Effect of Pooling Family Oral Fluids on the Probability of PRRSV RNA Detection by RT-rtPCR

Family oral fluids (FOFs) are an aggregate sample type shown to be a cost-efficient and convenient option for determining the porcine reproductive and respiratory syndrome virus (PRRSV) status of weaning age pigs. This study investigated the effect of pooling PRRSV-positive FOF samples with known PRRSV-negative FOF samples at different levels (1:3, 1:5, 1:10, 1:20) on the probability of PRRSV RNA detection by reverse-transcriptase real-time polymerase chain reaction (RT-rtPCR). Mathematical models were built to assess how much the probability of RT-rtPCR detection changed with increasing PCR-positive samples present within pools and how partially sampling a farrowing room influenced the probability of PCR detection of PRRSV RNA in pooled samples at different prevalence scenarios. A general example of a guideline for FOF-based sampling under different prevalence scenarios to detect PRRSV RNA by RT-rtPCR with at least 95% certainty is presented.At the sample level, the probability of detecting PRRSV RNA by RT-rtPCR decreased from 100% to 87%, 68% and 26% when diluting up to 1:20 for PRRSV positive FOF having initial Cycle threshold (Ct) below 34, between 34 and 36, or above 36, respectively. When the prevalence among crates is near-zero (1 or 2 positive out of 56), the most cost-efficient farrowing room sampling strategy to detect PRRSV RNA with at least 95% certainty was pooling FOF samples up to 1:10; at higher prevalence (≥ 3 of 56 crates positive), the most cost-efficient strategy was submitting samples in pools of 20. Subsampling a farrowing room for FOF pools was also demonstrated to be a valuable cost saving strategy. Overall, based on the conditions of this study, pooling FOFs up to 1:20 is recommended in situations of cost constraint and regardless of pooling level chosen, capturing as many litters as possible improves herd sensitivity.