The first HPAIV outbreak of the season: Assessment of the pathogenicity of the strain isolated in Autumn 2020 in Japan

Updated:January 12, 2021 (Tuesday)

-The characteristic feature of the Kagawa 2020 strain-

The National Institute of Animal Health, NARO (NIAH) had analyzed the pathogenicity of the influenza A virus H5N8 subtype that caused the highly pathogenic avian influenza (HPAI) outbreak in a poultry barn in Kagawa prefecture, located in western Japan. The results of the intravenous inoculation tests to chicken veins and the intranasal inoculation tests showed high mortality rates of the infected chickens but the survival time of these chickens tend to be longer than the other strains from the past. To notice the outbreak at the early stage, NIAH suggests farmers pay attention to the mortality increase within their poultry barns.


On 5 November 2020, an HPAI outbreak was observed in a poultry barn in Kagawa Prefecture, Japan. After examination, the influenza A H5N8 subtype was isolated from the dead chickens from the poultry barn (Kagawa 2020 strain). The further investigation conducted by NIAH proved the isolated strain having a similar amino acid sequence in hemagglutinin (HA) of highly pathogenic avian influenza virus (HPAIV). According to the World Organization for Animal Health (OIE) criteria, Kagawa 2020 strain was defined to be HPAIV. The result of the intravenous inoculation tests to chicken veins also supported this assessment. After the intravenous inoculation tests, the infected chickens showed 100 % mortality. This is above the HPAIV assessment standard set by OIE, a minimum of 75 % mortality.

As a result of the investigation, a characteristic feature of the Kagawa 2020 strain was discovered. The survival time of the infected chickens tend to be longer compared to the past HPAIV strains: HPAIV H5 subtype in 2004 (Yamaguchi 2004 strain) and in 2018 (Kagawa 2018 strain). NIAH conducted intranasal inoculation tests on four groups of chickens to estimate the necessary amount of the viruses to kill its hosts. There were five chickens per group, and the groups had been inoculated with a fluid of different viral titer: 102, 104, 105, and 106 of the 50 % Egg Infectious Dose (EID50) of Kagawa 2020 strain. In the group infected with 106 EID50, all the chickens were dead within six days. In the group infected with 105 EID50, four out of five chickens were dead within seven days. Whereas all the chickens in the other two groups infected with 102 and 104 EID50 had survived until the end of the 14-day-observation. Depression was observed in the infected chickens, and no other significant symptoms were noticed.

NIAH continues to investigate the virus excretion dynamics of the infected chickens.

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Reference Information

The chickens inoculated with 106 EID50 of Kagawa 2020 survived significantly longer than the other strains isolated in Japan in the past.