Porcine reproductive and respiratory syndrome virus infection in neonatal pigs characterised by marked neurovirulence

Neonatal pigs from three herds of pigs were somnolent and inappetent and ha d microscopic lesions : characterised by severe meningoencephalitis, necrot ic interstitial pneumonia and gastric muscular. inflammation. Porcine repro ductive and respiratory syndrome virus (PRRSV) infection was diagnosed and confirmed by virus isolation, fluorescent antibody examination of frozen lu ng sections, serology,. immunohistochemistry and in situ hybridisation. Eac h herd had a history of PRRSV infection and was using or had used a modifie d-live vaccine. The isolates from the affected pigs were genetically distin ct from the modified-live vaccine strain of the virus when compared by rest riction enzyme analysis and nucleotide : sequencing of PRRSV open reading f rames 5 and a. The virus was identified in macrophages or microglia of brai n lesions by immunohistochemical staining of brain sections with an anti-PR RSV monoclonal antibody and an anti-macrophage antibody. The replication of the virus in the brain was verified by in situ hybridisation. The meningoe ncephalitis induced by the virus in pigs from each of the herds was unusual ly severe and the brain lesions were atypical when compared with other desc riptions of encephalitis induced by the virus, which should therefore be co nsidered as a possible diagnosis for neonatal pigs with severe meningoencep halitis. In addition, field isolates of the virus which are capable of caus ing disease can emerge and coexist with modified-live vaccine virus in some pig herds.