Preliminary observations on the experimental transmission of chronic wasting disease (CWD) from elk and white-tailed deer to fallow deer

Genetic modulation of CWD prion propagation in cervid PrP Drosophila

Chronic wasting disease is a fatal prion condition of cervids such as deer, elk, moose and reindeer. Secretion and excretion of prion infectivity from North American cervids with this condition causes environmental contamination and subsequent efficient lateral transmission in free-ranging and farmed cervids. Variants of cervid PrP exist that affect host susceptibility to chronic wasting disease. Cervid breeding programmes aimed at increasing the frequency of PrP variants associated with resistance to chronic wasting disease may reduce the burden of this condition in animals and lower the risk of zoonotic disease. This strategy requires a relatively rapid and economically viable model system to characterise and support selection of prion disease-modifying cervid PrP variants. Here, we generated cervid PrP transgenic Drosophila to fulfil this purpose. We have generated Drosophila transgenic for S138 wild type cervid PrP, or the N138 variant associated with resistance to chronic wasting disease. We show that cervid PrP Drosophila accumulate bona fide prion infectivity after exposure to cervid prions. Furthermore, S138 and N138 PrP fly lines are susceptible to cervid prion isolates from either North America or Europe when assessed phenotypically by accelerated loss of locomotor ability or survival, or biochemically by accumulation of prion seeding activity. However, after exposure to European reindeer prions, N138 PrP Drosophila accumulated prion seeding activity with slower kinetics than the S138 fly line. These novel data show that prion susceptibility characteristics of cervid PrP variants are maintained when expressed in Drosophila, which highlights this novel invertebrate host in modelling chronic wasting disease.

Cervid Prion Protein Polymorphisms: Role in Chronic Wasting Disease Pathogenesis

Chronic wasting disease (CWD) is a prion disease found in both free-ranging and farmed cervids. Susceptibility of these animals to CWD is governed by various exogenous and endogenous factors. Past studies have demonstrated that polymorphisms within the prion protein (PrP) sequence itself affect an animal's susceptibility to CWD. PrP polymorphisms can modulate CWD pathogenesis in two ways: the ability of the endogenous prion protein (PrP^C) to convert into infectious prions (PrP^Sc) or it can give rise to novel prion strains. In vivo studies in susceptible cervids, complemented by studies in transgenic mice expressing the corresponding cervid PrP sequence, show that each polymorphism has distinct effects on both PrP^C and PrP^Sc. It is not entirely clear how these polymorphisms are responsible for these effects, but in vitro studies suggest they play a role in modifying PrP epitopes crucial for PrP^C to PrP^Sc conversion and determining PrP^C stability. PrP polymorphisms are unique to one or two cervid species and most confer a certain degree of reduced susceptibility to CWD. However, to date, there are no reports of polymorphic cervid PrP alleles providing absolute resistance to CWD. Studies on polymorphisms have focused on those found in CWD-endemic areas, with the hope that understanding the role of an animal's genetics in CWD can help to predict, contain, or prevent transmission of CWD.

Variation in the prion protein gene (PRNP) sequence of wild deer in Great Britain and mainland Europe

Susceptibility to prion diseases is largely determined by the sequence of the prion protein gene (PRNP), which encodes the prion protein (PrP). The recent emergence of chronic wasting disease (CWD) in Europe has highlighted the need to investigate PRNP gene diversity in European deer species, to better predict their susceptibility to CWD. Here we report a large genetic survey of six British deer species, including red (Cervus elaphus), sika (Cervus nippon), roe (Capreolus capreolus), fallow (Dama dama), muntjac (Muntiacus reevesii), and Chinese water deer (Hydropotes inermis), which establishes PRNP haplotype and genotype frequencies. Two smaller data sets from red deer in Norway and the Czech Republic are also included for comparison. Overall red deer show the most PRNP variation, with non-synonymous/coding polymorphisms at codons 98, 168, 226 and 247, which vary markedly in frequency between different regions. Polymorphisms P168S and I247L were only found in Scottish and Czech populations, respectively. T98A was found in all populations except Norway and the south of England. Significant regional differences in genotype frequencies were observed within both British and European red deer populations. Other deer species showed less variation, particularly roe and fallow deer, in which identical PRNP gene sequences were found in all individuals analysed. Based on comparison with PRNP sequences of North American cervids affected by CWD and limited experimental challenge data, these results suggest that a high proportion of wild deer in Great Britain may be susceptible to CWD.

