Detecting subpopulations after PRRS virus infection in large breeding herds using multiple serologic tests

Objective: To assess the presence of breeding herd subpopulations after infection with porcine reproductive and respiratory syndrome virus (PRRSV). Design and procedure: Ten swine herds were selected based on the inability to control postweaning PRRS problems using nursery depopulation or piglet vaccination. No vaccines had been used in breeding animals. The existence of subpopulations at a point in time and over time was assessed. Cross-sectional serologic profiles of randomly selected sows were collected and samples were analyzed for IgM and IgG indirect-fluorescent antibodies (IFA), and serum neutralization (SN) antibodies at a point in time. Based on the results, sows were classified into one of two subpopulations: 'not infected' (negative on all three tests) or 'acutely infected' (IgM or IgG positive at ≥1:64 and SN negative). The ability of subpopulations to persist over time was also assessed. In conjunction with the previously described subpopulations, another subpopulation, 'antibody decay,' was assessed. This classification was based on evidence of a previous infection, with subsequent antibody decay over time. Two herds from the original 10 were selected and specific sows were serially sampled three times over a 6-month period. Results:Sows were found to be'not infected' in eight herds (mean=26%, range=0%-52%) and 'acutely infected' in five herds (mean = 14%, range = 0%-36%). Further, all three subpopulations persisted over time and sows changed subpopulations, depending on whether viral infection took place. Implications: Breeding herd subpopulations may exist and may be a factor in maintaining persistent viral transmission in chronically infected farms.