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, edema in horses, fatalities, and pregnancy disorders in pigs. It has caused occasional outbreaks in horse populations in Japan, China, and India, and the endemic areas are gradually expanding, particularly in Asia and Oceania. In this study, we isolated a new GETV strain from Culex tritaeniorhynchus mosquitoes collected in Nagasaki Prefecture, Japan, in 2022. Phylogenetic analysis demonstrated that this new strain, 22IH8, was 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 similar to those in previous Chinese strains. These results indicate that strain 22IH8 may have recently invaded the Japanese archipelago from mainland Asia. The antiserum against the current vaccine strain MI-110 demonstrated high neutralization activity against the strain 22IH8, indicating the efficiency of the current vaccine for horses in Japan.