Evaluation of Type 1 Porcine Reproductive and Respiratory Syndrome Virus Vaccines based on Clinical, Virological, Immunological and Pathological Analyses

Abstract

The Porcine Reproductive and Respiratory Syndrome Virus was first recognized in the early 1900s as a cause of reproductive losses in sow and severe respiratory disorders in growing pigs. PRRSV is recognized by 2 genotypes: Type 1 (European genotype) and Type 2 (North American genotype). Although these two PRRSV genotypes produce the same disease symptoms, they are significantly different antigens. Until late 2000s, Type 2 PRRSV was the main genotype in Korea. After the first detection of Type 1 PRRSV in 2005, the proportion of Type 1 PRRSV has continuously increased. Until 2013, Type 2 PRRSV vaccines were the only vaccines available in Korea. However, the modified live virus (MLV) vaccines for Type 1 PRRSV became available in 2014. Therefore, in order to evaluate the efficacies of commercial vaccines, it is reasonable to measure the cell-mediated immunity, humoral immunity, pathological and clinical responses against these two types of PRRSV. A study was performed to compare the efficacies of two commercial Type 1 PRRSV MLV vaccines against heterologous Type 1 and Type 2 PRRSV challenge in growing pigs and boars. For the study of growing pigs, a total of 112 pigs were randomly divided into 7 groups

4 vaccinated and challenged groups, 2 non-vaccinated and challenged groups, and a negative control group. According to this study, commercial Type 1 PRRSV MLV vaccines could reduce the level of viremia against Type 1 PRRSV, but it could not reduce the level of viremia against Type 2 PRRSV. Low level of IL-10 and high level of Type 1 PRRSV-specific IFN-γ-SC were both detected within Type 1 PRRSV challenged groups, whereas high level of IL-10 and low number of Type 2 PRRSV-specific IFN-γ-SC were detected in vaccinated pigs within Type 2 PRRSV challenged groups. Type 1 PRRSV vaccines effectively reduced the lung lesions and Type 1 PRRSV nucleic acids in Type 1 PRRSV challenged groups. However, both the lung lesions and Type 2 PRRSV nucleic acids did not reduce in Type 2 PRRSV challenged groups. Accordingly, the Type 1 PRRSV commercial vaccines can provide partial protections against Type 1 PRRSV challenge but cannot provide effective protection against heterologous Type 2 PRRSV challenge. For the study of boars, a total of 35 purebred Landrace boars were randomly divided into 7 groups

4 vaccinated and challenged groups, 2 non-vaccinated and challenged groups, and a negative control group. In this study, the clinical symptoms were reduced by Type 1 PRRSV MLV vaccines after challenging with Type 1 and Type 2 PRRSV. Seminal shedding of PRRSV was independent of viremia. The reduction of Type 1 PRRSV seminal shedding coincided with the appearance of Type 1 PRRSV-specific IFN-γ-SC in Type 1 PRRSV-challenged groups. The frequencies of Type 1 PRRSV-specific IFN-γ-SC induced by Type 1 PRRSV vaccine were relatively high compared to Type 2 PRRSV-specific IFN-γ-SC induced by the same vaccine. The PRRSV MLV vaccine can provide a more effective protection against the same genotype than a different genotype in terms of seminal shedding of PRRSV in boars.