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H9N2 is a subtype of the species Influenza A virus (bird flu virus).〔(NAID NIH )〕 Over the years the H9N2 influenza strain caused illness in several children aged nine months to 5 years in Hong Kong with the latest occurring in December 2009.〔(Influenza A (H9N2) infection in a 5-year-old boy )〕〔(HK girl infected with rare but mild A/H9N2 flu virus )〕 ==Infection in birds== This the most common subtype of influenza viruses in chickens in China. H9N2 also causes a great economic loss for the poultry industry, even under the long-term vaccination programs. And the recent human infection with avian influenza virus revealed that H9N2 influenza virus is the gene donor for H7N9 and H10N8 viruses that are infecting humans nowadays. The crucial role of H9N2 viruses at the animal-human interface is due to the wide host range, adaptation in both poultry and mammals and extensive gene reassortment. In China, which is regarded as an epicenter of avian influenza viruses, the H9N2 virus has been detected in multiple avian species, including chicken, duck, quail, pheasant, partridge, pigeon, silky chicken, chukar and egret. Epidemiological and genetic studies revealed that the hemagglutinin (HA) gene of the H9N2 influenza viruses could be divided into Eurasian avian and American avian lineages. The Eurasian avian lineage involved three distinct lineages, including A/chicken/Beijing/1/94-like (BJ/94-like), A/quail/Hong Kong/G1/97-like (G1-like), and A/duck/Hong Kong/Y439/97 (Y439-like). 〔 (H9N2 influenza virus in China: a cause of concern ) 〕 ==Transmission from chicken to human== The H9N2 influenza virus can be transmitted by air droplet, dust, feed, or water. Chickens usually seemed to be healthy after the infection but some of them do show depression and ruffled feathers. The virus replicates itself in tracheas. It makes chickens more susceptible to secondary infections, especially Escherichia coli infections with a mortality rate of at least 10%. Also, the trachea or bronchi are easily embolized by mucus when the ventilation is poor, leading to severe respiratory disease and death. 〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Influenza A virus subtype H9N2」の詳細全文を読む スポンサード リンク
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