Apparent stability masks underlying change in a mule deer herd with unmanaged chronic wasting disease.

The contagious prion disease "chronic wasting disease" (CWD) infects mule deer (Odocoileus hemionus) and related species. Unchecked epidemics raise ecological, socioeconomic, and public health concerns. Prion infection shortens a deer's lifespan, and when prevalence (proportion of adults infected) becomes sufficiently high CWD can affect herd dynamics. Understanding population responses over time is key to forecasting long-term impacts. Here we describe unexpected stability in prevalence and abundance in a mule deer herd where CWD has been left unmanaged. High apparent prevalence (~30%) since at least 2005 likely drove observed changes in the proportion and age distribution of wild-type native prion protein (PRNP) gene homozygotes among deer sampled. Predation by mountain lions (Puma concolor) may be helping keep CWD in check. Despite stable appearances, prion disease nonetheless impairs adult survival and likely resilience in this deer herd, limiting its potential for growth despite refuge from hunter harvest and favorable habitat and winter conditions.

Use of a 2-aminothiazole to Treat Chronic Wasting Disease in Transgenic Mice.

Treatment with the 2-aminothiazole IND24 extended the survival of mice infected with mouse-adapted scrapie but also resulted in the emergence of a drug-resistant prion strain. Here, we determined whether IND24 extended the survival of transgenic mice infected with prions that caused scrapie in sheep or prions that caused chronic wasting disease (CWD; hereafter "CWD prions") in deer, using 2 isolates for each disease. IND24 doubled the incubation times for mice infected with CWD prions but had no effect on the survival of those infected with scrapie prions. Biochemical, neuropathologic, and cell culture analyses were used to characterize prion strain properties following treatment, and results indicated that the CWD prions were not altered by IND24, regardless of survival extension. These results suggest that IND24 may be a viable candidate for treating CWD in infected captive cervid populations and raise questions about why some prion strains develop drug resistance whereas others do not.

Complement protein C3 exacerbates prion disease in a mouse model of chronic wasting disease.

Accumulating evidence shows a critical role of the complement system in facilitating attachment of prions to both B cells and follicular dendritic cells and assisting in prion replication. Complement activation intensifies disease in prion-infected animals, and elimination of complement components inhibits prion accumulation, replication and pathogenesis. Chronic wasting disease (CWD) is a highly infectious prion disease of captive and free-ranging cervid populations that utilizes the complement system for efficient peripheral prion replication and most likely efficient horizontal transmission. Here we show that complete genetic or transient pharmacological depletion of C3 prolongs incubation times and significantly delays splenic accumulation in a CWD transgenic mouse model. Using a semi-quantitative prion amplification scoring system we show that C3 impacts disease progression in the early stages of disease by slowing the rate of prion accumulation and/or replication. The delayed kinetics in prion replication correlate with delayed disease kinetics in mice deficient in C3. Taken together, these data support a critical role of C3 in peripheral CWD prion pathogenesis.

Effects of chronic wasting disease on reproduction and fawn harvest vulnerability in Wisconsin white-tailed deer.

Chronic wasting disease (CWD) is a fatal, transmissible spongiform encephalopathy that affects free-ranging and captive North American cervids. Although the impacts of CWD on cervid survival have been documented, little is known about the disease impacts on reproduction and recruitment. We used genetic methods and harvest data (2002-04) to reconstruct parentage for a cohort of white-tailed deer (Odocoileus virginianus) fawns born in spring 2002 and evaluate the effects of CWD infection on reproduction and fawn harvest vulnerability. There was no difference between CWD-positive and CWD-negative male deer in the probability of being a parent. However, CWD-positive females were more likely to be parents than CWD-negative females. Because our results are based on harvested animals, we evaluated the hypothesis that higher parentage rates occurred because fawns with CWD-positive mothers were more vulnerable to harvest. Male fawns with CWD-positive mothers were harvested earlier (>1 mo relative to their mother's date of harvest) and farther away from their mothers than male fawns with CWD-negative mothers. Male fawns with CWD-positive mothers were also harvested much earlier and farther away than female fawns from CWD-positive mothers. Most female fawns (86%) with CWD-positive mothers were harvested from the same section as their mothers, while almost half of male and female fawns with CWD-negative mothers were farther away. We conclude that preclinical stages of CWD infection do not prohibit white-tailed deer from successfully reproducing. However, apparently higher harvest vulnerability of male fawns with CWD-positive mothers suggests that CWD infection may make females less capable of providing adequate parental care to ensure the survival and recruitment of their fawns.

