Approved by the AZA Board of Directors in 2003
In response to the concern for introduction of Chronic Wasting Disease (CWD), and the management and regulatory implications for zoological facilities, a set of guidelines have been drafted for AZA-accredited institutions by an ad hoc working group of the American Association of Zoo Veterinarian's Infectious Disease Committee in conjunction with the AZA Animal Health Committee (2003).
The objectives of these guidelines are to:
Although every individual institution needs to establish their own acceptable level of risk with regard to this issue, the implementation of these guidelines will show state and federal regulatory agencies dealing with AZA-accredited institutions that our community is both informed and proactive with regard to this issue. As USDA and state regulatory agencies develop mandatory programs for surveillance and eradication of CWD and cervid transport, these guidelines may require modification to comply with regulatory requirements for each individual institution. It is strongly encouraged that senior zoo staff establish a relationship with regulatory personnel responsible for monitoring CWD programs in their area in order to keep abreast of regulations being developed at both the State and Federal levels.
A full review of the Transmissible Spongiform Encephalopathies (TSE) was recently published in both the AZA Communique and the Journal of Zoo and Wildlife Medicine.  The causative agents of TSE's are described as unconventional proteinaceous infectious particles, or prions. Prions have yet to be fully characterized but it is thought that they are abnormal proteins that are capable of changing normal cellular prion proteins into abnormal shapes. These abnormal shapes cause conformational changes that result in changes in many of the properties of these proteins including: increased resistance to heat, pH and partial resistance to proteases.
Large-scale improper folding of protein sheets in tissues of the central nervous system is partially responsible for neurological clinical signs in all affected species. This improper folding, when it occurs in sufficient quantities, results in the formation of holes in brain tissue that gives it a spongy appearance under the microscope – hence the name spongiform encephalopathy.
Transmissible spongiform encephalopathies consist of numerous diseases in man and animals.  Those TSE's of concern to zoo and wildlife practitioners include scrapie (sheep and goats), bovine spongiform encephalopathy (BSE), transmissible mink encephalopathy (TME), feline spongiform encephalopathy (FSE), and chronic wasting disease (CWD) in deer (Odocoileus sp.) and elk (Cervus elaphus nelsoni). Kuru, classical Creutzfeld-Jakob disease (CJD), Gerstmann-Straussler syndrome (GSS), fatal familial insomnia, and variant Creutzfeld-Jakob disease (vCJD) are TSEs that affect humans.
All are characterized by a long incubation period, up to forty years in kuru, without the presence of clinical symptoms. The clinical phase results in a progressive degenerative neuropathy without the presence of an identifiable inflammatory process or specific signs of chronic viral infection. In addition, no virus-like or other micro-organism-like structure is present in the brains or CNS fluid of infected patients. All infections result in death in the species in which they occur. There is currently no vaccine or treatment for any of these diseases.
Chronic Wasting Disease (CWD) is a progressive, debilitating and invariably fatal disease of deer and elk.  It was first recognized in 1967 and clinical signs include progressive weight loss, behavior changes, and listlessness. It is classified as a transmissible spongiform encephalopathy (TSE) or "prion" disease. The disease has been found in greatest numbers among free-ranging deer and elk in north central Colorado and southeastern Wyoming; but cases have been found in free-ranging deer and/or elk in Nebraska, South Dakota, New Mexico, Utah, Illinois, and Wisconsin. In addition, CWD has been diagnosed in farmed elk and/or deer herds in several states including Colorado, Kansas, Minnesota, Montana, Nebraska, Oklahoma, South Dakota, and Wisconsin. In Canada, CWD has been identified in both free-ranging deer and privately owned elk in Saskatchewan, and farmed elk in Alberta.
CWD has affected mule deer (Odocoileus hemionus), white-tailed deer (O. virginianus), and elk (Cervus elaphus nelsoni). Other ruminants housed near infected cervids have not been naturally infected. It is not known whether other cervid species (North American or exotic) are susceptible to infection. Transmission between animals is most likely to occur through animal-to-animal contact and/or contamination of feed or water sources with saliva, urine or feces from a diseased animal. The route of transmission has not been definitively determined. It does not appear that CWD is transmitted to other ruminant species in contact with infected cervids.
Diagnosis is confirmed by immunohistochemical staining of the obex portion of the brainstem after death. Recently, three ELISA-based CWD diagnostic test kits have been approved by the USDA; these tests can be applied to lymph node, and are licensed for use in deer and elk. Although these will be used for surveillance of free-ranging cervids, they are not currently approved for use in captive cervid regulatory programs.
Current information indicates that CWD does not cause illness in people. The known prion diseases of humans and domestic animals appear to be caused by prions unrelated to CWD. It is unknown whether consumption of infected tissue may present a risk. Risk avoidance recommendations for those with close contact to susceptible animals (hunters, wildlife biologists, pathologists) include:
The USDA and some states have a voluntary herd-certified status program. A federal program for surveillance and eradication of CWD is currently being developed by the USDA. Several states have placed moratoria on imports and transport of cervids at the time of this writing. Check with your State Veterinarian for current information.
