The Thai Journal of Veterinary Medicine


Archived paraffin-embedded lymphoid tissue samples from seventy five experimental PRRSV-infected pigs were examined for PRRSV antigen and IFN-γ positive cells, using immunohistochemistry. Lymphoid tissues (5 pigs/group) including spleen, tracheobronchial lymph nodes and tonsil came from negative control pigs (group 1), low virulence strain of PRRSV (RespPRRS/ReproTM, modified-live virus vaccine) infected pigs (group 2) and high virulence strain of PRRSV (VR-2385) infected pigs(group 3). They were necropsied at 3, 7,10, 14 or 28 days post-inoculation (DPI). According to the study, microscopic lesions in the lymphoid tissues from pigs infected with the high virulence strain had more severe lesions, characterized by severe lymphoid depletion and necrosis, than those-infected with the low virulence strain. The most severe lesions in the spleen and lymph nodes were observed 14 DPI, but the tonsil had the most severe lesions at 7 DPI. In the spleen, lymph nodes and tonsil, a significant increase in the number of PRRSV-positive cells (p<0.05) was seen in the high virulence-inoculated pigs, when compared to the control pigs at 14 DPI. IFN-γ antigen was found in both lymphocytes and macrophages. In the spleen, a significant increase of IFN-γ positive cells was found at both 14 and 28 DPI in the high virulence group, whereas in the lymph nodes and tonsil, a significant increase of those cells was seen at 14 DPI. The results suggested that the increase in the number of PRRSV-positive cells in the high virulence group, correlated well with the severity of the lymphoid tissue lesions and that IFN-γ might be inhibiting the replication of PRRSV in the lymphoid tissue. However, the increase in the number of IFN-γ positive cells did not correlate with the decrease in the number of PRRSV positive cells.

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