New influenza (H7N9) has high case fatality rate

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Friday, 5 April, 2013

The new strain of influenza (H7N9) that emerged in China in March has killed 5 of the 11 confirmed cases, a case fatality rate of 45%. This is of major concern. The case fatality rate of the current pandemic of highly pathogenic avian influenza (HPAI) H5N1 is higher (often above 60%). However, transmission of H5N1 to humans is low owing to lack of suitable receptors in the upper respiratory tract of humans, and transmission usually requires close exposure to an ill bird or very rarely a person infected with H5N1.

Although birds are suspected to be the source of this strain of influenza A, there is no strong association with contact with birds for the human cases. However, there is a report that the National Avian Influenza Reference Laboratory has identified H7N9 avian influenza virus in specimens from a dove [pigeon] at the Shanghai Huai agricultural products wholesale market in Songjiang District. The genetic sequence analysis showed that the strains of low pathogenic avian influenza (LPAI) virus isolates were highly homologous with the H7N9 avian flu virus (ProMed 5 April 2013).

Cases have come from Shanghai, Anhui province, Jiangsu province and Zhejiang province. The widespread distribution of these cases is of concern. A map of cases (see above) is available from Pandemic Information News.

A HPAI outbreak involving H7N7 occurred in free-range poultry in NSW last year (see THS news item from 17 Nov 2012 for details). No humans became ill. This outbreak was eradicated.

For humans an influenza strain that is LPAI in birds, but highly pathogenic for humans, is arguably highly dangerous as birds will be aclinical reservoirs and the strain may spread widely in flocks undetected. The three conditions required for a pandemic of influenza are: 1) a strain with increased pathogenicity; 2) the strain transmits readily human to human; 3) the strain is new and the global population lacks immunity. The level of immunity to H7-9N7 strains in human populations is almost negligible. H9N7 already has met criteria #1 and #3. Its transmissibility human to human is a very important question.

WHO is making updates available through its Global Alert and Response site - http://www.who.int/csr/don/en/.

A FAQ page on influenza A H9N7 is available at http://www.who.int/influenza/human_animal_interface/faq_H7N9/en/index.html.

Posted by Rick Speare