Evaluation of the efficacy of a trivalent vaccine mixture against a triple challenge with Mycoplasma hyopneumoniae, PCV2, and PRRSV and the efficacy comparison of the respective monovalent vaccines against a single challenge

Oh, Taehwan; Park, Kee Hwan; Yang, Siyeon; Jeong, Jiwoon; Kang, Ikjae; Park, Changhoon; Chae, Chanhee

We have assessed the efficacy of a trivalent vaccine mixture against M. hyopneumoniae, PCV2, and PRRSV and compared it to the respective monovalent vaccinated and unvaccinated positive control groups. The trivalent vaccine mixture was able to reduce clinical signs, lung lesions, PRRSV and PCV2 viremia and improve weight gain compared to the unvaccinated positive control groups. The trivalent vaccine mixture, however, did not result in equal protection when compared against each respective monovalent vaccine, with the largest vaccine occurring within PRRSV. Although the PRRS vaccine in this study is widely used, an efficacious, commercially available PRRSV vaccine continues to be a global challenge, as PRRSV variant strains are continuously emerging from highly pathogenic outbreaks (particularly in Asia). Pigs vaccinated with the trivalent vaccine following a triple challenge significantly improved their growth performance when compared to the unvaccinated control pigs. In contrast, growth performance was better but not significantly different between M. hyopneumoniae- vaccinated and control pigs following M. hyopneumoniae challenge. These results are consistent with previous findings, where no significant difference on growth performance was observed between M. hyopneumoniae-vaccinated and unvaccinated pigs following an M. hyopneumoniae challenge [3, 4] . No significant difference on growth performance was also observed between PRRS-vaccinated and control pigs [5] . The possible reason for the lack of statistical significance could be that the growth performance of the pigs vaccinated with the monovalent vaccine was only evaluated for 6 weeks after a single challenge. In addition, growth retardation by a single challenge is likely not very severe. Therefore, the improvement on growth performance by the monovalent vaccines is not as drastic. A triple challenge is typically more severe than a single challenge. In addition, in a field study, infection with M. hyopneumoniae, PRRSV, and PCV2 increased the chance for opportunistic secondary bacterial infections which could result in further growth retardation. However, we did not observe a decrease in growth performance due to secondary bacterial infection in this experimental study. Taken together the results suggest that growth performance is an important parameter in PRDC cannot be controlled without controlling M. hyopneumoniae because M. hyopneumoniae infections exacerbate lung lesions caused by PRRSV and PCV2 in infected pigs [6, 7] . In addition, previous work has shown that an M. hyopneumoniae vaccine can reduce interstitial pneumonia caused by PRRSV [8] . Therefore, control of M. hyopneumoniae is the first step to control PRDC caused by the three challenge pathogens used in this study. The trivalent vaccine mixture and monovalent M. hyopneumoniae vaccine were able to elicit similar numbers of M. hyopneumoniae-specific IFN-γ-SC in vaccinated pigs. This is important since, the cellmediated immunity as measured by IFN-γ-SC has been shown to play an important role in controlling M. hyopneumoniae infection [9, 10] . Induction of cell-mediated immunity is also associated with a significant reduction in the amount of M. hyopneumoniae nasal shedding [11] . Vaccination with the trivalent product resulted in a comparable reduction of M. hyopneumoniae nasal shedding and lung lesions to that of the respective M. hyopneumoniae monovalent.