Chronic wasting disease: an evolving prion disease of cervids

Chronic wasting disease (CWD) is a relatively new and burgeoning prion epidemic of deer, elk, reindeer, and moose, which are members of the cervid family. While the disease was first described in captive deer, its subsequent discovery in various species of free-ranging animals makes it the only currently recognized prion disorder of both wild and farmed animals. In addition to its expanding range of host species, CWD continues to spread from North America to new geographic areas, including South Korea, and most recently Norway, marking the first time this disease was detected in Europe. Its unparalleled efficiency of contagious transmission, combined with high densities of deer in certain areas, complicates strategies for controlling CWD, raising concerns about its potential for spread to new species. Because there is a high prevalence of CWD in deer and elk, which are commonly hunted and consumed by humans, and since prions from cattle with bovine spongiform encephalopathy have been transmitted to humans causing variant Creutzfeldt-Jakob disease, the possibility of zoonotic transmission of CWD is particularly concerning. Here we review the clinical and pathologic features of CWD and its disturbing epidemiology, and discuss features that affect its transmission, including genetic susceptibility, pathogenesis, and agent strain variability. Finally, we discuss evidence that speaks to the potential for zoonotic transmission of this emerging disease.

Molecular Mechanisms of Chronic Wasting Disease Prion Propagation

Prion disease epidemics, which have been unpredictable recurrences, are of significant concern for animal and human health. Examples include kuru, once the leading cause of death among the Fore people in Papua New Guinea and caused by mortuary feasting; bovine spongiform encephalopathy (BSE) and its subsequent transmission to humans in the form of variant Creutzfeldt-Jakob disease (vCJD), and repeated examples of large-scale prion disease epidemics in animals caused by contaminated vaccines. The etiology of chronic wasting disease (CWD), a relatively new and burgeoning prion epidemic in deer, elk, and moose (members of the cervid family), is more enigmatic. The disease was first described in captive and later in wild mule deer and subsequently in free-ranging as well as captive Rocky Mountain elk, white-tailed deer, and most recently moose. It is therefore the only recognized prion disorder of both wild and captive animals. In addition to its expanding range of hosts, CWD continues to spread to new geographical areas, including recent cases in Norway. The unparalleled efficiency of the contagious transmission of the disease combined with high densities of deer in certain areas of North America complicates strategies for controlling CWD and raises concerns about its potential spread to new species. Because there is a high prevalence of CWD in deer and elk, which are commonly hunted and consumed by humans, the possibility of zoonotic transmission is particularly concerning. Here, we review the current status of naturally occurring CWD and describe advances in our understanding of its molecular pathogenesis, as shown by studies of CWD prions in novel in vivo and in vitro systems.

Temporal patterns of chronic wasting disease prion excretion in three cervid species

Chronic wasting disease (CWD) is the only naturally occurring transmissible spongiform encephalopathy affecting free-ranging wildlife populations. Transmission of CWD occurs by direct contact or through contaminated environments; however, little is known about the temporal patterns of CWD prion excretion and shedding in wild cervids. We tested the urine and faeces of three species of captive cervids (elk, mule and white-tailed deer) at 6, 12, 18 and 24 months after oral inoculation to evaluate the temporal, species- and genotype-specific factors affecting the excretion of CWD prions. Although none of the animals exhibited clinical signs of CWD during the study, we determined that all three cervid species were excreting CWD prions by 6 months post-inoculation. Faecal samples were consistently positive for CWD prions for all three cervid species (88 %), and were more likely to be positive than urine samples (28 %). Cervids with genotypes encoding for the prion protein (PRNP) that were considered to be more susceptible to CWD were more likely to excrete CWD prions (94 %) than cervids with genotypes considered to be less susceptible (64 %). All cervids with CWD prions in their urine also had positive faeces (n=5), but the converse was not true. Our study is the first to demonstrate CWD prion excretion in urine by asymptomatic elk and mule deer. Our results indicate that the excretion of CWD prions in faeces and, to a lesser extent, urine may provide an important avenue for depositing prions in the environment.