Survival patterns in white-tailed and mule deer after oral inoculation with a standardized, conspecific prion dose.

We orally inoculated white-tailed deer (Odocoileus virginianus) and mule deer (Odocoileus hemionus) with a standardized, conspecific prion dose and collected biologic samples throughout the disease course. Mule deer (PRNP genotype 225SS) and PRNP genotype 96GG white-tailed deer succumbed along similar trajectories, but 96GS- and 96SS-genotype individuals tended to survive longer.

Assessment of prospective preventive therapies for chronic wasting disease in mule deer.

We compared prion infection rates among mule deer (Odocoileus hemionus) receiving pentosan polysulfate, tannic acid, tetracycline HCl, or no treatment 14 days before to 14 days after (dpi) oral inoculation with tonsil tissue homogenate. All deer were infected, but the rapid disease course (230-603 dpi) suggested our challenge was overwhelming.

Evidence for distinct chronic wasting disease (CWD) strains in experimental CWD in ferrets.

Chronic wasting disease (CWD) is an evolving prion disease of cervids (deer, elk and moose) that has been recognized in North America and Korea. Infection of non-cervid reservoir or transport species in nature is not reported. However, the ferret (Mustela putorius furo) is susceptible to CWD after experimental inoculation. Here, we report that infection of ferrets with either of two ferret CWD isolates by various routes of exposure has revealed biologically distinct strain-like properties distinguished by different clinical progression and survival period. The isolates of ferret CWD were also differentiated by the distribution of the infectious prion protein (PrP(CWD)) in the brain and periphery, and by the proteinase K sensitivity of PrP(CWD). These findings suggest that diversity in prion conformers exists in CWD-infected cervids.

Estimating chronic wasting disease effects on mule deer recruitment and population growth.

Chronic wasting disease (CWD), a prion disease of mule deer (Odocoileus hemionus), accelerates mortality and in so doing has the potential to influence population dynamics. Although effects on mule deer survival are clear, how CWD affects recruitment is less certain. We studied how prion infection influenced the number of offspring raised to weaning per adult (≥2 yr old) female mule deer and subsequently the estimated growth rate (λ) of an infected deer herd. Infected and presumably uninfected radio-collared female deer were observed with their fawns in late summer (August-September) during three consecutive years (2006-2008) in the Table Mesa area of Boulder, Colorado, USA. We counted the number of fawns accompanying each female, then used a fully Bayesian model to estimate recruitment by infected and uninfected females and the effect of the disease on λ. On average, infected females weaned 0.95 fawns (95% credible interval=0.56-1.43) whereas uninfected females weaned 1.34 fawns (95% credible interval=1.09-1.61); the probability that uninfected females weaned more fawns than infected females was 0.93). We used estimates of prevalence to weight recruitment and survival parameters in the transition matrix of a three-age, single-sex matrix model and then used the matrix to calculate effects of CWD on λ. When effects of CWD on both survival and recruitment were included, the modeled λ was 0.97 (95% credible interval = 0.82-1.09). Effects of disease on λ were mediated almost entirely by elevated mortality of infected animals. We conclude that although CWD may affect mule deer recruitment, these effects seem to be sufficiently small that they can be omitted in estimating the influences of CWD on population growth rate.

A weighted surveillance approach for detecting chronic wasting disease foci.

A key component of wildlife disease surveillance is determining the spread and geographic extent of pathogens by monitoring for infected individuals in regions where cases have not been previously detected. A practical challenge of such surveillance is developing reliable, yet cost-effective, approaches that remain sustainable when monitoring needs are prolonged or continuous, or when resources to support these efforts are limited. In order to improve the efficiency of chronic wasting disease (CWD) surveillance in Colorado, United States, we developed a weighted surveillance system exploiting observed differences in CWD prevalence across demographic strata within infected mule deer (Odocoileus hemionus) populations. We used field data to estimate sampling weights for individuals from eight demographic strata distinguished by differences in apparent health, sex, and age. In this system, individuals from a sample source with high prevalence and low inclusion probability (e.g., clinical CWD "suspects") received >or=10.3 times more weight than those from a source with low prevalence and high inclusion probability (e.g., apparently healthy, hunter-harvested individuals). We simulated use of this alternative surveillance system for a deer management unit in Colorado and evaluated the potential effects of using biased weights on the probability of failing to detect CWD and on relative surveillance costs. We found that this system should be transparent, cost-effective, and reasonably robust to the inadvertent use of biased weights. By implementing this, or a similar, weighted surveillance system, wildlife agencies should be able to maintain or improve current surveillance standards while, perhaps, collecting and examining fewer samples, thereby increasing the efficiency and cost-effectiveness of ongoing CWD surveillance programs.

