Association between clinical signs and high serum titers of porcine reproductive and respiratory syndrome virus (PRRSV) in nursery pigs under field conditions

Summary Objective: To establish clinical categories to identify nursery pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV) and to quantify their PRRSV serum titers, in order to select the proper pigs for gilt acclimatization. Methods: Nursery pigs were studied during an acute outbreak of PRRS in a single herd. Four clinical categories were defined: healthy pigs (H), healthy pigs with superficial inguinal lymphadenopathy (HL), sick pigs (S), and sick pigs with superficial inguinal lymphadenopathy (SL). For each category, 40 pigs were selected from each of three different age groups (2, 4, and 6 weeks after weaning) and organized by clinical category. Blood was collected from each pig and tested by quantitative polymerase chain reaction assay for PRRSV and by ELISA for PRRSV antibodies. Results: Rectal temperatures and mean superficial lymph node widths were greatest at 4 weeks post weaning (PW), and ELISA titers were greater at 4 and 6 weeks PW than at 2 weeks PW. At 2 weeks PW, only one pig, classified SL, was viremic. At 4 and 6 weeks PW, 30 to 100% of pigs in each clinical category were viremic, but there was no significant association between occurrence of viremia and clinical category. The association between lymphadenopathy and PRRS viremia was not significant in pigs 4 weeks or 6 weeks PW. Implication: Nursery pigs with clinical signs of PRRS (fever, respiratory signs, lymphadenopathy) are potential sources of PRRSV for use in gilt acclimatization protocols; however, viremia cannot be predicted solely on the basis of these signs.