Risk factors associated with prolonged infection of porcine reproductive and respiratory syndrome virus determined by whole-herd sampling methods

Two naive sow herds became infected with porcine reproductive and respiratory syndrome virus (PRRSV) and were exposed by live virus inoculation (LVI) on week 0. Serum and tonsil scrapings were collected on week 20 post LVI and tested for PRRSV by reverse-transcriptase polymerase chain reaction (RT-PCR). For either sample type, non-negative animals were considered infected. Prolonged infection rate was 3.56% in herd 1 and 1.76% in herd 2. Age and breed were significant factors of prolonged infection. Twenty of thirty-five infected animals removed from herd 2 and resampled on week 27 were still PRRSV positive using RT-PCR on tonsil scrapings.