THE EPIDEMIOLOGY OF CHRONIC WASTING DISEASE ON SASKATCHEWAN CERVID FARMS (2002 – 2017)

Chronic wasting disease (CWD) is a naturally occurring fatal transmissible spongiform encephalopathy of cervids. In 1996, CWD was recognized in a herd of farmed elk in Saskatchewan (SK) and by the year 2000 the disease was detected in wild mule deer (Bollinger et al., 2004). Records of CWD in farmed elk and white-tailed deer (WTD) in SK from 2002 – 2017 were reviewed to: 1) summarize the epidemiology of the disease in farmed elk and WTD within the province, and 2) develop a quantitative risk assessment to determine the probability of there being at least one infected WTD or elk in a movement event dependent on the location of the farm within the province. Data collected and compiled by the Canadian Food Inspection Agency and the SK Ministry of Agriculture were used to describe the epidemiology of CWD in SK farm cervids from 2002 – 2017. During this time period a total of 56 farms were found to be infected with CWD and underwent an eradication process. Animal movements within the period were numerous, with immigration events occurring more frequently than emigration events. Infected farms averaged nine movement events over the 16-year time period (range 0 – 74 movement events), and movement events onto and off farms involving at least one known CWD-positive animal occurred in 13 and 10 of the 16 years, respectively. Sources of CWD transmission for case farms were determined for 84% (47/56) of farms, and were primarily linked to within-farm (68%, 15/47) and off-farm sources (32%, 15/47), while for the remaining 16% (9/56) of farms, the source was unproven. The 132MM and 96GG genotypes were most prominent for CWD detected in elk and WTD, respectively. Higher immunohistochemistry grading was observed for clinically infected animals (70% of WTD were Grade 3, and 58% elk were Grade 4). Since the initial evaluation period (1996 – 2002) of CWD on farmed elk, the median period prevalence of CWD on farms slightly decreased from 4.4% to 3.9%, and the median prevalence at the time of depopulation (point prevalence) for this review period was 2.6%. A quantitative risk assessment determined that the risk of an infected animal being involved in a movement event was dependent on the geographic location, sex and age of the animal. The quadrant sections Q1 (central west) and Q2 (central east) had the greatest probability of an infected animal being involved in a movement event for WTD and elk, respectively, while Q4 (southeast) had the lowest probability for both species. Captive WTD within SK had a greater mean probability of at least one infected animal (4.80%) being involved in a movement event compared to elk (3.22%) for the province as a whole. For elk, males less than the median age of CWD detection (70 mo) posed the greatest risk of contributing at least one infected animal in a shipment within the province, while for WTD the disease prevalence on WTD farms posed the greatest risk. We conclude that during this time period within-farm transmission was an increasingly important source of infection; however, the movement of infected animals among farms continued to play a role in the spread of disease within the province. Further investigation and identification of farm management factors associated with within-farm CWD transmission could help mitigate disease spread and decrease prevalence on SK cervid farms. The risk assessment study concluded that the geographic location, species, sex and age influence the potential risk of having at least one CWD infected animal involved in a movement event. Although additional risks (provincial surveillance program compliance, history of CWD detection on farm, duration and distance of shipment and quarantine protocol) were not investigated, further analysis involving added risks would be needed to further understand and lessen disease spread.

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