Serum samples provide the best sample to detect porcine reproductive and respiratory syndrome (PRRS) virus during acute infection, with the blood swab sample performing almost as well in research conducted by the University of Minnesota in collaboration with South Dakota State University, PIC and Boehringer Ingelheim Vetmedica, Inc.

Boar studs are regularly monitored for the presence of the PRRS virus, testing different biological samples by reverse-transcription polymerase chain reaction (RT-PCR).

Usually, samples are run in pools to reduce the cost of submitting individual samples, even though the impact of pooling on the sensitivity of RT-PCR is unknown.

To assess the impact of pooling and collection method on PRRS virus by detection, 29 boars were inoculated with a low virulent PRRS strain. Serum, blood swab and semen samples were obtained from each boar every two to three days for two weeks and tested by RT-PCR.

Eleven of the 29 boars did not appear to become infected from the inoculation. Data from the other 18 boars showed that serum provided the best sample, followed by blood swab.

Other results showed that semen samples failed to detect PRRS infection in most of the cases.

For most of the samples, pooling did not affect the performance of the test. But PRRS detection was missed in a small proportion of pooled samples. The impact of pooling on the sensitivity of PCR was higher in samples taken when infection started. Sensitivity was decreased by 6-8% when serum or blood swab samples were run in pools of five.

This decrease in sensitivity should be taken into account when designing surveillance protocols for boar studs by proportionally sampling more boars.

Researcher: Albert Rovira, DVM, University of Minnesota. Contact Rovira by phone (612) 625-7702, fax (612) 625-1210 or e-mail