Comparative Efficacy of Chimeric Porcine Circovirus (PCV) Vaccines against Experimental Heterologous PCV2d Challenges

SIMPLE SUMMARY: Disease caused by infection with porcine circovirus type 2 (PCV2), collectively known as porcine circovirus-associated disease (PCVAD), is one of the most important viral infectious diseases in pigs. To date, PCV2 has been classified into at least 8 genotypes, namely PCV2a, PCV2b, PCV2c, PCV2d, PCV2e, PCV2f, PCV2g, and PCV2h. Among these, PCV2a, PCV2b, and PCV2d are the predominant genotypes that have chronologically circulated and affected the global pig population. Application of the PCV2 vaccine is a key strategy in the prevention and control of PCV2 infection. However, to the best of our knowledge, little is known about the benefits of using the chimeric PCV2a-2b antigen-based vaccine in Thailand in experimental challenges with field isolates of PCV2d. The present study has demonstrated that the chimeric PCV1-2a-based vaccine and the chimeric PCV1-2a-2b-based vaccine are effective against Thai PCV2d inoculation. The present study further strengthens the use of the PCV2 vaccine as an important tool for prevention of PCVAD in pigs. ABSTRACT: The objective of this study was to evaluate the efficacy of two multivalent commercial porcine circovirus (PCV) vaccines against heterologous PCV2d challenges. A total of 24 crossbred male pigs aged 26 days selected from a specific pathogen-free herd were randomly divided into four groups (six pigs per group) and assigned as follows: negative control (unvaccinated/sham-challenge), vaccinated with chimeric PCV1-2a vaccine (PCV1-2a/PCV2d-challenge), vaccinated with chimeric PCV1-2a-2b vaccine (PCV1-2a-2b/PCV2d-challenge) and positive control (unvaccinated/PCV2d-challenge). At 21 days after vaccination, the pigs were intranasally and intramuscularly inoculated with either sham or field isolates of PCV2d (PCV2d/149/TH/2020). After being challenged, blood samples were obtained weekly and analyzed for levels of PCV2d viremia, neutralizing antibodies, and IgG against PCV2. At 30 days post-challenge (DPC), the pigs were euthanized and then subjected to pathological evaluations and molecular analysis. The results indicated that pigs in the PCV1-2a-2b/PCV2d-challenge and the PCV1-2a/PCV2d-challenge groups possessed significantly greater levels of PCV2d-neutralizing antibody titer when compared with the positive control group. Moreover, pigs in the PCV1-2a-2b/PCV2d-challenge group exhibited a lower degree of severity in terms of gross lesion scores and lower levels of PCV2 viremia when compared with the positive control group. This study demonstrated that vaccinating pigs with either the PCV1-2a or PCV1-2a-2b chimeric vaccines elicits a potent immune response against PCV2d infection and reduces viremia after PCV2d inoculation in pigs.