E. Tom Thorne

Chronic wasting disease of deer and elk: A review with recommendations for management

Chronic wasting disease (CWD) has emerged as an important disease of wild and farmed cervids in North America. Of the transmissible spongiform encephalopathies (TSEs), or prion diseases, CWD is the only 1 found in free-ranging species. Because the TSEs include infamous diseases like bovine spongiform encephalopathy (BSE) of cattle and variant Creutzfeldt-Jakob disease of humans, CWD by association has become a disease of interest beyond the parochial concerns where it is found. Consequently, wildlife managers are faced with developing programs for addressing CWD. Mule deer (Odocoileus hemionus), white-tailed deer (O. virginianus), and Rocky Mountain elk (Cervus elaphus nelsoni) are the only species known to be naturally susceptible to CWD. Although implications of CWD are not entirely clear at this time, we know that CWD is a fatal, contagious disease of mature reproductive segments of deer and elk populations. It has been endemic in free-ranging cervids in a core area of contiguous portions of southeastern Wyoming and northeastern Colorado, USA, for a minimum of 20 years and probably longer. The known geographic distribution of endemic CWD is relatively limited at this time, although as results of intensified surveillance become available, this may change. Foci of CWD in free-ranging deer have been identified distant from the core endemic area as far east as Wisconsin. Distribution has greatly expanded in the last decade or more via commerce in infected farmed elk; as a result, CWD recently has been found in multiple jurisdictions of the plains, foothills, and Rocky Mountains of western North America, and in South Korea. Studies of the biology and natural history of CWD over the last few years have resulted in a better understanding of its pathogenesis and epidemiology. Chronic wasting disease is transmitted horizontally from infected to susceptible cervids. Early involvement of alimentary tract-associated lymphoid tissues during incubation suggests plausible routes for transmission via feces or saliva. Residual environmental contamination also appears to be important in sustaining epidemics. Studies of CWD epidemiology led to development of models to help explain the history of CWD as well as forecast its impacts on deer and elk populations. Improved tests allow CWD to be diagnosed early in incubation, long before clinical signs appear. Where CWD is not known to occur, managers should be, and in some cases are, developing surveillance programs and regulations that prevent or reduce the likelihood that CWD will be introduced into their jurisdictions. Where CWD is already endemic, responsible agencies are conducting surveillance to assess status and trends in prevalence and geographic distribution, managing deer and elk populations to limit spread, and developing and evaluating techniques for further controlling and perhaps eradicating CWD. Programs for addressing the challenges of CWD management will require interagency cooperation, commitment of funds and personnel, and applied research.

CAUSES OF MORTALITY OF THE WYOMING TOAD

Wyoming toads (Bufo baxteri) that died from January 1989 to June 1996 were submitted to the Wyoming State Veterinary Laboratory (Laramie, Wyoming, USA) for postmortem evaluation. These consisted of 108 free-ranging toads and 170 animals from six captive populations. Ninety seven (90%) of 108 free-ranging toad carcasses were submitted during September and October. From 1989 to 1992, 27 (77%) of 35 mortalities in the captive populations occurred in October, November, and December. From 1993 to 1996, mortality in captive toads occurred without a seasonal pattern and coincided with changes in hibernation protocols that no longer mimicked natural cycles. Cause of mortality was determined in 147 (53%) of the 278 cases. Mycotic dermatitis with secondary bacterial septicemia was the most frequent diagnosis in 104 (71%) of 147 toads. Basidiobolus ranarum was found by microscopic examination of skin sections in 100 (96%) of 104 of these mortalities. This fungus was isolated from 30 (56%) of 54 free-ranging and 24 (48%) of 50 captive toads. This research documents the causes of mortality for both free-ranging and captive endangered Wyoming toads over a 7 yr period.

Comparative Vaginal Cytology of the Estrous Cycle of Black-Footed Ferrets (Mustela Nigripes), Siberian Polecats (M. Eversmanni), and Domestic Ferrets (M. Putorius Furo):

Vaginal cytology and vulva size were used to characterize the reproductive cycle of female black-footed ferrets (Mustela nigripes), Siberian polecats (M. eversmanni), and domestic ferrets (M. putorius furo). Emphasis was on black-footed ferrets because of the need to breed these critically endangered animals and on Siberian polecats because of the close taxonomic relationship to black-footed ferrets. Vaginal cytology of the 3 species of ferret is similar. Proestrus was characterized by an increasing percentage of superficial epithelial cells and enlargement of the vulva. During estrus, superficial cells were usually ≥ 90% of epithelial cells in the vaginal lavage and after several days were fully keratinized. Neutrophils were more common during all stages of the estrous cycle in domestic ferrets than they were in the other species. Following copulation, percentage of superficial calls in the vagina declined and vulva swelling subsided. Large cells, probably of uterine symplasma origin, were observed in vaginal lavages following whelping or pseudopregnancy. Vaginal cytology is extremely useful in the reproductive management of black-footed ferrets and Siberian polecats. Knowledge of normal vaginal cytology could be applied to the diagnosis of female reproductive abnormalities in all 3 species.

Brucella abortus strain RB51 vaccination in elk. I. Efficacy of reduced dosage.

