| Porcine reproductive and respiratory syndrome virus (PRRSV) infection commonly induces a high frequency of disease and mortality in suckling and nursery-age pigs. Many different secondary bacterial diseases have been reported in PRRSV-infected piglets. To confirm that PRRSV infection increases piglet's susceptibility to secondary bacterial infection, PRRSV-free sows were experimentally infected with PRRSV at 98-day gestation. Piglets from PRRSV-infected sows showed high frequency of lesions in lung, brain, lymph nodes, spleen, and thymus. During two identical experiments, twenty out of 22 in-utero PRRSV-infected piglets died within one week after challenge with Streptococcus suis type 2 on day 5 after birth, compared to one of 18 piglets infected with PRRSV only, and five of 23 piglets infected with Streptococcus suis type 2 only. These results suggest that late-term in-utero infection by PRRSV exacerbates the infection and disease caused by Streptococcus suis type 2. Abnormal changes of T cell subsets were observed in the peripheral blood lymphocytes of piglets infected in-utero with PRRSV. Piglets infected with PRRSV had a significant increase in peripheral blood CD8+ cells, and a decrease in CD4+ cells and CD4+:CD8 + cell ratios on 0, 7, and 14 days after birth. These pigs also had a significant increase in CD4+CD8+ cells. The thymuses of piglets infected with PRRSV were severely atrophied, which was characterized by cortical thymocyte depletion. The percentage of CD4 +CD8+ double positive thymocytes was significantly decreased in PRRSV-infected piglets at 0, 7, and 14 days of age. There were no significant differences in the percentage of CD4+CD8 -- and CD4--CD8+ single positive thymocytes between PRRSV-infected and non-infected piglets on the three time points, except an increase in CD4--CD8+ at birth. Analysis of cytokine mRNA production by peripheral blood mononuclear cells revealed that IL-6, IL-10, and IFNgamma mRNA expression was significantly increased in PRRSV-infected piglets at 0 and 14 days of age as compared to non-infected controls. There were no significant differences in IL-2, IL-4, and IL-12 mRNA expression between PRRSV-infected piglets and controls at 0, and 14 days of age. The IL-10/IL-12 ratio was significantly decreased in PRRSV-infected piglets at 0, and 14 days of age. No differences were observed in lymphocyte subsets, cytokine mRNA expression, or delayed type hypersensitivity response between PRRSV-infected and non-infected pigs at 9 weeks of age. These results suggest that certain immune responses of piglets infected in-utero with PRRSV are suppressed, resulting an increased susceptibility to secondary bacterial pathogens such as Streptococcus suis in the first two weeks of life. The immune functions seem to return to normal by 9 weeks of age. |