Introduction of CWD into a zoological institution may occur through:
Regulations for interstate transport of cervids are currently being developed by the USDA and many states. Due to the rapid changes in state and federal regulations pertaining to cervid movement, it is critical that a designated staff member obtain current information from the appropriate regulatory agencies. Jurisdiction over captive cervids varies between states but is assigned to either the State's Department of Agriculture (DOA), or equivalent, and/or Department of Fish and Game, or equivalent.
The currently proposed USDA program will be designed to restrict interstate movement in order to provide basic minimum standards for the federal programs (Plan for Assisting State, Federal Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive Cervids, USDA-APHIS, June 26, 2002). State regulatory programs may implement additional controls if deemed necessary.
We recommend that the following guidelines and definitions be applied to cervid and other ungulate species:
Documentation of a disease surveillance program (based on the AZA requirements for medical records and necropsies) and provisions to exclude wildlife may suffice to meet the same criteria established for "CWD accredited free herds" of farmed cervids. Discussion with regulatory agencies may allow AZA institutions to transfer cervids between institutions (similar to the exemption for TB testing of cervids transferred between AZA facilities). As the federal program is developed, AAZV will continue to work with the USDA on behalf of AZA member institutions. However, it is expected that program species (Odocoileus hemionus, O. virginianus, Cervus elaphus nelsoni) will be required to meet all federal requirements, regardless of source.
Only those AZA-accredited institutions that have documentation meeting standards for a "CWD accredited free herd" should ship cervids to non-accredited facilities. Receiving facilities should meet all institutional and AZA requirements for disposition.
Herd "CWD-free" certification programs will vary between states, and may be more stringent than the USDA federal program. Currently, there is no officially approved antemortem (performed on living animal) test for CWD. Diagnosis is based on histopathologic changes in brain tissue using immunohistochemical (IHC) staining of the obex (brainstem). There are approximately 26 veterinary diagnostic laboratories certified by USDA to perform CWD testing (contact your state veterinarian for details). In 2003, the USDA approved three ELISA-based CWD diagnostic test kits (Bio-Rad, VMRD, IDEXX).
Although these tests are incorporated into surveillance of free-ranging cervids, they are not currently approved for captive cervid regulatory programs. In order to align with planned federal certification status for captive cervids, AZA institutions should maintain cervid herds for a minimum of 5 years with no evidence of CWD on necropsy using approved testing methods.
The national surveillance plan for farmed cervids (USDA) includes mandatory death reporting and CWD testing (by a certified laboratory) of all animals, except calves, that are slaughtered or die on the premises. It is not known at this time whether these plans will be applied to zoos in a regulatory fashion. Record keeping requirements for AZA accreditation should be adequate to meet inventory requirements; however, additional diagnostic testing may need to be implemented.
Experimental studies have indicated that IHC staining of lymphoid tissue obtained via tonsil biopsy may provide an antemortem test for CWD.  It appears that this may be useful to detect early infection in deer but may not be sensitive in elk, due to differences in CWD pathogenesis in this species. Therefore, lymphoid tissue (especially from tonsils) should be considered for testing from "high risk" or "at risk" species to contribute to the database. Details of collection and submission procedures should be obtained directly from the designated laboratory.
AZA accreditation standards require member institutions to perform necropsy examinations on all collection animals to determine the cause of death (see www.aza.org/accreditation). This does not always require removal and examination of the brain (specifically, the obex). It is strongly recommended that all zoo cervids over the age of 12 months that die or are euthanized should have the head (specifically, obex of the brain) submitted to NVSL, or a certified laboratory for CWD testing. In addition, tonsil and retropharyngeal lymph nodes should be collected for possible future testing.
Management and Surveillance of Free-Ranging Cervids on Zoo Grounds
Free-ranging cervids pose a potential risk to zoo collection animals. Although the actual risk cannot be quantified at this time, it is real. Therefore, every effort should be made to exclude these animals from zoo grounds. Maintenance of appropriate wildlife exclusion methods such as perimeter fences should be used to minimize contact between wildlife and collection animals. Any free-ranging cervids that die or are euthanized around the zoo grounds or surrounding areas should be screened for CWD.
USDA and State regulatory eradication programs
The USDA began its CWD eradication program for farmed elk and deer herds in fiscal year 2003 (Plan for Assisting States, Federal Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive Cervids, USDA-APHIS, June 26, 2002). The National Park Service will continue its targeted surveillance and removal of cervids exhibiting clinical signs of CWD. The Department of the Interior will work with states in hunter surveillance programs. Although enrollment of captive cervid herds is voluntary, herds that have program species (red deer, elk, mule deer, white-tailed deer) will not be permitted to move animals interstate if they are not certified.
