Antigenic and genetic analyses of avian influenza viruses isolated in Japan

H5 and H9 influenza virus isolates in Japan were genetically and antigenically characterized and the birds infected with the virus were pathologically examined. In 2005, H5N2 influenza viruses were isolated form layer chickens in Ibaraki Prefecture, Japan and the Ibaraki prefectural governor declared the end of the H5N2 outbreak in June, 2006, 1 year after the first detection. Sixteen H5N2 viruses were isolated from 9 farms of the affected 41 farms. Phylogenetic and antigenic analysis of the isolates showed that these isolates were closely related to the H5N2 strains prevalent in Central America that have been circulating since 1994. Experimental infection of chickens with the index isolate (A/chicken/Ibaraki/1/2005 (H5N2)) demonstrated that this virus replicated efficiently in the respiratory tract without clinical signs. Pathological findings of the chickens indicated that the virus efficiently replicated in salivary epithelial cells. The virus was transmitted among the chickens in separated cage. This result indicated that the infection to chickens spread by droplet transmission in the outbreaks. In 2008, H5N1 highly pathogenic avian influenza virus was isolated from whooper swans (Cygnus cygnus) found dead in Hokkaido, Japan. Pathological findings indicate that the swan died due to severe congestive edema in the lungs. Phylogenetic analysis of the HA genes of the isolates revealed that these are the progeny viruses of the isolates Clade 2.3.2 from poultry and wild birds in China, Russia, Korea, and Hong Kong. Antigenic analyses indicated that the viruses are different from the H5N1 viruses isolated from wild birds and poultry before 2007. The chickens vaccinated with A/duck/Hokkaido/Vac-1/2004 (H5N1) survived 14 days after challenge with the isolate, although small amount of the challenge virus was recovered from the tissues of the birds. These findings indicate that the H5N1 highly pathogenic avian influenza virus are circulating in wild birds in addition to domestic poultry in Asia and showing antigenic variation that may be due to vaccination. Recently, H9N2 influenza viruses were isolated from poultry and mammals in Asian countries. The hemagglutinins (HAs) of H9 influenza viruses isolated from birds and mammals of different species were antigenically and genetically analyzed. Based on the reactivity patterns with the panel of 8 monoclonal antibodies, 21 H9N2 virus strains isolated from birds and mammals were divided into 7 antigenically distinct groups. The present findings indicate that H9N2 viruses are genetically and antigenically varied. The vaccinations to poultry in some countries resulted in antigenic variation of H5 and H9 influenza viruses isolated in Asia. These result indicate that the stamping-out policy is most important to control for avian influenza.