Management of chronic wasting disease in ranched elk: conclusions from a longitudinal three-year study
N.J. Haley,D.M. Henderson,R. Donner,S. Wyckoff,K. Merrett,J Tennant,
Pages 76-87 | Received 09 Oct 2019, Accepted 28 Jan 2020, Published online: 07 Feb 2020
Chronic wasting disease is a fatal, horizontally transmissible prion disease of cervid species that has been reported in free-ranging and farmed animals in North America, Scandinavia, and Korea. Like other prion diseases, CWD susceptibility is partly dependent on the sequence of the prion protein encoded by the host’s PRNP gene; it is unknown if variations in PRNP have any meaningful effects on other aspects of health. Conventional diagnosis of CWD relies on ELISA or IHC testing of samples collected post-mortem, with recent efforts focused on antemortem testing approaches. We report on the conclusions of a study evaluating the role of antemortem testing of rectal biopsies collected from over 570 elk in a privately managed herd, and the results of both an amplification assay (RT-QuIC) and conventional IHC among animals with a several PRNP genotypes. Links between PRNP genotype and potential markers of evolutionary fitness, including pregnancy rates, body condition, and annual return rates were also examined. We found that the RT-QuIC assay identified significantly more CWD positive animals than conventional IHC across the course of the study, and was less affected by factors known to influence IHC sensitivity – including follicle count and PRNP genotype. We also found that several evolutionary markers of fitness were not adversely correlated with specific PRNP genotypes. While the financial burden of the disease in this herd was ultimately unsustainable for the herd owners, our scientific findings and the hurdles encountered will assist future CWD management strategies in both wild and farmed elk and deer.
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Return rate of CWD positive animals In the 2017 sampling period, 315 animals were tested for CWD antemortem. Of those, 71 tested positive by IHC, RT-QuIC, or both. Thirty-four infected animals (thirty-three cows and one bull) were euthanized and confirmed CWD positive post-mortem, with the remaining thirty-seven animals (Twenty-six bulls and eleven cows) released back onto the property. Of those animals which were released, four were harvested in the fall of 2017 and were found to be CWD positive post-mortem. Three of the remaining thirty (10%) returned for the 2018 sampling period, the remainder were lost in the field and went untested. All three were 132ML heterozygous animals, each positive again on antemortem testing in year three. In contrast, 120 of the 329 animals negative by antemortem testing were harvested in the fall of 2017, with 26 found to be CWD positive (21.7%). Of the remaining 209 animals, 141 returned for the 2018 sampling period (67.5%), with the remaining animals presumed lost in the field, untested. (Table 1 and Figure 1)
Over the course of the entire study, four of forty-nine CWD positive 132MM animals released back onto the property (8.2%) returned for a second year of sampling. Nine of thirty-three 132ML animals positive for CWD returned for a second year of sampling (27%), a return rate that was significantly greater than that of CWD positive 132MM animals (risk ratio: 3.34, p = 0.03). The lone 132LL cow identified during the course of antemortem testing in year two did not return for sampling in year three. Cumulatively, just 13 of 82 animals identified as CWD positive and released onto the property returned the following year (15.9%). These low rates of yearly return are in stark contrast to the cumulative return rate for CWD negative animals. For animals homozygous for the 132M allele, 107/144 returned in year two, and 28/69 returned in year three (63% overall). For 132ML heterozygous animals, 170/221 returned in year two, with 62/181 returning in year three (58% overall). Twenty-five of thirty-five animals homozygous for the 132L allele returned in year two, with fifteen of thirty-five returning in year three (57% overall). Cumulatively, 60% of animals negative for CWD returned the following year – a yearly return rate nearly over 3.5 times that of CWD positive animals (risk ratio: 3.62, p < 0.001; 95% confidence interval 1.96–6.69).
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While reports on the management of chronic wasting disease in wild deer and elk are many and varied [33–43], rare is the case presented for managing the disease in farmed cervids. Almost without exception, farmed cervids are immediately placed under quarantine and eventually depopulated when CWD is discovered on site [12]. This manuscript reports our efforts to manage CWD in a large elk herd, in a controlled setting with endemic CWD, through the use of annual live animal testing and targeted culling of CWD positive cows. Although the herd owners were presented with additional management directives, including culling of CWD positive bulls and those animals positive by an amplification assay (RT-QuIC), they were not implemented due to concern regarding its potential impact on hunting revenue. Ultimately, we could not completely evaluate our management practices, as the herd was slowly depopulated after the final sampling period due to the financial burden brought by the disease.