Experimental transmission of chronic wasting disease (CWD) of elk (Cervus elaphus nelsoni), white-tailed deer (Odocoileus virginianus), and mule deer (Odocoileus hemionus hemionus) to white-tailed deer by intracerebral route.

To compare clinical and pathologic findings of chronic wasting disease (CWD) in a natural host, 3 groups (n = 5) of white-tailed deer (WTD) fawns were intracerebrally inoculated with a CWD prion of WTD, mule deer, or elk origin. Three other uninoculated fawns served as controls. Approximately 10 months postinoculation (MPI), 1 deer from each of the 3 inoculated groups was necropsied and their tissues were examined for lesions of spongiform encephalopathy (SE) and for the presence of abnormal prion protein (PrP(d)) by immunohistochemistry (IHC) and Western blot (WB). The remaining deer were allowed to live until they developed clinical signs of the disease which began approximately 18 MPI. By 26 MPI, all deer were euthanatized on humane grounds. Obvious differences in clinical signs or the incubation periods were not observed between the 3 groups of deer given CWD. In 1 of 3 nonclinical deer euthanatized at 10 MPI, minimal microscopic lesions of SE were seen in the central nervous system (CNS) tissues, and PrP(d) was observed by IHC in tissues of all 3 deer. In the clinical deer, CNS lesions of SE and PrP(d) accumulations were more severe and extensive. It is concluded that the 3 sources of CWD prion did not induce significant differences in time to clinical disease or qualitative differences in signs or lesions in WTD. However, this observation does not imply that these CWD agents would necessarily behave similarly in other recipient species.

Experimental chronic wasting disease (CWD) in the ferret.

Chronic wasting disease (CWD), a prion disease of North American deer, elk and moose, affects both free-ranging and captive cervids. The potential host range for CWD remains uncertain. The susceptibility of the ferret to CWD was examined experimentally by administering infectious brain material by the intracerebral (IC) or oral (PO) route. Between 15 and 20 months after IC inoculation, ferrets developed neurological signs consistent with prion disease, including polyphagia, somnolence, piloerection, lordosis and ataxia. Upon first sub-passage of ferret-adapted CWD, the incubation period decreased to 5 months. Spongiform change in the neuropil was most marked in the basal ganglia, thalamus, midbrain and pons. The deposition of PrP(CWD) was granular and was occasionally closely associated with, or localized within, neurons. There were no plaque-like or perivascular PrP aggregates as seen in CWD-infected cervids. In western blots, the PrP(CWD) glycoform profile resembled that of CWD in deer, typified by a dominant diglycosylated glycoform. CWD disease in ferrets followed IC but not PO inoculation, even after 31 months of observation. These findings indicate that CWD-infected ferrets share microscopical and biochemical features of CWD in cervids, but appear to be relatively resistant to oral infection by primary CWD inoculum of deer origin.

Epidemiology of an outbreak of chronic wasting disease on elk farms in Saskatchewan.

