potential to destroy the domestic cervid industry should it be discovered in Alberta. Thus, all feasible methods of prevention must be practiced, and policies for compensation and certified-free herds should be established. Only through strict controls and proper management practices will government and producers be able to prevent the possible collapse of the cervid industry.
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