Terry J. Kreeger

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.

EPIZOOTIOLOGY OF CHRONIC WASTING DISEASE IN FREE-RANGING CERVIDS IN COLORADO AND WYOMING

Surveillance and epidemic modeling were used to study chronic wasting disease (CWD), a transmissible spongiform encephalopathy that occurs naturally among sympatric, free-ranging deer (Odocoileus spp.) and Rocky Mountain elk (Cervus elaphus nelsoni) populations in contiguous portions of northeastern Colorado and southeastern Wyoming (USA). We used clinical case submissions to identify endemic areas, then used immunohistochemistry to detect CWD-infected individuals among 5,513 deer and elk sampled via geographically-focused random surveys. Estimated overall prevalence (prevalence, 95% confidence interval) in mule deer (4.9%, 4.1 to 5.7%) was higher than in white-tailed deer (2.1%, 0.5 to 3.4%) or elk (0.5%, 0.001 to 1%) in endemic areas; CWD was not detected in outlying portions of either state. Within species, CWD prevalence varied widely among biologically- or geographically-segregated subpopulations within the 38,137 km2 endemic area but appeared stable over a 3-yr period. The number of clinical CWD cases submitted from an area was a poor predictor of local CWD prevalence, and prevalence was typically ≥1% before clinical cases were first detected in most areas. Under plausible transmission assumptions that mimicked field data, prevalence in epidemic models reached about 1% in 15 to 20 yr and about 15% in 37 to 50 yr. Models forecast population declines once prevalence exceeded about 5%. Both field and model data supported the importance of lateral transmission in CWD dynamics. Based on prevalence, spatial distribution, and modeling, we suggest CWD has been occurring in northeastern Colorado and southeastern Wyoming for >30 yr, and may be best represented as an epizootic with a protracted time-scale.

Brucella abortus strain RB51 vaccination in elk. II. Failure of high dosage to prevent abortion.

Brucella abortusstrain RB51 is used as a vaccine because it induces antibodies that do not react on standard serologic tests for brucellosis allowing differentiation between vaccination and infection. Strain RB51 was evaluated in captive elk (Cervus elaphus) to determine if vaccination protected against abortion following experimental challenge. Thirty elk were vaccinated intramuscularly with 1.0 × 1010 colony-forming units (CFU) of strain RB51 in March 1998. Fourteen of these were given a booster dose of 1.13 × 1010 CFU exactly 1 yr later. All vaccinated elk seroconverted via a modified dot blot assay to strain RB51 with the booster group having higher titers (P ≤0.001). Seventeen other elk served as unvaccinated controls. All elk were bred and determined pregnant using pregnancy-specific protein B analysis. Elk were challenged in March 2000 with 1.1 × 107 CFU of B. abortusstrain 2308 administered intraconjunctivally and all elk seroconverted to strain 2308. Fifteen of 17 control elk aborted; 16 of 16 elk given a single vaccination aborted (P = 0.44); and 13 of 14 elk given a booster aborted (P = 0.86). There were two viable calves in the control group and one in the booster group. Strain 2308 was recovered from fetuses and nonviable calves in all groups. Based on the results of this and other studies, the use of strain RB51 to prevent abortion in elk cannot be recommended.

Observations on the Use of GonaconTM in Captive Female elk (Cervus Elaphus)

Overabundant populations of elk (Cervus elaphus) are a significant concern in some areas of the western United States because of potential ecologic damage and spread of brucellosis to domestic livestock. Brucella abortus is transmitted among elk through direct contact with aborted fetuses, placentas and associated fluids, or postpartum discharge of infected animals. Because transmission of brucellosis is dependent on pregnancy, contraception of cows could be used for both disease and population management. The objective of this study was to evaluate the contraceptive efficacy of a gonadotropin-releasing hormone vaccine (GonaConTM) in female elk. In September 2004, cows were given a single immunization of either 1,000 μg (n=12) or 2,000 μg (n=10) of GonaConTM and compared with a group of adjuvant-treated controls (n=15). In November 2004, 2005, and 2006, cows were grouped with bulls for the breeding season. Blood samples were taken in February 2005 and March 2006 and 2007 for pregnancy testing, progesterone assays, and antibody titers. For cows given 1,000 μg GonaConTM the percentages that were infertile for 2005, 2006, and 2007 were 86%, 90%, and 100%, respectively, compared with 90%, 100%, and 100% for cows given 2,000 μg GonaConTM. Rates of infertility for control cows were 23%, 28%, and 0% (P<0.0001). The results indicated that either dose of GonaConTM prevented pregnancy of elk cows for at least 3 yr. We concluded that GonaConTM use for population management of elk warrants consideration as part of a strategy to control brucellosis.