An outbreak of chronic wasting disease (CWD) in farmed elk in Saskatchewan from 1996 to 2002 was reviewed to 1, determine the progression of CWD from infection to death in farmed elk; 2, assess animal risk factors for CWD infection in farmed elk; 3, assess farm management and exposure risk factors for within herd CWD transmission; and 4, assess the suitability of the Canadian Food Inspection Agency's (CFIA) current disease control policy for CWD in light of the findings. The results from animal movement tracing, animal testing, and a farm management questionnaire were used. The duration of CWD (time from exposure to death of a CWD test-positive animal) was between a mean minimum of 19 months and a mean maximum of 40 months. Age and sex were not associated with CWD infection, except that adult elk (> or = 2 y) were more likely to be infected than young elk (< 18 mo) (RR = 2.3, 95% CI 1.6-3.5). Elk calves born in the last 18 mo prior to the death or diagnosis of their dam were at higher risk if their dams died of CWD (RR = 4.1, 95% CI 1.5-11.4) or exhibited clinical signs of CWD (RR = 8.3, 95% CI 2.7-25.7). Significant risk factors for transmission of CWD on elk farms were the introduction from an infected farm of trace-in elk that died of CWD (RR = 13.5, 95% CI 2.0-91) or developed clinical signs of CWD (RR = 7.1, 95% CI 0.93-54) and the elapsed time in years since the incursion of CWD (OR = 5.6, 95% CI 1.8-17.4). The assumptions on which CFIA's disease control policies were based were validated, but based on this new information, quarantine in cases where exposure to preclinical elk has occurred could be considered as an alternative to whole herd eradication.

Human prion disease and relative risk associated with chronic wasting disease.

The transmission of the prion disease bovine spongiform encephalopathy (BSE) to humans raises concern about chronic wasting disease (CWD), a prion disease of deer and elk. In 7 Colorado counties with high CWD prevalence, 75% of state hunting licenses are issued locally, which suggests that residents consume most regionally harvested game. We used Colorado death certificate data from 1979 through 2001 to evaluate rates of death from the human prion disease Creutzfeldt-Jakob disease (CJD). The relative risk (RR) of CJD for CWD-endemic county residents was not significantly increased (RR 0.81, 95% confidence interval [CI] 0.40-1.63), and the rate of CJD did not increase over time (5-year RR 0.92, 95% CI 0.73-1.16). In Colorado, human prion disease resulting from CWD exposure is rare or nonexistent. However, given uncertainties about the incubation period, exposure, and clinical presentation, the possibility that the CWD agent might cause human disease cannot be eliminated.

Oral transmission of chronic wasting disease in captive Shira's moose.

Three captive Shira's moose (Alces alces shirasi) were orally inoculated with a single dose (5 g) of whole-brain homogenate prepared from chronic wasting disease (CWD)-affected mule deer (Odocoileus hemionus). All moose died of causes thought to be other than CWD. Histologic examination of one female moose dying 465 days postinoculation revealed spongiform change in the neuropil, typical of transmissible spongiform encephalopathy. Immunohistochemistry staining for the proteinase-resistant isoform of the prion protein was observed in multiple lymphoid and nervous tissues. Western blot and enzyme-linked immunosorbent assays provided additional confirmation of CWD. These results represent the first report of experimental CWD in moose.

Retrospective investigation of chronic wasting disease of cervids at the Toronto Zoo, 1973-2003.

The occurrence of chronic wasting disease (CWD) at the Toronto Zoo was investigated retrospectively, based on an examination of management, animal health, and postmortem records, and immunohistochemical studies. Records of animal movements, clinical signs, and postmortem findings were examined for all cervids 1973-2003. All available samples of fixed, wax-embedded lymphoid or central nervous system tissue from cervids that died at the Toronto Zoo from 1973 to 2003, > 12 months of age, were tested, using prion protein immunostaining. Chronic wasting disease prion antigen was detected in 8 of 105 animals tested: 7 mule deer and 1 black-tailed deer. The most likely method of introduction was the importation of CWD-infected animals from a zoo in the United States. Animal-to-animal contact and environmental contamination were the most likely methods of spread of CWD at the zoo. No mule deer left the Toronto Zoo site, and the last animal with CWD died in 1981. Historic findings and ongoing testing of cervids indicate that the Toronto Zoo collection has very low risk of currently being infected with CWD.

Dynamics of prion disease transmission in mule deer.

Chronic wasting disease (CWD), a contagious prion disease of the deer family, has the potential to severely harm deer populations and disrupt ecosystems where deer occur in abundance. Consequently, understanding the dynamics of this emerging infectious disease, and particularly the dynamics of its transmission, has emerged as an important challenge for contemporary ecologists and wildlife managers. Although CWD is contagious among deer, the relative importance of pathways for its transmission remains unclear. We developed seven competing models, and then used data from two CWD outbreaks in captive mule deer and model selection to compare them. We found that models portraying indirect transmission through the environment had 3.8 times more support in the data than models representing transmission by direct contact between infected and susceptible deer. Model-averaged estimates of the basic reproductive number (R0) were 1.3 or greater, indicating likely local persistence of CWD in natural populations under conditions resembling those we studied. Our findings demonstrate the apparent importance of indirect, environmental transmission in CWD and the challenges this presents for controlling the disease.