Bovine brucellosis is a serious zoonotic disease affecting some populations of Rocky Mountain elk (Cervus elaphus nelsoni) and bison (Bison bison) in the Greater Yellowstone Area, USA. The fear that elk and/or bison may spread Brucella abortusto livestock has prompted efforts to reduce or eliminate the disease in wildlife. Brucella abortusstrain RB51 (RB51) vaccine has recently been approved for use in cattle. Unlike strain 19 vaccine, RB51 does not cause false positive reactions on standard brucellosis serologic tests. If effective, it may become the vaccine of choice for wildlife. In February 1995, 45 serologically negative female elk calves were trapped and taken to the Sybille Wildlife Research and Conservation Education Unit near Wheatland, Wyoming, USA. In May 1995, 16 of these elk calves were hand-vaccinated with 1 × 109 colony forming units (CFU) of RB51, 16 were vaccinated with 1 × 108 CFU RB51 by biobullet, and 13 were given a saline placebo. The elk were bred in fall of 1996 and they were challenged with 1 × 107 CFU of B. abortusstrain 2308 by intraconjunctival inoculation in March 1997. Thirteen (100%) control elk aborted, 14 (88%) hand-vaccinated elk aborted, and 12 (75%) biobullet vaccinated elk aborted or produced nonviable calves. These results suggest that a single dose of 1 × 108 to 1 × 109 CFU RB51 does not provide significant protection against B. abortusinduced abortion in elk. However, the vaccine appears to be safe at this dose and additional study may reveal a more effective RB51 vaccine regimen for elk.

VALIDATION OF A BRUCELLA ABORTUS COMPETITIVE ENZYME-LINKED IMMUNOSORBENT ASSAY FOR USE IN ROCKY MOUNTAIN ELK (CERVUS ELAPHUS NELSONI)

Brucellosis caused by infection with Brucella abortus is present in some elk (Cervus elaphus nelsoni) of the Greater Yellowstone Area (parts of Wyoming, Montana, and Idaho, USA). Since 1985, the Wyoming Game and Fish Department has vaccinated elk on elk feedgrounds in northwestern Wyoming during the winter months using B. abortus strain 19 (strain 19). Analysis of this vaccination program is hampered by the inability of standard serologic tests to differentiate between strain 19 vaccinated elk and those exposed to field strain B. abortus. In 1993, a competitive enzyme-linked immunosorbent assay (cELISA) was licensed to serologically differentiate between strain 19 vaccinated cattle and cattle exposed to field strain B. abortus. Seven groups of elk sera representing various B. abortus exposure histories were used to validate the cELISA test for elk. The cELISA test differentiated strain 19 vaccinated elk from elk that were challenged with B. abortus strain 2308, a pathogenic laboratory strain. The specificity of the cELISA was 96.8% for elk vaccinated with strain 19 only and sampled between 6 mo and 2 yr post vaccination, or with no B. abortus exposure. The sensitivity of the cELISA was 100%. The cELISA test will be useful in evaluating sera collected from elk in vaccinated, brucellosis endemic herds in the Greater Yellowstone Area.

SAFETY OF BRUCELLA ABORTUS STRAIN RB51 IN BULL ELK

Some of the elk (Cervus elaphus nelsoni) of the Greater Yellowstone Area (Wyoming, Idaho, Montana; USA) are infected with Brucella abortus, the bacterium that causes bovine brucellosis. Brucella abortus strain RB51 vaccine is being considered as a means to control B. abortus induced abortions in cow elk. However, the most probable vaccination strategies for use in free-ranging elk might also result in some bull elk being inoculated, thus, it is important to insure that the vaccine is safe in these animals. In the winter of 1995, 10 free-ranging bull elk calves were captured, tested for B. abortus antibodies, and intramuscularly inoculated with 1.0 × 109 colony forming units (CFU) of B. abortus strain RB51. Blood was collected for hemoculture and serology every 2 wk after inoculation for 14 wk. Beginning 4 mo postinoculation and continuing until 10 mo postinoculation elk were serially euthanized, necropsied, and tissues collected for culture and histopathology. These elk cleared the organism from the blood within 6 wk and from all tissues within 10 mo. No lesions attributable to B. abortus were found grossly and only minimal to mild lymphoplasmacytic epididymitis was found in a few elk on histologic examination. In a separate study, six adult bull elk from Wind Cave National Park (South Dakota, USA) were taken to a ranch near Carrington (North Dakota, USA). Three were orally inoculated with approximately 1.0 × 1010 CFU of RB51 and three were inoculated with corn syrup and saline. Ninety days post-inoculation semen was examined and cultured from these bulls. Strain RB51 was not cultured from their semen at that time. There were no palpable abnormalities in the genital tract and all elk produced viable sperm. Although they contain small sample sizes, these studies suggest that B. abortus strain RB51 is safe in bull elk.

Spontaneous Tumors of Free-Ranging Terrestrial Mammals of North America

Published information on tumors of free-ranging wild animals is limited. Case reports are scattered throughout the literature in journals as well as project reports from wildlife management agencies, preceedings of meetings and theses and dissertations. Much information on occurrence of tumors in wildlife remains buried in files of wildlife biologists, veterinary pathologists and diagnostic laboratories.

Safety of Brucella abortus Strain RB51 in Deer Mice

Brucella abortus strain RB51 is an approved brucellosis vaccine for use in cattle that may have potential as an oral vaccine for use in elk (Cervus elaphus) and/or bison (Bison bison). This study was designed to determine effects of strain RB51 on deer mice (Peromyscus maniculatus), a nontarget species that could have access to treated baits in a field situation. In February 1994, 90 mice were orally dosed or intraperitoneally injected with 1 × 108 colony forming units strain RB51 and 77 controls were similarly dosed with sterile saline. At weekly intervals through early April 1994, 4 to 6 mice from each group were euthanized, gross necropsies performed, spleens and uteruses cultured, and tissues examined histologically. All orally inoculated mice cleared the infection by 6 wk post-inoculation (PI). While most of the injected mice cleared the infection by 7 wk PI, a few required 9 wk. There were minimal adverse effects attributable to strain RB51. Apparently, strain RB51 would not negatively impact P. maniculatus populations if it were used in a field situation. Also, deer mice appear to be able to clear the vaccine in 6 to 9 wk, thus the probability of these mice transmitting the vaccine to other animals is low.