Evaluation of the utility of testicular-only processing fluid for porcine reproductive and respiratory syndrome virus diagnostics and the effect of sample pooling on the test results

Introduction
The testicular-only processing fluid (TOPF) obtained from piglet testicles after castration could be an alternative sample for porcine reproductive and respiratory syndrome (PRRS) laboratory diagnosis. If this matrix were proved useful, testing it would spare piglets the stress of blood drawing and eliminate some labour required to take blood samples. The aim of the study was to evaluate the utility of TOPF for this diagnostic purpose.
Material and Methods
Serum-and-TOPF pairs from male piglets and sera from female piglets were tested using commercial ELISA and real-time RT-PCR kits. For the pooling simulation, 10 μL aliquots of TOPF separated into low-, moderately and highly positive were mixed with appropriate volumes of negative TOPF samples. This simulated pools of 5, 10, 20, 40 and 80 samples containing 1 positive for serological analyses and pools of 10, 20, 40, 80, 160 and 320 samples containing 1 positive in molecular analyses.
Results
The percentages of anti-porcine reproductive and respiratory syndrome virus (PRRSV) antibodies were statistically significantly different (P-value < 0.05) between boar sera (69.55%) and TOPF (54.49%), as well as between gilt sera (74.52%) and TOPF. However, after adjusting the cut-off value, no significant differences were noted. The RNA of PRRSV was detected in 21.26% of male sera, 15.23% of TOPFs and 17.00% of female sera. Pooled sample testing revealed discrepancies in positive results associated with the pool size and original sample positivity strength.
Conclusion
TOPF samples can be a valuable matrix for laboratory PRRS diagnosis in piglets. However, it is important to be aware of the potential for false-negative results.