Detection of porcine reproductive and respiratory syndrome virus in semen and serum of boars during the first six days after inoculation

Objectives: To determine, during the first 6 days post inoculation, when porcine reproductive and respiratory syndrome virus (PRRSV) can be detected in serum or semen by polymerase chain reaction (PCR); the impact of pooling on detection of PRRSV by PCR; and the possible association between rectal temperature and detection of PRRSV in serum by PCR. Materials and methods: Forty mature boars (four groups of 10) were inoculated intranasally with PRRSV variant MN 30-100. Serum and semen samples were collected on a rotating basis from one group every 12 hours for 6 days and tested for PRRSV by PCR. Rectal temperatures were recorded for all 40 boars at 12-hour intervals. Results: Serum samples became PCR-positive before semen samples. During the first 6 days after inoculation, serum was PRRSV-positive in 36 of 40 boars, and semen was PRRSV-positive in four of 40 boars. Median time to detection was 36 and 48 hours for nested PCR and Taqman PCR, respectively. Results were inconsistent when a positive semen sample was pooled with negative semen. Elevated rectal temperature was not associated with PCR-positive serum or semen results. Implications: Under the conditions of this study, PCR is more sensitive and detects PRRSV-infected boars earlier in serum than in semen. Pooling of positive semen samples provides variable PCR results. Rectal temperatures are not correlated with PCR-positive results. Sampling techniques are needed to more easily obtain serum samples for PCR from boars once or twice weekly at the time of semen collection.