Maternal antibodies to Bartha-K61 vaccine did not protect newborn piglets from challenge with novel recombinant pseudorabies virus.

The Bartha-K61 vaccine greatly helped to control or eliminate Pseudorabies virus (PRV) from pig populations in many countries and has been widely used worldwide to this day. Since the emergence of PRV variants in 2011, clinical production data suggest that controlling or eliminating PRV has become increasingly difficult. In this study, the PRV variant CH/JLHF/2022 was isolated from a farm in Jilin Province, China, where Bartha-K61 was administered in 2022. This strain is a recombinant strain composed of one major parent and four minor parents, belonging to the GII subtype, with a genome-wide homology of 90.7-98 % with other strains. Compared with the existing strains, new amino acid mutations were discovered for the first time in four important proteins, altering the corresponding protein structures. It is worth noting that a mutation in one of the proteins altered the epitope. In addition, serum neutralization experiments confirmed that CH/JLHF/2022 could evade antibodies produced by the Bartha-K61 vaccine, while the PRV variants were subjected to immune pressure. In vivo experiments showed that after inoculation of expectant sows with Bartha-K61 vaccine, newborn piglets were protected by the vaccine in the case of infection with the conventional variant. However, virus replication and excretion were observed in piglets infected with CH/JLHF/2022, and it damages the colonic intestinal barrier and reduces the length and density of jejunal villi. In conclusion, the newly emerged PRV variants have evolved from incomplete vaccine protection to a 100 % mortality rate in newborn piglets. This study provides new ideas for the evolution of PRV to evade immunity.