***> at present, no PrPC allele conferring absolute resistance in cervids has been identified.
J Gen Virol. 2017 Nov; 98(11): 2882–2892.
Published online 2017 Oct 23. doi: 10.1099/jgv.0.000952
PMCID: PMC5845664
PMID: 29058651
Estimating chronic wasting disease susceptibility in cervids using real-time quaking-induced conversion
Chronic wasting disease (CWD) resistance in cervids is often characterized as decreased prevalence and/or protracted disease progression in individuals with specific alleles; at present, no PrPC allele conferring absolute resistance in cervids has been identified.
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In summary, evaluating the amplification rates and efficiencies of recombinant PrPC substrates by RT-QuIC could be a useful tool for estimating the susceptibility of rare or newly discovered PRNP alleles, allowing researchers to target specific alleles for downstream evaluation in challenge studies. In the face of an ever-expanding CWD-endemic area, it is increasingly important to characterize the natural susceptibility of these alleles, as well as their geographical distribution and the evolutionary basis for their rarity. Do the QGAK, 225F and 225Y alleles represent recent, random anomalies, or are they more primitive mutations that adversely affect reproductive fitness? Perhaps they are an indication that cervids with these rare alleles were themselves once the target of a primordial prion strain. While some evidence has been presented for distinct strains of CWD, little is known about their geographical distribution or virulence in cervid hosts of diverse PRNP backgrounds. It is possible that, with the appropriate framework, RT-QuIC could allow for the discrimination of known and novel prion strains. Without further research into disease management and prevention, including resistance, the only certainty seems to be that CWD will continue its insidious spread, with further discoveries in new hosts and geographical locations.
P-145 Estimating chronic wasting disease resistance in cervids using real time quaking- induced conversion
Nicholas J Haley1, Rachel Rielinqer2, Kristen A Davenport3, W. David Walter4, Katherine I O'Rourke5, Gordon Mitchell6, Juergen A Richt2 1
Department of Microbiology and Immunology, Midwestern University, United States; 2Department of Diagnostic Medicine and Pathobiology, Kansas State University; 3Prion Research Center; Colorado State University; 4U.S. Geological Survey, Pennsylvania Cooperative Fish and Wildlife Research Unit; 5Agricultural Research Service, United States Department of Agriculture; 6Canadian Food Inspection Agency, National and OlE Reference Laboratory for Scrapie and CWO
In mammalian species, the susceptibility to prion diseases is affected, in part, by the sequence of the host's prion protein (PrP). In sheep, a gradation from scrapie susceptible to resistant has been established both in vivo and in vitro based on the amino acids present at PrP positions 136, 154, and 171, which has led to global breeding programs to reduce the prevalence of scrapie in domestic sheep. In cervids, resistance is commonly characterized as a delayed progression of chronic wasting disease (CWD); at present, no cervid PrP allele conferring absolute resistance to prion infection has been identified. To model the susceptibility of various naturally-occurring and hypothetical cervid PrP alleles in vitro, we compared the amplification rates and efficiency of various CWD isolates in recombinant PrPC using real time quaking-induced conversion. We hypothesized that amplification metrics of these isolates in cervid PrP substrates would correlate to in vivo susceptibility - allowing susceptibility prediction for alleles found at 10 frequency in nature, and that there would be an additive effect of multiple resistant codons in hypothetical alleles. Our studies demonstrate that in vitro amplification metrics predict in vivo susceptibility, and that alleles with multiple codons, each influencing resistance independently, do not necessarily contribute additively to resistance. Importantly, we found that the white-tailed deer 226K substrate exhibited the slowest amplification rate among those evaluated, suggesting that further investigation of this allele and its resistance in vivo are warranted to determine if absolute resistance to CWD is possible. ***at present, no cervid PrP allele conferring absolute resistance to prion infection has been identified.
PRION 2016 CONFERENCE TOKYO
***at present, no cervid PrP allele conferring absolute resistance to prion infection has been identified.
''There are no known familial or genetic TSEs of animals, although polymorphisms in the PRNP gene of some species (sheep for example) may influence the length of the incubation period and occurrence of disease.''
c) The commonest form of CJD occurs as a sporadic disease, the cause of which is unknown, although genetic factors (particularly the codon 129 polymorphism in the prion protein gene (PRNP)) influence disease susceptibility. The familial forms of human TSEs (see Box 1) appear to have a solely genetic origin and are closely associated with mutations or insertions in the PRNP gene. Most, but not all, of the familial forms of human TSEs have been transmitted experimentally to animals. There are no known familial or genetic TSEs of animals, although polymorphisms in the PRNP gene of some species (sheep for example) may influence the length of the incubation period and occurrence of disease.
