Submitted to: American Association of Swine Practitioners Proceedings
Publication Type: Proceedings
Publication Acceptance Date: February 27, 1999
Publication Date: N/A
Technical Abstract: Porcine reproductive and respiratory syndrome (PRRS) is a disease of swine caused by the PRRS virus (PRRSV). Acute infections are usually recognized by a sudden onset of abortions and the delivery of stillborn and weak pigs that can affect 10-50% of the sow herd. Most acutely-infected herds develop a chronic PRRSV infection that can result in a cyclic pattern of small clusters of abortions 6 to 12 months apart referred to as "PRRS mini-breaks". Why these occur is unknown, however, we suspect PRRSV mutation on the farm may be one factor that contributes to their occurrence. This paper describes our preliminary efforts to test the hypothesis that PRRSV can infect a farm, produce an acute disease that is followed by chronic disease, and during this time span the virus mutates into a new form that may be able to avoid the herd immunity that was developed against the original virus. The viral "escape mutant" then may cause the occasional PRRS mini-break. We recovered PRRSV from a farm durin an acute infection and then again 14 months later during a PRRS mini-break. At the genetic level the 2 isolates are similar and based on cross-protection studies in pigs during acute infection the isolates appear to be antigenically similar. Additional studies evaluating protection during chronic infection in pigs are underway.