Relationship between weekly porcine reproductive and respiratory syndrome virus exposure in breeding herds and subsequent viral shedding and mortality in the nursery

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Objective: Describe the relationship of weekly breeding herd status based on processing fluid (PF) testing for porcine reproductive and respiratory syndrome virus (PRRSV) RNA by quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) on subsequent viral shedding and cumulative mortality during the nursery phase. Materials and methods: Weekly cohorts (n = 121) of newborn piglets were classified into PRRSV exposure groups according to PRRSV detection in PF: low (quantification cycles [Cq] ≤ 27), medium (27 < Cq ≤ 34), high (34 < Cq ≤ 37), and negative (Cq > 37). At 6 weeks of age, oral fluids (OF) were collected from a subset of 41 cohorts, tested by qRT-PCR, and results used to classify the nursery shedding status into the same aforementioned categories. Cumulative nursery mortality was recorded for all 121 cohorts and compared between the different PRRSV exposure groups. Test agreement was assessed between PF and OF results of 41 cohorts. Moreover, the effect of 4:1 OF pooling on the probability of testing qRT-PCR-positive was evaluated. Results: The nursery mortality for low Cq cohorts was 3.40 percentage points (range, 3.28-3.99) higher than other exposure groups. Overall, Cq values were higher in PF than in OF samples, and fair agreement (κ = 0.2398) between PF and OF was encountered. Compared to individual samples, 4:1 OF pooling resulted in 100% specificity and 76.92% sensitivity. Implications: Weekly PF testing for PRRSV allowed for exposure group classification for each pig batch produced, which was a good predictor of subsequent cumulative nursery mortality.