Evidence of an antigenic shift among Palyam serogroup orbiviruses.

The Japanese isolates of Palyam serogroup viruses isolated from 1985 to 2001 were investigated for the genome sequence of segments 2 and 7 and were phylogenetically analyzed in comparison with Australian and African isolates of the same serogroup. The nucleotide sequences of segment 7 were highly conserved within Japanese isolates (95.1 to 100%) and between Japanese and Taiwanese isolates (96.0 to 100%), whereas the identities between Japanese and Taiwanese isolates and Australian and African isolates were fairly conserved (84.2 to 92.0%). Phylogenetic analysis based on segment 7 revealed three clusters according to geographical origin. As a result of the nucleotide sequence analysis of segment 2, which encodes a serotype-specific antigen, Japanese isolates were classified into two groups by genome length and nucleotide identities. Four of the nine Japanese isolates were categorized into the same group as prototype strain K-47 of the Chuzan virus, and the remaining isolates were categorized into the same group as the D'Aguilar virus and Nyabira virus. Phylogenetic analysis based on segment 2 revealed two clusters, the cluster containing Chuzan virus and the cluster containing the D'Aguilar and Nyabira viruses. To examine the antigenic relationship among viruses categorized in different clusters, we conducted a cross-neutralization test. KSB-29/E/01, isolated in 2001 in Japan, was neutralized by antiserum not only to strain B8112 of D'Aguilar virus but also to Chuzan virus. These results indicated that genetically and antigenically unique characteristics of KSB-29/E/01 were attributed to genetic reassortment of segment 2 between Chuzan virus and D'Aguilar virus.