Delwyn P. Keane

SPATIAL EPIDEMIOLOGY OF CHRONIC WASTING DISEASE IN WISCONSIN WHITE-TAILED DEER

Chronic wasting disease (CWD) is a fatal, emerging disease of cervids associated with transmissible protease-resistant prion proteins. The potential for CWD to cause dramatic declines in deer and elk populations and perceived human health risks associated with consuming CWD-contaminated venison have led wildlife agencies to embark on extensive CWD control programs, typically involving culling to reduce deer populations. We characterized the spatial distribution of CWD in white-tailed deer (Odocoileus virginianus) in Wisconsin to facilitate CWD management. We found that CWD prevalence declined with distance from a central location, was locally correlated at a scale of 3.6 km, and was correlated with deer habitat abundance. The latter result is consistent with patterns expected for a positive relationship between density and prevalence of CWD. We recommend management activities focused on culling in geographic areas with high prevalence to have the greatest probability of removing infected individuals. Further research is needed to elucidate the factors involved in CWD spread and infection rates, especially the role of density-dependent transmission.

Demographic patterns and harvest vulnerability of chronic wasting disease infected white-tailed deer in Wisconsin

Abstract Chronic wasting disease (CWD) is a fatal disease of white-tailed deer (Odocoileus virginianus) caused by transmissible protease-resistant prions. Since the discovery of CWD in southern Wisconsin in 2001, more than 20,000 deer have been removed from a >2,500-km2 disease eradication zone surrounding the three initial cases. Nearly all deer removed were tested for CWD infection and sex, age, and harvest location were recorded. Our analysis used data from a 310-km2 core study area where disease prevalence was higher than surrounding areas. We found no difference in harvest rates between CWD infected and noninfected deer. Our results show that the probability of infection increased with age and that adult males were more likely to be infected than adult females. Six fawns tested positive for CWD, five fawns from the core study area, including the youngest (5 months) free-ranging cervid to test positive. The increase in male prevalence with age is nearly twice the increase found in females. We concluded that CWD is not randomly distributed among deer and that differential transmission among sex and age classes is likely driving the observed patterns in disease prevalence. We discuss alternative hypotheses for CWD transmission and spread and, in addition, discuss several possible nonlinear relationships between prevalence and age. Understanding CWD transmission in free-ranging cervid populations will be essential to the development of strategies to manage this disease in areas where CWD is found, as well as for surveillance strategies in areas where CWD threatens to spread.

Chronic wasting disease in a Wisconsin white- tailed deer farm

In September 2002, chronic wasting disease (CWD), a prion disorder of captive and wild cervids, was diagnosed in a white-tailed deer (Odocoileus virginianus) from a captive farm in Wisconsin. The facility was subsequently quarantined, and in January 2006 the remaining 76 deer were depopulated. Sixty animals (79%) were found to be positive by immunohistochemical staining for the abnormal prion protein (PrPCWD)in at least one tissue; the prevalence of positive staining was high even in young deer. Although none of the deer displayed clinical signs suggestive of CWD at depopulation, 49 deer had considerable accumulation of the abnormal prion in the medulla at the level of the obex. Extraneural accumulation of the abnormal protein was observed in 59 deer, with accumulation in the retropharyngeal lymph node in 58of 59 (98%), in the tonsil in 56 of 59 (95%), and in the rectal mucosal lymphoid tissue in 48 of 58 (83%). The retina was positive in 4 deer, all with marked accumulation of prion in the obex. One deer was considered positive for PrPCWD in the brain but not in the extraneural tissue, a novel observation in white-tailed deer. The infection rate in captive deer was 20-fold higher than in wild deer. Although weakly related to infection rates in extraneural tissues, prion genotype was strongly linked to progression of prion accumulation in the obex. Antemortem testing by biopsy of rectoanal mucosal-associated lymphoid tissue (or other peripheral lymphoid tissue) may be a useful adjunct to tonsil biopsy for surveillance in captive herds at risk for CWD infection.

