From mouse to pig: Is PRV vaccine safe across two species?

•Bartha-K61 PRV vaccine was proved to be highly pathogenic to mice by propagation in vivo for five continuous passages.•There was no safety concern of Bartha-K61 PRV vaccine on pigs after the vaccine virus was passaged in mice for 5 times.•The safety of 5th mouse-derived PRV on pigs may suggest mice may play a minimal role in the derivation of PRV vaccine-like field viruses that are believed to cause disease in young pigs.

Pseudorabies (PR) is an economically important viral disease of pigs which can infect numerous species of mammals including rodents. Commercial PR vaccines have been widely used worldwide to control and eradicate this disease. However, some PRV vaccines such as Bartha-K61 were occasionally reported to be lethal to mice. Since mice are commonly found in pig farms, the safety issue of PRV live vaccine across different species was never addressed. In this study, PRV vaccine strain Bartha-K61 was in vivo propagated in mice for five passages. The mortality of mice ranged from 80%-100% at each passage of PRV infection. The fifth passage of PRV was used to infect piglets to test its virulence on this species. The infected piglets clinically behaved normally and survived by the end of study (terminated at 10 days post-infection). Histopathologically, there was infiltration of eosinophile granulocyte in tonsil and lung and no other changes were observed in other organs of infected pigs. Immunohistochemistry staining results showed that PRV antigen was only found in lung sample of one piglet. Therefore, the above results suggested there was no safety concern of Bartha-K61 PRV vaccine on pigs after the vaccine virus was passaged in mice for 5 times. The result of this study may suggest that mice may play a minimal role in the derivation of PRV vaccine-like field viruses that are believed to cause disease in young pigs.