The National Agriculture and Food Research Organization (NARO) has classified the causative virus for 84 cases of high pathogenicity avian influenza (HPAI) that occurred in the domestic poultry facilities during the 2022 season (fall 2022 to spring 2023) into 3 groups and 17 genotypes. It was found that 14 of these genotypes contained genes derived from various avian influenza viruses (AIV) found in wild birds. While all 17 genotyped virus strains were highly lethal to chickens, the average number of days required for death varied depending on the strain, ranging from 2 to 6.2 days. Wild birds are strongly suggested to be involved in the emergence and spread of various viruses in the 2022 season. Therefore, we need to continue to closely monitor outbreak trends and be vigilant against the invasion of the virus into Japan and farms.
Overview
From October 28, 2022 to April 7, 2023, 84 cases of HPAI, which is the highest number of outbreaks ever, have occurred at domestic poultry facilities raising chickens, ducks etc. Approximately 17.71 million poultry were slaughtered to prevent the spread of the disease and it also had an impact on people's eating habits, such as the supply and demand of chicken eggs and the soaring prices.
NARO analyzed the hemagglutinin (HA) gene segment of 83 H5N1 and 1 H5N2 high pathogenicity avian influenza viruses (HPAIV) isolated in the 2022 season and found that they were classified into three groups. AIV are maintained through repeated infections in populations of wild waterfowl such as ducks and it is speculated that they invade the country during migration. Three groups of H5 subtype viruses, which are identical to those found in the viruses derived from poultry, were detected in the 242 cases of wild birds, wild bird feces, and environmental samples such as lake water collected from September 25, 2022 to April 20, 2023, and 155 cases have been analyzed so far.
Furthermore, when the genotypes of the viruses in poultry outbreaks were analyzed, they were classified into 17 types. Those of the 17 genotypes, 14 were found in which some gene segments were replaced by AIV derived from various wild birds in Japan and overseas. This suggests that these viruses may have emerged because of genetic reassortment caused by repeated infections in wild bird populations.
As a result of experimental infections to chickens by nasal inoculation, which is the natural infection route, with representative virus strains of 17 classified genotypes, it was revealed that there were differences in the infection fatality rate and the number of days from infection to death depending on the genotype. Furthermore, from the estimated amino acid sequence analysis of the viruses detected in poultry during the 2022 season, it is thought that there is a low possibility that these virus strains will directly infect humans and cause disease, or that antiviral drugs will be less effective.
It has been suggested that the viruses detected in poultry during the 2022 season may have become more diverse in their genotypes as a result of increased virus infection frequency in wild bird populations and genetic reassortment among wild birds. Wild birds play a major role in the emergence, maintenance, and propagation of various viruses, and as infections continue worldwide, we must continue to be even more vigilant against virus invasion into farms.
