Porcine reproductive and respiratory syndrome virus does not exacerbate Mycoplasma hyopneumoniae infection in young pigs.

The purpose of this study was to determine if Porcine Reproductive and Respiratory Syndrome (PRRS) virus infection altered the severity of acute Mycoplasma hyopneumoniae (MH) infection in young pigs. Twenty five, 3-week-old male pigs were randomly assigned by litter and weight to one of 3 groups. Groups 1 (PRRS only, n = 5) and 2 (PRRS + MH, n = 10) were inoculated intranasally with PRRS virus (IN-5 isolate, 10(5) TCID50) and viremia in all pigs was confirmed by virus isolation from serum 3 days later. Group 3 (MH only, n = 10) was inoculated at the same time with virus free culture media. Seven days after virus inoculation, Groups 2 and 3 were inoculated intratracheally with MH (strain P-5722-3, 10(7) CCU). All pigs were euthanized and necropsied 28 days later, when maximum lesions of mycoplasmosis occurs. Pigs in group 1 did not cough and had no gross lung lesions, but were still viremic at necropsy. MH was isolated from all pigs in groups 2 (avg. log 5.2 +/- 1.3) and 3 (avg. log 5.1 +/- 1.5), but differences were not significant (P = 0.87). Similarly, there were no differences in average days coughing (8.9 +/- 2.8 v 11.2 +/- 4.5, P = 0.17), grossly pneumonic lung (16.5% v 17%, P = 0.91), or microscopic lung lesion scores (10.1 +/- 2.6 v 11.1 +/- 1.9, P = 0.35) between pigs in groups 2 and 3. Under these experimental conditions, PRRS virus infection did not increase the severity of experimental Mycoplasma hyopneumoniae infection in young pigs.