Chronic Wasting Disease (CWD) Susceptibility of Several North American Rodents That Are Sympatric with Cervid CWD Epidemics

ABSTRACT Chronic wasting disease (CWD) is a highly contagious always fatal neurodegenerative disease that is currently known to naturally infect only species of the deer family, Cervidae. CWD epidemics are occurring in free-ranging cervids at several locations in North America, and other wildlife species are certainly being exposed to infectious material. To assess the potential for transmission, we intracerebrally inoculated four species of epidemic-sympatric rodents with CWD. Transmission was efficient in all species; the onset of disease was faster in the two vole species than the two Peromyscus spp. The results for inocula prepared from CWD-positive deer with or without CWD-resistant genotypes were similar. Survival times were substantially shortened upon second passage, demonstrating adaptation. Unlike all other known prion protein sequences for cricetid rodents that possess asparagine at position 170, our red-backed voles expressed serine and refute previous suggestions that a serine in this position substantially reduces susceptibility to CWD. Given the scavenging habits of these rodent species, the apparent persistence of CWD prions in the environment, and the inevitable exposure of these rodents to CWD prions, our intracerebral challenge results indicate that further investigation of the possibility of natural transmission is warranted.

Comparison of Retropharyngeal Lymph Node and Obex Region of the Brainstem in Detection of Chronic Wasting Disease in White-Tailed Deer (Odocoileus Virginianus)

Chronic wasting disease (CWD) in Wisconsin was first identified in February 2002. By April 2005. medial retropharyngeal lymph node (RLN) tissues had been examined from over 75,000 white-tailed deer for the presence of CWD by either immunohistochemical (IHC) staining for the prion protein associated with CWD (PrPres) or by using enzyme-linked immunosorbent assays with confirmation of positives by IHC staining and had been detected in 469 animals. Obex tissue was also available from 438 of the CWD-positive animals and was CWD positive by IHC staining in 355 (81%). To verify whether false-negative results were possible examining only RLN, both obex and RLN samples were examined for CWD by IHC staining from 4,430 of the white-tailed deer harvested from an area in Wisconsin where the overall deer CWD prevalence was approximately 6.2%. Two hundred and fourteen of the 269 positive deer (79.6%) had deposits of PrPres in both obex and lymphoid tissues. 55 (20.4%) had deposits only in lymphoid tissue, and there were no deer that had deposits only in obex.

Validation of Use of Rectoanal Mucosa-Associated Lymphoid Tissue for Immunohistochemical Diagnosis of Chronic Wasting Disease in White-Tailed Deer (Odocoileus virginianus)

ABSTRACT The examination of rectoanal mucosa-associated lymphoid tissue (RAMALT) biopsy specimens for the diagnosis of transmissible spongiform encephalopathies has been described in sheep, elk, and small numbers of mule and white-tailed deer. Previous sample numbers have been too small to validate examination of this type of tissue as a viable antemortem diagnostic test. In this study, we examined RAMALT collected postmortem from 76 white-tailed deer removed from a farm in Wisconsin known to be affected by chronic wasting disease (CWD) and from 210 free-ranging white-tailed deer harvested from an area in Wisconsin where the overall prevalence of CWD among the deer was approximately 4 to 6%. The results of immunohistochemical (IHC) staining of the RAMALT sections were compared to the results of IHC staining of sections from the brain stem at the convergence of the dorsal motor nucleus of the vagus nerve, sections of the medial retropharyngeal lymph nodes (RLNs), and sections of tonsil (sections of tonsil only from captive animals were tested). The sensitivities of the IHC staining test with RAMALT sections were 81% for the captive animals and 91% for the free-ranging animals. False-negative results were usually associated with early infection, indicated by a low intensity of immunostaining in the obex and/or a polymorphism at PRNP codon 96. While the RLN remains the tissue of choice for use for the diagnosis of CWD in white-tailed deer, the results of the present study further support the use of RAMALTs collected antemortem as an adjunct to testing of tonsil biopsy specimens and surveillance by necropsy for the screening of farmed deer which have been put at risk through environmental exposure or exposure to deer with CWD.

Diagnostic accuracy of rectal mucosa biopsy testing for chronic wasting disease within white-tailed deer (Odocoileus virginianus) herds in North America: Effects of age, sex, polymorphism at PRNP codon 96, and disease progression

