Quantification of early detection surveillance in PRRS-free regions

In May 2022, a national control program targeting porcine reproductive and respiratory syndrome virus was launched in Denmark, and the first regions are approaching a PRRS-free status. Hence, the question now arises as to how the surveillance should be performed to identify new incursions of the PRRS virus in PRRS-free regions as early as possible. The aim of the present study was to quantify the early detection sensitivity of the current and alternative PRRS surveillance systems in Denmark at a regional level. The current PRRS surveillance system is composed of a notifiable surveillance component and an active serological surveillance component, consisting of monthly sampling (breeding and multiplier farms) or yearly sampling (production farms). The results from the present study estimated that the current surveillance system would have a 21.5 % [16.0 %;32.4 %]_min;max probability of detecting an incursion of PRRS into a PRRS-free region. If monthly serum samples were taken on all farms, the probability would be increased to 87.7 % [87.4 %;88.6 %]_min;max. Adding a syndromic surveillance component to the existing surveillance system resulted in an early detection sensitivity of 23.9 % [17.6 %;34.6 %]_min;max, while targeted risk-based sampling resulted in probabilities of 27.8 % [17.9 %;33.8 %]_min;max and 62.6 % [17.9 %;79.5 %]_min;max for quarterly or monthly sampling frequencies, respectively. Results from the present study indicate a relatively low chance of detecting newly infected farms within the first month in a PRRS-free region with the current surveillance system. The probability of early detection of PRRS can be increased either by increasing the sampling frequency on all farms or by using targeted risk-based sampling.