CHRONOLOGICAL IMMUNOHISTOCHEMICAL DETECTION AND LOCALIZATION OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS IN GNOTOBIOTIC PIGS

An immunogold-silver immunohistochemical technique was used to determi ne the chronological distribution and localization of porcine reproduc tive and respiratory syndrome virus (PRRSV) in experimentally infected gnotobiotic pigs. Thirty-two pigs were randomly allocated to infected (n = 24) or control (n = 8) groups. Pigs in infected groups were inoc ulated at 3 days of age by nasal instillation of PRRSV isolate ATCC VR -2332 (total dose = 10(2.64) TCID50), and control pigs were exposed in the same manner to uninfected cell culture supernatant. Three infecte d and one control pigs were euthanatized at 12 hours and at 1, 2, 3, 5 , 7, 14, and 21 days postexposure (DPE). Bronchiolar epithelial cells, arteriolar endothelial cells, monocytes, and interstitial, alveolar, and intravascular macrophages stained for PRRSV antigen at 12 hours po stexposure. Staining for PRRSV antigen in endothelial cells, monocytes , and alveolar, interstitial, and intravascular macrophages was most i ntense and widespread in lung sections from 14 and 21 DPE. In the hear t, macrophages in the interstitial and subendocardial spaces and endot helial cells in a few arterioles stained for PRRSV antigen at 14 and 2 1 DPE. Tonsillar macrophages and mucosal epithelium stained for PRRSV antigen at 12 hours postexposure and sporadically with less intensity in subsequent sampling periods. In the nasal turbinate, PRRSV antigen was identified in macrophages within the mucosal epithelium at 12 hour s postexposure and again at 14 and 21 DPE. There was focal staining fo r PRRSV antigen in the choroid plexus in one pig at 14 DPE. Based on t he results of this experiment, the pathogenesis of PRRSV infection in gnotobiotic pigs can be described as initial virus entry through nasal epithelial, tonsillar, and pulmonary macrophages, with viremia occurr ing by 12 hours postexposure followed by the development of pneumonia, myocarditis, encephalitis, rhinitis, vasculitis, and lymphoid necrosi s. Although PRRSV can infect macrophages in heart, tonsil, turbinate, and choroid plexus, pulmonary macrophages are predominantly and consis tently infected and are the predominant cells for virus replication in gnotobiotic pigs.