''There are no known familial or genetic TSEs of animals, although polymorphisms in the PRNP gene of some species (sheep for example) may influence the length of the incubation period and occurrence of disease.''
ADOPTED: 26 September 2019 doi: 10.2903/j.efsa.2019.5863 Update on chronic wasting disease (CWD) III
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3.2.1.2. Non-cervid domestic species
The remarkably high rate of natural CWD transmission in the ongoing NA epidemics raises the question of the risk to livestock grazing on CWD-contaminated shared rangeland and subsequently developing a novel CWD-related prion disease. This issue has been investigated by transmitting CWD via experimental challenge to cattle, sheep and pigs and to tg mouse lines expressing the relevant species PrP.
For cattle challenged with CWD, PrPSc was detected in approximately 40% of intracerebrally inoculated animals (Hamir et al., 2005, 2006a, 2007). Tg mice expressing bovine PrP have also been challenged with CWD and while published studies have negative outcomes (Tamguney et al., 2009b), unpublished data provided for the purposes of this Opinion indicate that some transmission of individual isolates to bovinised mice is possible (Table 1).
In small ruminant recipients, a low rate of transmission was reported between 35 and 72 months post-infection (mpi) in ARQ/ARQ and ARQ/VRQ sheep intracerebrally challenged with mule deer CWD (Hamir et al., 2006b), while two out of two ARQ/ARQ sheep intracerebrally inoculated with elk CWD developed clinical disease after 28 mpi (Madsen-Bouterse et al., 2016). However, tg mice expressing ARQ sheep PrP were resistant (Tamguney et al., 2006) and tg mice expressing the VRQ PrP allele were poorly susceptible to clinical disease (Beringue et al., 2012; Madsen-Bouterse et al., 2016). In contrast, tg mice expressing VRQ sheep PrP challenged with CWD have resulted in highly efficient, life-long asymptomatic replication of these prions in the spleen tissue (Beringue et al., 2012).
A recent study investigated the potential for swine to serve as hosts of the CWD agent(s) by intracerebral or oral challenge of crossbred piglets (Moore et al., 2016b, 2017). Pigs sacrificed at 6 mpi, approximately the age at which pigs reach market weight, were clinically healthy and negative by diagnostic tests, although low-level CWD agent replication could be detected in the CNS by bioassay in tg cervinised mice. Among pigs that were incubated for up to 73 mpi, some gave diagnostic evidence of CWD replication in the brain between 42 and 72 mpi. Importantly, this was observed also in one orally challenged pig at 64 mpi and the presence of low-level CWD replication was confirmed by mouse bioassay. The authors of this study argued that pigs can support low-level amplification of CWD prions, although the species barrier to CWD infection is relatively high and that the detection of infectivity in orally inoculated pigs with a mouse bioassay raises the possibility that naturally exposed pigs could act as a reservoir of CWD infectivity.
3.2.1.3. Other species
Studies have demonstrated that the CWD agent(s) can be transmitted by the IC route in several species of rodents, such as voles (Subfamily Arvicolinae), deer mice (Peromyscus maniculatus), mice and hamsters (Subfamily Cricetinae). The susceptibility was, however, variable, being high in voles and deer mice but lower in mice and hamsters (Raymond et al., 2007; Heisey et al., 2010; Kurt et al., 2011; Di et al., 2013; Lee et al., 2013). Mink (subfamily Mustelinae) (Harrington et al., 2008), ferrets (Mustela putorius) (Bartz et al., 1998; Sigurdson et al., 2008) and cats (Mathiason et al., 2013) were susceptible to IC challenge with NA CWD sources, while CWD transmitted poorly to raccoons (Procyon lotor) by the IC route (Moore et al., 2019).
