This document provides a short overview of the disease cycle and transmission of Chronic Wasting Disease (CWD)among the elk herd in Colorado. It also outlines the steps the State of Colorado is taking to ensure detection of the disease in wild and domestic herds of bovine animals.
The Colorado Department of Agriculture has taken strong preventative measures to protect Colorado's domesticated deer and elk populations from chronic wasting disease. Chronic Wasting Disease (CWD), a disease of deer and elk, is one of the transmissible spongiform encephalopathy (TSE) diseases, i.e. scrapie, bovine spongiform encephalopathy (BSE), mink encephalopathy, feline encephalopathy, and Creutzfeldt-Jakob Disease (CJD) in humans. TSE diseases are thought to be associated, if not caused by, abnormal prion protein. Prion protein is normally produced in all mammals, even though the function of the protein is poorly understood. Scientists know that when the abnormal prion protein (PrPres) enters a susceptible host the normal prion protein (PrP) is slowly converted to the PrPres form; also, cells of the susceptible host will begin to produce PrPres. PrPres is very resistant to degradation by most disinfectants, heat, and environmental processes. PrPres differs from normal PrP by the way it is folded during its transcription at the cellular mitochondrial membrane. Apparently, there is an alteration of amino acids in a given loci that causes the abnormal folding.
The TSE diseases have similar transmissibility within their own host specie by injection and oral route. CWD is thought to be transmitted primarily orally through contaminated feed and water from CWD infected animals, particularly the end-stage clinical CWD animal. Environmental contamination with the CWD PrPres appears to be a major way the disease maintains itself in a cervid population. Both the excretion and secretions from infected animals, as well as the carcasses of animals that succumbed to the disease factor into this environmental contamination. Chronic wasting disease pathogenesis is not fully understood, although pathogenesis studies have shown when elk are infected orally with CWD-infected material, the tonsils and head lymph nodes will have PrPres present, by immunohistochemical (IHC) staining, two months after oral dosage. At four months the mesenteric lymph nodes and pyre's patches have PrPres fibrils. The dorsal motor nucleus of the Vagus located at the obex will show staining for PrPres as early as six months. Clinical signs develop between 14 and 30 months post infection. Deer follow a similar pattern of disease progression, however, deer are much more susceptible to the disease as compared to elk. Currently, the definitive diagnosis depends on hallmark spongiform lesions in the brain by post mortem histopathologhy, PrPres staining with a monoclonal antibody IHC, and the Western blot. The obex is the preferred specimen for the diagnostic tests.
Colorado's Alternative Livestock facilities have been and continue to be under mandatory surveillance of all elk mortalities, i.e. natural, slaughter, and hunt park kills. Through this process two premises with one elk each have been found to be positive for CWD. The first diagnosed CWD case was in a small herd of eleven animals, which was depopulated and indemnified at 50% of market value. The second case was disclosed in a large operation that had been submitting CWD samples for thirty months when a 21-month-old heifer died after routine processing, and was apparently in good health prior to her death. A herd plan was developed for the later herd with risk groups assigned according to animals exposure to the positive CWD heifer and her environment (pen). Thirty-five heifer pen mates were depopulated with indemnity. All thirty-five heifers' brains, head and mesenteric lymph nodes were negative to both histopathology and IHC staining for CWD, and the carcasses were allowed to enter the human food chain.
However, it was the slaughter of the heifers that prompted the Food Safety and Inspection Service (FSIS) to do a CWD risk assessment for human safety and slaughter facility contamination by the PrPres. Interestingly, this action alerted the FDA that they have purview over elk meat, which prompted their TSE advisory committee to evaluate CWD as a potential human health risk. The TSE committee ruled there was insufficient scientific data and the limited information did not indicate a human health risk. But, the FDA and the committee would continue to monitor information and consumers of elk and venison for variant CJD. A limited CWD PrPres research has been carried out with velveted antler, but no PrPres has been disclosed in antlers of CWD-positive bull elk. South Korea is the major market for velvet, however, they have halted importation of U.S. velvet due to concern about CWD.
To date, five states have diagnosed CWD in captive elk herds. They are Colorado, Nebraska, Montana, Oklahoma and South Dakota. Canadian farms in Saskatchewan province currently number 18 farms, which have had cases of CWD. Elk movements are blamed for the CWD extension into captive elk herds.
CWD Specimen Collection
Colorado's CWD surveillance program depends on good specimen collection and preservation. Certainly, ambient temperature, and the ability to find mortalities in a timely manner dramatically influence specimen quality, but assuring that the obex with the dorsal motor nucleus of the vagus is included is paramount for an appropriate CWD examination. The State Veterinarian's Office has and will continue to provide CWD specimen collection training. Encouraging elk producers to refrigerate elk heads as soon as possible is helpful in maintaining the brain in reasonable condition. All mortalities are required to have a brain or obex specimen submission regardless of its condition. The immunohistochemical staining can be performed on degenerated samples, since the PrPres remains and does not break down. Alternatively, the western blot can be run for the PrPres.
Practitioners that collect elk CWD specimens must include all ID with the specimen, producers name, address, and phone number. The absence of this information prevents the producer from being credited for the their submissions and assuring that there is a "chain of custody". When possible the ears should be submitted with the ID intact with the obex or brain specimen.
Upon request, personnel from the State Veterinarian's Office will provide regional CWD specimen collection and preservation training for veterinarians. Generally, two to three week notices will be required prior to a training session. Please, contact the State Veterinarian's Office at (303) 239-4161.