Pathological Responses of Red Foxes to Capture in Box Traps

We documented the physiological responses of captive-raised red foxes (Vulpes vulpes) to capture in box (i. e., live) traps. The behavior of captured foxes was video recorded, and heart rate and body temperature were monitored via radio telemetry throughout an 8-hour restraint period. Endocrine, biochemical, hematological, and pathological samples were collected. Responses of foxes caught in box traps were compared to the responses reported by Kreeger at al. (1990c) for untrapped (i.e., control) foxes and foxes caught in padded- and unpadded-jaw foothold traps. Heart rate and body temperature increased after foxes were caught in box traps, but never significantly exceeded mean pretrapped levels. Foxes caught in box traps were physically active for 35.7±8.8 (SE)% of the restraint period. The majority of this activity consisted of pacing in the trap. Foxes caught in box traps had higher (P<0.003) adrenocorticotropin and cortisol values than untrapped foxes, and lower (P<0.001) β-endorphin and cortisol levels than foxes caught in foothold traps. Bilirubin, alkaline phosphatase, lactate dehydrogenase, and aspartate aminotransferase levels for foxes caught in box traps were elevated (P<0.01) above levels of untrapped foxes. Foxes caught in box traps had lower (P<0.004) alkaline phosphatase, lactate dehydrogenase, creatine kinase, and aspartate aminotransferase levels than foxes caught in foothold traps. Because foxes caught in box and padded-jaw foothold traps had no limb damage, the observed biochemical differences between animals caught in these traps were likely due to psychogenic factors associated with limb restraint and differences in the intensity of exertion (i.e., pacing vs. digging). Hematological profiles of foxes caught in box traps or foothold traps were similar, but trapped foxes had higher (P<0.01) leukocyte counts and significant neutrophilia and lymphopenia compared to untrapped foxes. The gross and histopathological findings for foxes caught in box and foot hold traps were generally similar. Foxes caught in box traps had higher (P<0.05) incidences of adrenal and renal congestion and lung hemorrhage than did untrapped foxes. We conclude that factors associated with limb restraint directly contribute to the trauma experienced by trapped red foxes and, therefore, foxes caught in box traps undergo less trauma than foxes that are restrained by a limb in a padded- or unpadded-jaw foothold trap

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.

SAFETY AND EFFICACY OF BRUCELLA ABORTUS STRAIN RB51 VACCINE IN CAPTIVE PREGNANT ELK

Brucella abortus strain RB51 is a laboratory-derived rough mutant of virulent B. abortus strain 2308 used as a vaccine because it induces antibodies that do not react on standard brucellosis serologic tests. Strain RB51 vaccine was evaluated in pregnant captive elk (Cervus elaphus) to determine (1) if it induced abortion and (2) if it protected against abortion following subsequent challenge. The time period of this study (February–June, 1998) was similar to field conditions where elk are vaccinated and possibly exposed to B. abortus. Fourteen elk were randomly and equally divided into vaccinated and control groups. The vaccinated group was vaccinated intramuscularly with 1.03 × 1010 colony-forming units (CFU) of strain RB51 and seroconverted postvaccination. Antibodies to strain RB51 were detected by a modification of an existing dot-blot assay. Both groups were challenged 40 days postvaccination with 9.8 × 106 CFU of B. abortus strain 2308 administered intraconjunctivally. The first abortion occurred 38 days postchallenge. Abortion occurred in all control elk and in five of seven vaccinated elk 5 to 12 wk postchallenge (P = 0.23). Mixed strain RB51 and 2308 infections were present in fetuses and vaginas from the vaccinated group whereas only strain 2308 was cultured from control group fetuses and vaginal swabs. Further evaluation of strain RB51 will be necessary to determine if it will be safe and efficacious in free-ranging pregnant elk.

Immobilization of white-tailed deer with xylazine hydrochloride and ketamine hydrochloride and antagonism by tolazoline hydrochloride.

Fourteen penned and 17 free-ranging white-tailed deer (Odocoileus virginianus Rafinesque) were singularly or repeatedly immobilized with 100 mg xylazine hydrochloride (HC1) and 300 mg ketamine HC1. The mean times from intravenous injection to ambulation for 1.0, 2.0, and 4.0 mg/kg body weight doses of tolazoline HCl were 13.5, 10.5, and 9.2 min. Deer not receiving tolazoline HCl recovered in an average of 168 min. Heart rates significantly (P < 0.001) increased from 47 to 83 beats/min after tolazoline HCl administration, representing a return to normal rate. Tolazoline HCl had no effect on respiratory rate. A total of 85 reversals with tolazoline HCl resulted in no apparent adverse reactions.

Chronic Wasting Disease Drives Population Decline of White-Tailed Deer

Chronic wasting disease (CWD) is an invariably fatal transmissible spongiform encephalopathy of white-tailed deer, mule deer, elk, and moose. Despite a 100% fatality rate, areas of high prevalence, and increasingly expanding geographic endemic areas, little is known about the population-level effects of CWD in deer. To investigate these effects, we tested the null hypothesis that high prevalence CWD did not negatively impact white-tailed deer population sustainability. The specific objectives of the study were to monitor CWD-positive and CWD-negative white-tailed deer in a high-prevalence CWD area longitudinally via radio-telemetry and global positioning system (GPS) collars. For the two populations, we determined the following: a) demographic and disease indices, b) annual survival, and c) finite rate of population growth (λ). The CWD prevalence was higher in females (42%) than males (28.8%) and hunter harvest and clinical CWD were the most frequent causes of mortality, with CWD-positive deer over-represented in harvest and total mortalities. Survival was significantly lower for CWD-positive deer and separately by sex; CWD-positive deer were 4.5 times more likely to die annually than CWD-negative deer while bucks were 1.7 times more likely to die than does. Population λ was 0.896 (0.859–0.980), which indicated a 10.4% annual decline. We show that a chronic disease that becomes endemic in wildlife populations has the potential to be population-limiting and the strong population-level effects of CWD suggest affected populations are not sustainable at high disease prevalence under current harvest levels.

Chemical Immobilization of Red Foxes (Vulpes vulpes)

Red foxes (Vulpes vulpes) were immobilized with one of the following drug combinations: ketamine/xylazine (n = 22), ketamine/promazine (n = 35), ketamine/midazolam (n = 13), or tiletamine/zolazepam (n = 22). Foxes given ketamine/xylazine had the shortest induction and longest recovery times relative to other drug combinations, whereas foxes given ketamine/midazolam had the longest induction times. Recommended doses for the various combinations are given. Foxes given ketamine/xylazine were given either 0.1, 0.2, 0.4 mg/kg yohimbine, or saline 40 min after anesthetic induction. Administration of yohimbine significantly shortened arousal and recovery times relative to control values (P < 0.001).