Antigen distribution of porcine reproductive and respiratory syndrome virus (PRRSV) in Thai crossbred pigs using immunohistochemistry

Twenty, 3-week-old, crossbred pigs from a herd free from porcine reproductive and respiratory syndrome virus (PRRSV) were inoculated intranasally and intramusculary (1 ml each 10**(2.5) TCID sub(50)/ml) with Thai PRRSV isolates of the US genotype (0NP1) (n=9) or the EU genotype (02SB3) (n=8). Three pigs were used as negative controls. Three infected and one control group pig were euthanatized 5, 9 or 15 days postinoculation (DPI) to detect and to compare the location and the amount of PRRSV-antigen present. Immunohistochemical evaluation of formalin-fixed tissues revealed PRRSV-antigen in the pulmonary macrophages of the lungs (6/9, 8/8 and 0/3 of the 01NP1-infected, 02SB3-infected and control pigs, respectively). Macrophages staining for PRRSV-antigen in the spleen were seen in 1/9, 4/8 and 0/3 of the 01NP1-infected, 02SB3-infected and control groups. Other tissues and cells in which PRRSV-antigen was detected included follicular macrophages in the lymph nodes, macrophages in the liver, Peyeris patches, kidney, tonsil, turbinate bones, and heart. No PRRSV-antigen was detected in the brain of any PRRSV-inoculated pigs. Significantly, more PRRSV-antigen was detected in the spleen (P LT 0.05) of the 02SB3-infected pigs than in the spleen of 01NP1-infected pigs. The results suggested that PRRSV-antigen of the EU genotype was seen in a greater proportion of tissues despite showing less clinical signs. It could be that the EU genotype is well adapted to the pigs and may persist in the pigs for a longer period of time.