Relative vulnerability of chronic wasting disease infected mule deer to vehicle collisions.

We estimated chronic wasting disease (CWD) prevalence among vehicle-killed mule deer (Odocoileus hemionus) in select data analysis units (DAUs) in northern Colorado, USA, and compared these with estimated CWD prevalence among mule deer of the same sex sampled in the vicinity of collision sites to assess relative vulnerability of CWD-infected individuals to vehicle collisions. Twenty-five of 171 vehicle-killed mule deer tested positive for CWD (overall prevalence=0.146, 95% confidence interval [CI]=0.097-0.208); 173 of 2,317 deer sampled in the vicinity of these vehicle-killed deer tested positive (overall prevalence=0.075, 95% CI=0.064-0.085). In nine of ten DAU x sex comparisons, relative risk of CWD infection tended to be higher among vehicle-killed deer (range of estimated relative risks=1.6-15.9). Spongiform encephalopathy was detected in 12 of 20 (60%; 95% CI=39-81%) CWD-positive deer killed by vehicles and in 79 of 180 (44%; 95% CI=37-52%) CWD-positive deer detected via random sampling (relative risk=1.37; 95% CI=0.92-2.03), suggesting that infected deer killed by vehicles tended to be in later stages of disease than those killed by hunters. Our data offer evidence that CWD-infected mule deer may be relatively vulnerable to vehicle collisions. It follows that sampling of vehicle-killed mule deer may be exploited to increase efficiency of surveillance programs designed to detect new foci of CWD infection; moreover, evidence of increased susceptibility to vehicle collisions may aid in understanding vulnerability of CWD-infected individuals to other forms of death, particularly predation.

Chronic wasting disease of elk: transmissibility to humans examined by transgenic mouse models.

Chronic wasting disease (CWD), a prion disease affecting free-ranging and captive cervids (deer and elk), is widespread in the United States and parts of Canada. The large cervid population, the popularity of venison consumption, and the apparent spread of the CWD epidemic are likely resulting in increased human exposure to CWD in the United States. Whether CWD is transmissible to humans, as has been shown for bovine spongiform encephalopathy (the prion disease of cattle), is unknown. We generated transgenic mice expressing the elk or human prion protein (PrP) in a PrP-null background. After intracerebral inoculation with elk CWD prion, two lines of "humanized" transgenic mice that are susceptible to human prions failed to develop the hallmarks of prion diseases after >657 and >756 d, respectively, whereas the "cervidized" transgenic mice became infected after 118-142 d. These data indicate that there is a substantial species barrier for transmission of elk CWD to humans.

Epidemiology of chronic wasting disease in captive white-tailed and mule deer.

The natural occurrence of chronic wasting disease (CWD) in a 1993 cohort of captive white-tailed deer (Odocoileus virginianus) afforded the opportunity to describe epidemic dynamics in this species and to compare dynamics with those seen in contemporary cohorts of captive mule deer (O. hemionus) also infected with CWD. The overall incidence of clinical CWD in white-tailed deer was 82% (nine of 11) among individuals that survived >15 mo. Affected white-tailed deer died or were killed because of terminal CWD at age 49-76 mo (x = 59.6 mo, SE = 3.9 mo). Epidemic dynamics of CWD in captive white-tailed deer were similar to dynamics in mule deer cohorts. Incidence of clinical CWD was 57% (4/7) among hand-raised (HR) and 67% (4/6) among dam-raised (DR) mule deer; affected HR mule deer succumbed at 64-86 mo of age (x = 72 mo; SE = 5 mo), and affected DR mule deer died at age 31-58 mo (x = 41.3 mo; SE = 6.1 mo). Sustained horizontal transmission of CWD most plausibly explained epidemic dynamics, but the original source of exposures could not be determined. Apparent differences in mean age at CWD-caused death among these cohorts may be attributable to differences in the timing or intensity of exposure to CWD, and these factors appear to be more likely to influence epidemic dynamics than species differences. It follows that CWD epidemic dynamics in sympatric, free-ranging white-tailed and mule deer sharing habitats in western North American ranges also may be similar.