Thesis (M.Sc.)--Chulalongkorn University, 2015

To evaluate the efficacy of a new PRRS MLV of lineage 8 against a Thai HP-PRRSV (10PL01), eighty-one, PRRSV-free piglets were obtained from a PRRS-free herd and were divided into two experiments with the major difference of infection timing after vaccination, 42 days in experiment 1 (n= 42) and 28 days in experiment 2 (n= 39). Each experiment had similar protocol containing three groups including a negative control, unvaccinated challenged and vaccinated challenged groups. Pigs in vaccination groups were immunized with A PRRS MLV of lineage 8 at 3 weeks of age. Then, unvaccinated challenged and vaccinated challenged groups were intranasally inoculated with a Thai HP-PRRSV (10PL01). Vaccinated challenged pigs showed significantly lower levels of mean rectal temperatures, clinical severity, lung lesion scores, and viral titers in serum and lung tissue compared to the unvaccinated challenged pigs (p < 0.05). Vaccinated challenged pigs had higher survival rate than those of unvaccinated challenged pigs in both experiments. It should be noted that pigs challenged 42 days after vaccination showed a better performance than pigs challenged 28 days after vaccination. In conclusion, PRRS MLV of lineage 8 was able to improve the survival rate against the Thai HP-PRRSV infection in both 42 days and 28 days vaccination-to-infection protocols.