Genetic, phylogenetic, and serological analysis of a Getah virus strain isolated from Culex tritaeniorhynchus mosquitoes in Nagasaki, Japan in 2022

Getah virus (GETV), belonging to the genus Alphavirus within the family Togaviridae, is a mosquito-borne virus that causes fever, rash, and edema in horses and fatalities and pregnancy disorders in pigs. It has caused occasional outbreaks in horse populations in Japan, China, and India, and endemic areas are gradually expanding, particularly in Asia and Oceania. In this study, we isolated a new GETV strain from Culex tritaeniorhynchus mosquitoes collected from Nagasaki Prefecture, Japan in 2022. Phylogenetic analysis revealed that this new strain, 22IH8, is more closely related to previous Chinese strains than to the strains prevalent in Japan in the 2010s. Furthermore, some amino acid substitutions in the viral proteins of strain 22IH8 were found to be common with those in previous Chinese strains. These results suggest that 22IH8 strain may have recently invaded the Japanese archipelago from mainland Asia. The antiserum against the current vaccine strain, MI-110, showed high neutralization activity against the 22IH8 strain indicating the efficiency of the current vaccine for horses in Japan.