An effective live animal diagnostic test is needed to assist in the control of chronic wasting disease (CWD), which has spread through captive and wild herds of white-tailed deer (Odocoileus virginianus) in Canada and the United States. In the present study, the diagnostic accuracy of rectal mucosa biopsy sample testing was determined in white-tailed deer from 4 CWD-infected captive herds. Specifically, the current study compared the immunohistochemical detection of disease-associated prion protein in postmortem rectal mucosa biopsy samples to the CWD status of each deer as determined by immunodiagnostic evaluations of the brainstem at the obex, the medial retropharyngeal lymph node, and the palatine tonsil. The effects of age, sex, genotype, and disease progression were also evaluated. Diagnostic sensitivity on rectal biopsy samples for CWD in white-tailed deer ranged from 63% to 100%; the pooled estimate of sensitivity was 68% with 95% confidence limits (95% CLs) of 49% and 82%. However, diagnostic sensitivity was dependent on genotype at prion protein gene (PRNP) codon 96 and on disease progression as assessed by obex grade. Diagnostic sensitivity was 76% (95% CLs: 49%, 91%) for 96GG deer but only 42% (95% CLs: 13%, 79%) for 96GS deer. Furthermore, diagnostic sensitivity was only 36% for deer in the earliest stage of disease (obex grade 0) but was 100% for deer in the last 2 stages of preclinical disease (obex grades 3 and 4). The overall diagnostic specificity was 99.8%. Selective use of antemortem rectal biopsy sample testing would provide valuable information during disease investigations of CWD-suspect deer herds.

Nor98 scrapie identified in the United States.

A distinct strain of scrapie identified in sheep of Norway in 1998 has since been identified in numerous countries throughout Europe. The disease is known as Nor98 or Nor98-like scrapie, among other names. Distinctions between classic scrapie and Nor98 scrapie are made based on histopathology and immunodiagnostic results. There are also differences in the epidemiology, typical signalment, and likelihood of clinical signs being observed. In addition, sheep that have genotypes associated with resistance to classic scrapie are not spared from Nor98 disease. The various differences between classic and Nor98 scrapie have been consistently reported in the vast majority of cases described across Europe. The current study describes in detail the pathologic changes and diagnostic results of the first 6 cases of Nor98 scrapie disease diagnosed in sheep of the United States.

Surveillance for transmissible spongiform encephalopathy in scavengers of white-tailed deer carcasses in the chronic wasting disease area of Wisconsin.

Chronic wasting disease (CWD), a class of neurodegenerative transmissible spongiform encephalopathies (TSE) occurring in cervids, is found in a number of states and provinces across North America. Misfolded prions, the infectious agents of CWD, are deposited in the environment via carcass remains and excreta, and pose a threat of cross-species transmission. In this study tissues were tested from 812 representative mammalian scavengers, collected in the CWD-affected area of Wisconsin, for TSE infection using the IDEXX HerdChek enzyme-linked immunosorbent assay (ELISA). Only four of the collected mammals tested positive using the ELISA, but these were negative when tested by Western blot. While our sample sizes permitted high probabilities of detecting TSE assuming 1% population prevalence in several common scavengers (93%, 87%, and 87% for raccoons, opossums, and coyotes, respectively), insufficient sample sizes for other species precluded similar conclusions. One cannot rule out successful cross-species TSE transmission to scavengers, but the results suggest that such transmission is not frequent in the CWD-affected area of Wisconsin. The need for further surveillance of scavenger species, especially those known to be susceptible to TSE (e.g., cat, American mink, raccoon), is highlighted in both a field and laboratory setting.

SURVEILLANCE TO DETECT CHRONIC WASTING DISEASE IN WHITE-TAILED DEER IN WISCONSIN

Chronic wasting disease (CWD), a prion disease affecting North American cervids, has been discovered in at least 12 states and provinces throughout the continent. Since 2002, a number of states and provinces have initiated surveillance programs to detect CWD in native cervid populations. However, many questions remain about the appropriate methods, geographic scope, and number of samples required for an effective CWD surveillance program. We provide an improved statistical method to calculate the probability of detecting CWD in primary sample units (e.g., county or deer management unit) that also considers deer abundance and the nonrandom distribution of CWD and hunter harvests. We used this method to analyze data from a statewide CWD detection program conducted in Wisconsin during the autumns of 2002 and 2003 to determine the distribution of CWD in white-tailed deer (Odocoileus virginianus). Deer heads were collected at hunter registration stations, and brainstem (obex) and retropharyngeal lymph nodes were removed for disease testing. Our analysis includes samples from >35,000 deer collected outside the known affected area. The probability of detecting chronic wasting disease at a prevalence of 1% varied from 0.89 to ≥0.99 among the 56 primary sample units. Detection probabilities for 1% CWD prevalence were >0.9 in 55 primary sample units, and >0.99 in 10. Detection probabilities will be higher in areas where CWD prevalence exceeds 1%. CWD-positive deer were detected in eight primary sample units surrounding the known affected area during surveillance activities. Our approach provides a novel statistical technique to accommodate nonrandom sampling in wildlife disease surveillance programs.