Induction of dual infections in newborn and three- Week-old pigs by use of two plaque size variants of porcine reproductive and respiratory syndrome virus

Objective - To determine whether pigs can be dually infected with different plaque variants of porcine reproductive and respiratory syndrome virus (PRRSV). Animals - 2 pregnant sows, 20 newborn pigs, and 20 three-week-old pigs. Procedure - The 2 late-term pregnant sows were inoculated with the PRRSV small-plaque variant MN-Hs, and their pigs were challenge exposed at 7 days of age with the PRRSV large-plaque variant MN-HI. In addition, twelve 3-week-old pigs were inoculated with MN-Hs virus. Two groups of the pigs were challenge exposed with MN-HI virus at 14 and 42 days after initial inoculation. Virus was isolated from the pigs at various intervals, plaque sizes were examined, and serologie testing was performed. Results - From the 2 groups of ten 7-day-old pigs, small-plaque PRRSV was isolated from 8 and 10 pigs. After subsequent challenge exposure, dual infections were diagnosed in 3 of 10 inoculated and 4 of 10 contact control pigs. In 3-week-old pigs infected with MN-Hs virus, dual infections were documented in 2 of 6 pigs when challenge exposed 14 days after initial infection. Virus was not isolated from sera of another 6 pigs challenge exposed 42 days after initial infection. Serum neutralization antibody was detected in all 6 pigs at challenge exposure. Conclusions - Dual infections were documented in viremic newborn pigs and 3-week-old pigs, using 2 plaque variants of PRRSV.