If enrolled from initiation of the program, interstate transport will be permitted after the first year of surveillance if no cases of CWD have been detected. Herds that are enrolled at later dates will be required to meet the full 5-year surveillance period before moving animals interstate. Interstate and intrastate movement of non-program cervid species will be subject to state regulations. It is strongly recommended that all AZA institutions comply with voluntary requirements for CWD certification for any cervid herds in their collection, including non-program cervid species.
If an institution participates in a voluntary herd certification program, remember that CWD is a reportable disease and results of CWD positive cases will be communicated through a network of certified testing laboratories to the USDA. Therefore, cervids from AZA institutions that test positive will be reported by the testing laboratory. In addition, veterinarians at the specific institutions will be responsible for contacting the appropriate state agencies (as designated by the state; this could include the state veterinarian, department of fish and wildlife, board of animal health, etc.) should a positive be found. Those institutions with CWD positive cervids will fall under the jurisdiction of federal and state eradication programs; we recommend creating a relationship with your regulatory authorities now to enhance communication should a suspect or positive case be found.
Currently, the USDA defines a positive case of CWD as any animal found positive by IHC staining of appropriate tissues. Suspects include any animal from a "high risk" species with compatible clinical signs. "Trace back" and "trace forward" investigations would be conducted by regulatory personnel to determine source and possibly exposed animals. Based on documentation already present in AZA-accredited institutions, sources and dispositions of animals can be identified, including enclosures and animal contacts.
Options for herds with a positive case include depopulation or quarantine and surveillance. Depopulation may not be desirable in the case of genetically valuable individuals. However, quarantine would result in restrictions on movements of cervids to and from, and possibly within, the institution. Additional restrictions on use of the premises, surveillance and testing, and quarantine for at least 60 months from the last known case would be imposed on the institution.
Individual institutions will need to determine the most appropriate plan with their regulatory agencies. Herd management plans are required for CWD positive and exposed herds. These plans include provisions for depopulation or quarantine, disposition of carcasses, decontamination, and future use of the premises (Plan for Assisting States, Federal Agencies, and Tribes in Managing Chronic Wasting Disease in Wild and Captive Cervids, USDA-APHIS, June 26, 2002).
Carcass disposal and decontamination
Carcass disposal of cervids may be regulated by state or local municipalities. Methods for handling CWD positive carcasses include incineration, tissue digestion, and burial using an engineered landfill (USAHA CWD Workshop, St. Louis, Missouri, October 22, 2002). Zoos are encouraged to discuss changes in current procedures with their local and state regulatory agencies. Wisconsin has developed a CWD carcass disposal risk assessment document. Guidelines for farm clean-up and disinfection have been created for use in Saskatchewan.
Based on other prion diseases and the link between contaminated pastures and CWD outbreaks, environmental contamination presents a major risk factor. Whether environments can be completely disinfected is questionable. Until effective cleaning and disinfection protocols can be identified, it is recommended that cervids should not be introduced to facilities or exhibits where CWD has occurred for at least 5 years. It is also critical that free-ranging cervids and other wildlife be excluded from these areas.
Other biosecurity measures that should be addressed for AZA-accredited institutions include staff education addressing zoonotic concerns, carcass disposal methods, and exhibit and facility contamination and decontamination. Currently, CWD has not been linked to human disease, despite recent media reports. AAZV has developed a Chronic Wasting Disease Fact Sheet (in PDF) that outlines general precautions that should be followed to minimize risks.
We (M. Miller, D. Travis) wish to thank AAZV, Disney's Animal Programs, and Lincoln Park Zoo for support to attend the USAHA meetings and for time to keep current on diseases of concern to the zoo veterinary community. We also wish to thank Drs. Robyn Barbiers, Julie Langenberg, Bob Cook, Wilbur Amand, Don Janssen, and Bruce Rideout for their advice and comments on this manuscript. We especially appreciate the discussions with Drs. Michael Gilsdorf and Dean Goeldner (USDA NAHP) regarding CWD and zoos.
Williams, E.S., M.W. Miller, T.J. Kreeger, R.H. Kahn, and E.T. Thorne. 2002. Chronic wasting disease of deer and elk. J. Wildl. Managem. 66(3): 551-563.
Wolfe, L.L., M.M. Conner, T.H. Baker, V.J. Dreitz, K.P. Burnham, E.S. Williams, N.T. Hobbs, and M.W. Miller. 2002. Evaluation of tonsillar biopsy data for estimating chronic wasting disease prevalence in free-ranging mule deer. Proc. Wildl. Dis. Confer. Arcata, California. p. 126.
Travis, D.A. and M. Miller. 2003. A Review of the transmissible spongiform encephalopathies and recommendations for surveillance of chronic wasting disease in zoos. Journal of Zoo and Wildlife Medicine 34(2): 125-133.