Expert elicitation on the uncertainties associated with chronic wasting disease

A high degree of uncertainty exists for chronic wasting disease (CWD) transmission factors in farmed and wild cervids. Evaluating the factors is important as it helps to inform future risk management strategies. Expert opinion is often used to assist decision making in a number of health, science, and technology domains where data may be sparse or missing. Using the "Classical Model" of elicitation, a group of experts was asked to estimate the most likely values for several risk factors affecting CWD transmission. The formalized expert elicitation helped structure the issues and hence provide a rational basis for estimating some transmission risk factors for which evidence is lacking. Considered judgments regarding environmental transmission, latency of CWD transmission, management, and species barrier were provided by the experts. Uncertainties for many items were determined to be large, highlighting areas requiring more research. The elicited values may be used as surrogate values until research evidence becomes available.

The role of genetics in chronic wasting disease of North American cervids

Chronic wasting disease (CWD) is a major concern for the management of North American cervid populations. This fatal prion disease has led to declines in populations which have high CWD prevalence and areas with both high and low infection rates have experienced economic losses in wildlife recreation and fears of potential spill-over into livestock or humans. Research from human and veterinary medicine has established that the prion protein gene (Prnp) encodes the protein responsible for transmissible spongiform encephalopathies (TSEs). Polymorphisms in the Prnp gene can lead to different prion forms that moderate individual susceptibility to and progression of TSE infection. Prnp genes have been sequenced in a number of cervid species including those currently infected by CWD (elk, mule deer, white-tailed deer, moose) and those for which susceptibility is not yet determined (caribou, fallow deer, sika deer). Over thousands of sequences examined, the Prnp gene is remarkably conserved within the family Cervidae; only 16 amino acid polymorphisms have been reported within the 256 amino acid open reading frame in the third exon of the Prnp gene. Some of these polymorphisms have been associated with lower rates of CWD infection and slower progression of clinical CWD. Here we review the body of research on Prnp genetics of North American cervids. Specifically, we focus on known polymorphisms in the Prnp gene, observed genotypic differences in CWD infection rates and clinical progression, mechanisms for genetic TSE resistance related to both the cervid host and the prion agent and potential for natural selection for CWD-resistance. We also identify gaps in our knowledge that require future research.

Rapid detection of CWD PrP: comparison of tests designed for the detection of BSE or scrapie

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) mainly affecting cervids in North America. The accumulation of an abnormal form of host-encoded prion protein (PrP(CWD) ) in the CNS and lymphoid tissues is characteristic of the disease and known to be caused by pathogenic prion proteins (PrP(res) ), which are thought to be transmitted mainly by contact with body fluids, such like saliva. Species known to be naturally infected by CWD include Rocky Mountain elk (Cervus elaphus nelsoni), white-tailed deer (Odocoileus virginianus) and mule deer (Odocoileus hemionus). Recently, large-scale disease eradication or control programs have been attempted to curtail the spread of disease. But reports of diseased free-ranging and farmed cervids in many locations in the USA and Canada are still continuing. The goal of this study was to find sensitive rapid test systems that are reliably able to detect CWD-associated PrP(CWD) in cervids, thereby reviewing an important control tool in case the disease spreads further and reaches Europe. Seven tests, originally developed for the detection of other TSE diseases such as Scrapie and bovine spongiform encephalopathy, including two Western blots, four enzyme-linked immunosorbent assays (ELISAs), and one lateral flow device, were included in this study. All seven tests evaluated were able to detect pathogenic prion proteins (PrP(CWD) ) in Northern American infected animals and distinguish physiologic prion protein (PrP(c) ) in brainstem (obex region) and lymph node samples from North American and European cervids, respectively. However, the specificity and sensitivity of the tests differed significantly. Highly sensitive tests for the detection of prion proteins are an important tool both for the design of effective disease surveillance and control strategies and the safety of the food chain. Thus, this study contributes to the emergency preparedness against CWD.

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.