3.2.2. European isolates
The host range of CWD in Europe has been much less investigated so far, due to its recent identification. Among the cervid species involved in the CWD epidemics in North America, only some species (such as moose and reindeer) inhabit Europe; mule deer, white-tailed deer and elk/wapiti are American cervid species, although a few populations of white-tailed deer have been introduced into Europe. Others cervids that mainly inhabit Europe are red deer and roe deer. After the first detection in a reindeer in Norway in 2016 (Benestad et al., 2016), CWD has been detected in wild reindeer, moose and one red deer in Norway (Mysterud and Edmunds, 2019), in a moose in Finland in March 2018 and in three moose in Sweden in March, May and September 2019. CWD has not been detected
Chronic Wasting Disease (CWD) III www.efsa.europa.eu/efsajournal 14 EFSA Journal 2019;17(11):5863
so far in wild roe deer, fallow deer or white-tailed deer nor in any farmed cervid species. However, for fallow deer and white-tailed deer, the number of animals tested by the surveillance systems is still very low.
The potential host range of European CWD strains is under investigation by bioassay experiments in a range of model species; most of these studies are ongoing and there are no published data available so far. Data from the experiments that are known to be ongoing in different laboratories have been gathered for the purposes of this Opinion. Overall, reindeer CWD, moose CWD and red deer CWD brain isolates (and LRS isolates from some selected cases) are being tested for transmissibility in mice, hamsters, bank voles and in a range of tg mouse lines expressing PrP sequences from: cervids (Q226 or E226 deer PrP variants), small ruminants (ARQ, VRQ, AHQ and ARR PrP polymorphic variants), cattle, pig, vulture and human (M129 and V129 PrP polymorphic variants) (see Section 3.3.1). Importantly, in most of these animal models, the transmissibility of European CWD isolates will be directly comparable with the outcome of similar (published or ongoing) experiments with CWD isolates from North America.
While most of these studies are still ongoing, some experiments with CWD isolates from Europe have already produced evidence of transmission in some recipient species (Table 1). These include bank voles, conventional laboratory mice and tg mice expressing cervid PrP, sheep PrP and mouse PrP. The same rodent models are also susceptible to NA CWD isolates and will therefore allow comparative strain typing of NA and European CWD strains in due course. There is no strong evidence so far for rodent models being widely susceptible to NA isolates but not to European isolates or vice versa.
Table 1 summarises more than 500 ongoing, published or unpublished primary transmission experiments of NA or European CWD isolates from different cervids into various rodent models, which have been gathered following the requests described in Section 2.2. The CWD isolates are grouped according to geographical origin and cervid species, with each column summarising the results obtained with one or more CWD isolates from a given species and country. Rodent models are grouped according to the PrP species expressed. Some species have polymorphic PrP sequences, so more than one PrP sequence per species has been modelled. In these cases, each row summarises the data obtained with more than one PrP variant of a given species. Therefore, conventional mice include wt mice expressing PRNPa or PRNPb mouse PrP variants; bank voles include two genetic lines with different amino acids at codon 109 (Bv109M and Bv109I); tg-cervidPrP mice include mouse lines expressing several cervid PrP variants (the deer wt Q226, the elk wt E226, the WTD variant S96, the elk variant M132); tg-sheepPrP mice include mouse lines expressing the ARQ, VRQ, AHQ or ARR small ruminant PrP variants; finally, tg-humanPrP mice include mouse lines expressing M or V at the human PrP polymorphic codon 129. Therefore, each box in the Table 1 summarises the outcome of bioassay experiments with one or more CWD isolates (from the same species and origin) in one or more recipient rodent models (expressing PrP from a single given species).
Chronic Wasting Disease (CWD) III www.efsa.europa.eu/efsajournal 15 EFSA Journal 2019;17(11):5863
Most of the studies conducted by molecular/biochemical methods are still ongoing. The preliminary data obtained by molecular/biochemical methods were difficult to summarise and will not be reported in the present Opinion. This was mainly due to lack of detail in the results obtained by direct PrPSc analyses (PrPres typing, conformational stability, proteinase K resistance, which are intended to investigate CWD strains) and to the different methodological approaches employed in amplification assays (PMCA and RT-QuIC). The information gathered by this activity shows that experiments aimed at modelling the species barrier for NA or European CWD isolates into different animal species, including humans, are underway in different laboratories and will be of help for understanding the potential host range of CWD strains.
3.2.3 Impact of the PRNP gene on transmissibility
Polymorphisms in the PRNP gene are known to influence susceptibility/resistance to prion disease in both small ruminants and humans (for recent review, see EFSA BIOHAZ Panel 2014, EFSA BIOHAZ Panel, 2017; Diack et al., 2014). Effects of host PRNP polymorphisms on CWD susceptibility/resistance have also been described in a number of cervid species (reviewed in EFSA BIOHAZ Panel, 2017, 2018).
