Abstract
Identifying the sources of ongoing and novel disease outbreaks is critical for understanding the diffusion of epizootic diseases. Identifying infection sources is difficult when few physical differences separate individuals with different origins. Genetic assignment procedures show great promise for assessing transmission dynamics in such situations. Here, we use genetic assignment tests to determine the source of chronic wasting disease infections in free‐ranging white‐tailed deer (Odocoileus virginianus) populations. Natural dispersal is thought to facilitate the geographic diffusion of chronic wasting disease, but egression from captive cervid populations represents an alternative source of infection that is difficult to detect due to physical similarities with wild deer. Simulated reference populations were created based on allele frequencies from 1,912 empirical microsatellite genotypes collected in four sampling subregions and five captive facilities. These reference populations were used to assess the likelihood of ancestry and assignment of 1,861 free‐ranging deer (1,834 noninfected and 27 infected) and 51 captive individuals to captive or wild populations. The ancestry (Q) and assignment scores (A) for free‐ranging deer to wild populations were high (average Q wild = 0.913 and average A wild = 0.951, respectively), but varied among subregions (Q wild = 0.800–0.947, A wild = 0.857–0.976). These findings suggest that captive egression and admixture are rare, but risk may not be spatially uniform. Ancestry and assignment scores for two free‐ranging deer with chronic wasting disease sampled in an area where chronic wasting disease was previously unobserved in free‐ranging herds indicated a higher likelihood of assignment and proportion of ancestry attributable to captive populations. While we cannot directly assign these individuals to infected facilities, these findings suggest that rare egression events may influence the epizootiology of chronic wasting disease in free‐ranging populations. Continued disease surveillance and genetic analyses may further elucidate the relative disease risk attributable to captive and wild sources.
Highlights
While natural dispersal is thought to be a driver of the geographic diffusion of chronic wasting disease in free-ranging populations (Green, Manjerovic, Mateus-Pinilla, & Novakofski, 2014; Hefley et al, 2017), interactions with infected captive individuals represent an alternative source of infection where captive and wild herds are kept in sympatry
TA B L E 1 Sample sizes (n), genetic summary statistics (NA = average number of alleles per locus, HE = Nei's unbiased estimated of heterozygosity, and PA = number of private alleles), average ancestry to the wild cluster (Qwild), and average assignment score to the wild cluster (Awild) for wild and captive whitetailed deer sampled in the mid-Atlantic region of the United States simulations, despite variance in sample size among herds and regardless of disease status, because all captive populations were characterized by small sample sizes (n = 1–16)
Correlation was speculated in previous outbreaks, our results demonstrate that captive egression may have contributed to a novel chronic wasting disease outbreak in a county in subregion 3 where the disease was undetected in wild populations prior to 2017 and that was geographically separated from previously detected disease foci
Summary
The geographic distribution and diffusion of wildlife diseases are commonly related to the probability of contact among infected and susceptible individuals or populations (Ostfeld, Glass, & Keesing, 2005). While natural dispersal is thought to be a driver of the geographic diffusion of chronic wasting disease in free-ranging populations (Green, Manjerovic, Mateus-Pinilla, & Novakofski, 2014; Hefley et al, 2017), interactions with infected captive individuals represent an alternative source of infection where captive and wild herds are kept in sympatry. A recent study from Pennsylvania, a state with over 1,000 captive cervid herds, found that more than 50 percent of herds in the region have participated in the transfer of at least one deer between facilities (Rorres et al, 2017) Such translocations, combined with the intentional or unintentional release of infected individuals, may pose a risk to adjacent free-ranging populations and facilitate the spread of chronic wasting disease into novel areas (Gerhold & Hickling, 2016). The following specific scenarios regarding origin were assessed for 27 cases of chronic wasting disease: (1) The infected individual had a potential origin from the free-ranging population, (2) the infected individual had a potential origin from a captive deer herd, or (3) the infected individual potentially shares ancestry with a captive herd
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