Dual infection of a Thai isolate HP-PRRSV and the pdmH1N1 2009 SIV in weanling pigs.

After the introduction of the pandemic H1N1 swine influenza virus (pdmH1N1 SIV) in Thai swine farms in 2009, a mild clinical outcome was observed and its role in porcine respiratory disease complex (PRDC) is questionable. A highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) has become endemic in Thai swine farms, recently. This study was conducted to determine the pathogenicity of the pdmH1N1 SIV when co-infected with a HP-PRRSV. Thirty-two weanling pigs from a free PRRSV, SIV, porcine circovirus type 2 (PCV-2) and Mycoplasma spp. commercial farm were randomly divided into 4 groups; Negative, PRRSV, the SIV and Co-infection groups. Pigs in the PRRSV and Co-infection groups were challenged intranasally with the Thai isolate HP-PRRSV at 0 day post HP-PRRSV challenge (dphc) and pigs in the SIV and Co-infection groups were inoculated intratracheally with the pdmH1N1 SIV at 6 dphc. The 1st and 2nd necropsies were at 2 and 4 days post SIV challenge (dsc). A comparison of the lung lesion scores between the PRRSV and Co-infection groups at 2 dsc showed no statistical difference, but at 4 dsc, the lung lesion scores of the Co-infection group had higher average scores than those of the PRRSV group. Similarly, the SIV viral load in the lung of the Co-infection group a showed higher viral load at 4 dsc, compared to the only SIV-infected group. The results suggest that low pathogenic pdmH1N1 SIV could enhance the lung severity when co-infected with the HP-PRRSV leading to PRDC mimicking the field scenarios.