COMPARISON OF THE ANTIGEN DISTRIBUTION OF 2 US PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS ISOLATES WITH THAT OF THE LELYSTAD-VIRUS

One hundred 4-week-old cesarean-derived colostrum-deprived pigs were i noculated with one of two different US porcine reproductive and respir atory syndrome virus (PRRSV) isolates (VR2385, VR2431) or the European Lelystad virus to detect and compare the location and amount of virus antigen. Interstitial pneumonia, myocarditis, lymphadenopathy, and en cephalitis were consistently seen in all three groups; however, diseas e and lesions were more severe in the VR2385 group. Immunohistochemica l evaluation of formalin-fixed tissues revealed virus antigen in alveo lar macrophages in lungs of 22/25, 14/25, 14/25, and 0/25 of the VR238 5, VR2431, Lelystad, and control pigs, respectively. Follicular macrop hages and dendritic cells in the lymph nodes of 14/25, 10/25, 10/25, a nd 0/25 pigs from the VR2385, VR2431, Lelystad, and control groups, re spectively, stained positive for virus antigen. Similar cells in the t onsils from 25/25, 21/25, 23/25, and 0/25 pigs from the VR2385, VR2431 , Lelystad, and control groups, respectively, stained positive for vir us antigen. Other tissues and cells in which virus antigen was detecte d included macrophages and endothelial cells in the heart, macrophages , and interdigitating cells in the thymus, macrophages and dendritic c ells in the spleen and Peyer's patches, and macrophages in hepatic sin usoids, renal medullary interstitium, and adrenal gland. PRRSV persist ed in macrophages in the lung, tonsil, lymph node, and spleen for at l east 28 days. Significantly more PRRSV antigen was detected in the lun g (P < 0.01), lymph nodes (P less than or equal to 0.05), and tonsils (P < 0.05) of the VR2385 pigs than was detected in the same tissues of the VR2431 and Lelystad pigs. The cell types in which PRRSV antigen w as detected and the distribution of PRRSV antigen-positive cells withi n particular tissues and organs were generally similar for the differe nt virus inoculation groups despite differences in virulence of the is olates.