Effect of vaccination with a modified-live porcine reproductive and respiratory syndrome virus vaccine on dynamics of homologous viral infection in pigs.

OBJECTIVE: To determine effects of vaccination protocols with modified-live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine on persistence and transmission of virus in pigs infected with a homologous isolate and determine clinical and virologic responses following heterologous viral challenge. ANIMALS: Four hundred forty 6- to 8-week-old PRRSV-naïve pigs. PROCEDURES: Pigs were allocated into 5 groups. Groups A to D were inoculated with wild-type PRRSV VR2332. Group A (positive control pigs) received PRRSV only. Groups B, C, and D received modified-live PRRSV vaccine (1, 2, or 3 doses). Group E served as a negative control group. To evaluate viral transmission, sentinel pigs were introduced into each group at intervals from 37 to 67, 67 to 97, and 97 to 127 days postinoculation (DPI). To evaluate persistence, pigs were euthanized at 37, 67, 97, or 127 DPI. To assess clinical and virologic response after challenge, selected pigs from each group were inoculated at 98 DPI with a heterologous isolate (PRRSV MN-184). RESULTS: Mass vaccination significantly reduced the number of persistently infected pigs at 127 DPI. Vaccination did not eliminate wild-type PRRSV; administration of 2 or 3 doses of modified-live virus vaccine reduced viral shedding after 97 DPI. Previous exposure to wild-type and vaccine virus reduced clinical signs and enhanced growth following heterologous challenge but did not prevent infection. CONCLUSIONS AND CLINICAL RELEVANCE: Findings suggest that therapeutic vaccination may help to reduce economic losses of PRRSV caused by infection; further studies to define the role of modified-live virus vaccines in control-eradication programs are needed.

Beta-glucan enhancement of T cell IFNgamma response in swine.

Beta-glucan has been shown to enhance anti-tumor and anti-infection functions in animals. Pigs at 4 months of age were infected with porcine reproductive and respiratory syndrome virus (PRRSV), and peripheral blood monocytes (PBMC) were isolated for the detection of interferon gamma (IFNgamma)-producing cells. We found that soluble high molecular weight beta-glucan could increase IFNgamma-producing cell frequency in a dose-dependent manner in the enzyme-linked immunospot assay (ELISPOT) in the absence of antigenic restimulation. A concentration as low as 1.6 microg/ml gave a significant increase and a similarly high enhancement was achieved at concentrations from 3.2 to 100 microg/ml. In PRRSV-specific IFNgamma ELISPOT, soluble beta-glucan elicited increased PRRSV-specific responses at concentrations from 3.2 to 50 microg/ml, but not at 100 microg/ml, whereas insoluble beta-glucan had no effect. Soluble beta-glucan augmented the porcine cellular immune response in an antigen-independent fashion, whereas insoluble beta-glucan had no activity. This finding suggests that soluble beta-glucan may enhance innate antiviral immunity against PRRSV.