Clinical, pathogenic and immunologic investigation of severe porcine reproductive and respiratory syndrome in swine

A severe and atypical form of porcine reproductive and respiratory syndrome (PRRS) has been experienced in mid-western swine farms since 1996. Highly virulent PRRSV with or without newly emerged H3N2 swine influenza virus (SIV) were considered as likely cause of atypical PRRS.; Clinical investigation was made on twelve 4.000-sow units under a large integrated farm with acute outbreaks of sow abortion and mortality. Many PRRSV were recovered from 5 PRRSV endemic units, while SIV H3N2 was isolated from all 12 herds during this clinical episode. Herds positive to both PRRSV and H3N2 suffered reproductive failure more severe than the herds positive to only SIV. Based on this finding, it may be suggested that a combined infection of PRRSV and SIVH3N2 could result in similar clinical signs of atypical PRRS.; Experimental infections were conducted to answer whether PRRSV alone or with SIV H3N2 could induce atypical PRRS. Following experimental infection with atypical PRRSV isolates, severe clinical signs were not found in 3-week old pigs and mid-term gestation sows. Two mid-gestation sows that were inoculated with an atypical PRRSV followed by H3N2 did not show severe clinical signs, but one of them aborted.; A serum neutralization (SN) test was used to determine serologic relationship among PRRSV isolates. Most of the antisera used were able to neutralize selected PRRSV isolates, although resulting SN titers were varied. This indicates that the selected isolates were serologically related. Multiple inoculations with different strains of PRRSV induced antibodies that had broader neutralizing ability and the highest SN antibody titer.; Sows with pre-existing SN antibody (titers of ≥1:4) did not become viremic after a PRRSV challenge. PRRSV infection or minimal virus replication may have occurred despite absence of viremia. Nevertheless, PRRSV infected sows were relatively resistant to PRRSV re-infection. Pre-existing SN titers may be used as an indicator of protective immunity to the PRRSV strain homologous to the antibody.