However, deer and elk wild-type PrP primary structures are equivalent, except at residue 226, which is glutamate in elk and glutamine in deer. The effect of this difference on CWD pathogenesis has been recently investigated using a gene-targeting approach in which the mouse PrP coding sequence was replaced with elk or deer PrP. The results obtained following experimental challenge with deer and elk CWD inocula from NA showed that the resulting GtE226 and GtQ226 mice had distinct kinetics of disease onset, with incubation times shorter in GtE226 than in GtQ226 mice, indicating that amino acid differences at PrP residue 226 dictate the selection and propagation of divergent strains in deer and elk with CWD. As prion strain properties largely dictate host range potential, these findings suggest that prion strains from elk and deer might pose distinct risks to sympatric species or humans exposed to CWD (Bian et al., 2019).
The most common cervid species in Europe (moose, red deer, reindeer and roe deer) share the same PrP primary structure, i.e. Q226. However, red deer PrP is polymorphic at residue 226 and can therefore code for either Q226 or E226. Interestingly, CWD cases detected so far in four Norwegian moose, the first Swedish moose and one Norwegian reindeer are all homozygous for Q226 (Benestad, 2019b,c,d), but the CWD case in red deer is instead homozygous for E226 (Vikøren et al., 2019). The impact of these differences in PrP genotype on the transmissibility and strain properties of European CWD isolates is currently under investigation using GtE226 and GtQ226 mice (Bian et al., 2019). PRNP genotypes of the other reindeer from Norway and of the other moose cases in Sweden and Finland are not in the public domain.
Data on the transmissibility of CWD in species with different PrP sequences obtained by in vivo or in vitro modelling allow the investigation of the structural basis of the transmission barriers for CWD. This in turn could provide hints for predicting, to some extent, the susceptibility of non-cervid species to CWD. Taken together, studies with CWD isolates from NA suggest that the 165–175 sequence similarity between cervid and host PrP is one important factor governing the susceptibility of different species to CWD (reviewed by Kurt et al., 2016). In particular, polymorphisms at N/S170 in the recipient species might be important for susceptibility, with species that have N170 being more susceptible than those with S170 (Kurt et al., 2016). However, this must not be seen as an absolute rule, as species having S170 in their PrP, such as squirrel monkeys, have also been reported to be susceptible to CWD. It is however pertinent to note that all livestock species and humans have PrP sequences with S170, so they should not be considered among the species with supposedly high susceptibility to NA CWD isolates. Ongoing experiments in rodent models seem to indicate a similar trend for European CWD isolates, as rodent models, apparently more susceptible to European CWD isolates such as bank voles and tg mice expressing deer PrP, are N170.
Little information is currently known about the genetics of either wild or farmed cervid populations in Europe. A recent published study of several deer species (mostly in Great Britain) reported that red deer showed the most PRNP gene variation, with polymorphisms at codons 98, 168, 226 and 247 and marked variability in genotype frequencies in different regions. Other deer species showed less variation, with roe and fallow deer having identical PRNP gene sequences in all the animals sampled. Based on comparison with PRNP sequences of NA cervids affected by CWD and limited experimental challenge data, the authors conclude that a high proportion of wild deer in Great Britain may be susceptible to CWD (Robinson et al., 2019). A similar conclusion was reached by a previous study of 715 genotyped cervids (red deer, roe deer and chamois) from the UK and Italy (Peletto et al., 2009).
3.2.4. Concluding remarks
• The transmission of prions between species is limited by the ‘transmission barrier’ and the amino acid sequence of the host PrP plays a very key role in the overall susceptibility to TSE. Even for prions deriving from the same species, the host range may vary according to the prion strain, implying that different CWD strains might have different host ranges and different potential for transmitting to livestock species and to humans.
• Whether the natural host range of CWD in NA extends beyond the family Cervidae is currently unclear and no natural infections have been reported so far in other wildlife species (e.g. predators and scavengers) with overlapping geographical ranges.
• NA CWD has been transmitted experimentally to cattle and sheep, but with incomplete attack rates. The species barrier appears higher for pigs, although challenged animals can support low-level prion amplification.
• Experimental transmission to tg mice and other rodent models shows some difference in the host ranges of different isolates but, particularly for the European isolates, many bioassays are still ongoing and data are not yet available.
• The number of strains, the strain diversity, the prevalence and the potential host range of disease in both NA and Europe CWD may be underestimated.
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