Molecular and Epidemiological Analyses of Human Adenovirus Type 7 Strains Isolated from the 1995 Nationwide Outbreak in Japan

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Abstract
The adenovirus type 7 (Ad7) isolates from the 1995 nationwide outbreak in Japan were genetically and seroepidemiologically analyzed in comparison with Japanese Ad7 strains isolated before 1995 to determine their genome type and to speculate on their origin and causative factors of the outbreak. Twenty-six Ad7 isolates from the outbreak were identified by restriction enzyme analysis as the Ad7d2 genome type, while 22 Ad7 strains sporadically isolated in Japan before 1995 were identified as Ad7d. Partial nucleotide sequencing of the E3 region of Ad7d2 revealed a nucleotide substitution of G to A at position 265, resulting in the absence of the Bst EII site and making Ad7d2 distinct from Ad7d. In Hiroshima City, Japan, no Ad7 was isolated from 1982 to 1994, but 43 and 50 Ad7 strains were isolated in 1995 and 1996, respectively. A seroepidemiological study of 251 serum samples collected in 1989 in Hiroshima City showed that only 2.8% of the samples were positive for Ad7. These results indicate that the 1995 outbreak of Ad7 in Japan was caused by the Ad7d2 genome type, which might have been introduced from outside Japan. The results also suggest that the low mass immunity in Japan was critical for the outbreak and that the mutation in the E3 region in Ad7d2 may have influenced transmission.