鳥インフルエンザ(H5N1)ウイルスの疫学的考察

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1. Modes of transmission and the origin of H5N1 virusesAvian Influenza (A.I.) is transmitted by infected birds and their excrements. Also, AI is mechanically transmitted by surface means via contaminated food, water, feeds, soil, vehicles, humans, animals, flies, feathers etc. AI viruses can be spread by national or international trades of infected birds and contaminated products. Wild birds, especially migratory waterfowl, are a recognized source and reservoir for all subtypes of AI viruses. Some mammals such as dogs and cats are susceptible to the virus, but they are usually considered as the dead ends.In 1996, H5N1 virus was first detected in Guangdong Province, China. In 1997 the virus became widespread in poultry markets in Hong Kong, and killed 6 of 18 infected persons. The virus was wiped out by culling all domestic poultry in Hong Kong. In 2002, a new H5N1 genotype appeared again in Hong Kong, and the variant strains spread across Southeast Asia and South Asia between 2002 and 2007. The viruses can be divided into several clades such as V1, V2, V3 and Indonesian clades. The strains of H5N1 virus appeared in Korea (2003) and Japan (2004) were closely related to Guangdong strain/174/04 which is distinct from the abov 4 clades.In April 2005, a new variant H5N1 virus, which caused high mortality in both wild birds and poultry, was observed in Quinghai Lake, China. The virus was spread westward through migratory birds into Siberia, Kazakhstan and Turkey. This unprecedented mortality of wild birds associated with H5N1 viruses opened a new window for its movement within wild and domestic birds across Eurasia, the Near East and Africa. Virus strains are divided into 3 clades (EMA 1, 2, 3). The virus isolated in 2007 in Japan is closely related to one of those viruses of Quinghai origin (EMA clades).2. AI situation in EuropeThe European Union decided to make risk assessments of H5N1 virus entering via migratory birds into Europe, and active and passive surveillance for AI virus in wild birds started in July 2005. The conclusion of this study indicated a high risk of introducing the virus via migratory birds, and also a risk of the infection to become enzootic in Europe.The EU encouraged each member country (a) to make an extensive survey of AI viruses in both wild and domestic birds, (b) to vaccinate zoo birds and poultry that cannot be kept in houses (c) to keep all domestic birds in closed housing in high risk areas or zones and (d) to vaccinate domestic birds that cannot be housed.Between 2005 and 2006, H5N1 viruses were detected in wild birds in 25 countries. AI outbreaks in poultry farms were reported from 13 countries in Eastern Europe, and 4 countries in Western Europe (Sweden, Denmark, France and Germany). It is considered that migratory birds played a major role in spreading H5N1 viruses in Europe.The results of the risk control measures in Western Europe can be summarized as follows : (i) It was successful to protect the zoo birds by vaccination, but several birds died due to trauma of vaccination.(ii) Surveillance of wild birds was useful in improving early warning systems for poultry producers, and was effective in reducing the exposure risks of poultry.(iii) Mass culling of poultry and ornamental birds could be avoided.In 2007, H5N1 virus surfaced again in Hungary, UK, Czech R., Germany and France. It seems that H5N1 viruses became enzootic in some countries in Eastern Europe including Russia.(View PDF for the rest of the abstract.)
獣医疫学会の論文
2008-01-20

著者

小澤義博
国際獣疫事務局

鳥インフルエンザ(H5N1)ウイルスの疫学的考察

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