Phylogenetic and antigenic characterization of newly isolated porcine epidemic diarrhea viruses in Japan

To evaluate the mechanism by which a large outbreak of porcine epidemic diarrhea (PED) occurred in Japan, where the majority of sows are vaccinated, we isolated two new strains of PED virus (PEDV) from the intestines of piglets and found that they showed greater similarity to US isolates (group II PEDV) than to the Japanese vaccine strain (group I PEDV). We compared the antigenicity of the vaccine type strain and newly isolated strains by means of a neutralization test using sera from a number of pigs from various farms; the results revealed that they are antigenically similar. This is the first report of the similarity of group I and II viruses using sera from individual pigs vaccinated with group I virus. These data suggest that the large outbreak of PED in Japan cannot be attributed to inefficient vaccination but may be due to the extremely high virulence of the newly appearing viruses.