UN health agency seeks details on potentially ominous bird flu infection in pigs

The United Nations health agency is seeking further details from China about the first-ever reported natural infection of pigs with a highly infectious strain of bird flu, a development that could be of particular concern as it would increase opportunities for the emergence of a new human influenza virus with pandemic potential.

The World Health Organization (WHO) request follows the presentation of initial evidence by a researcher from China’s Harbin Veterinary Research Institute that pigs from farms in parts of China have been infected with the highly pathogenic avian influenza (H5N1), the strain that has already killed some two dozen people in Asia this year and resulted in the deaths or culling of more than 100 million birds.

Confirmation of H5N1 infection in pigs would be especially worrying since they have been implicated in the emergence of new influenza viruses responsible for two of the previous century’s influenza pandemics, including the so-called Spanish flu pandemic of 1918-20 which is estimated to have killed some 20 million people worldwide. Pigs are known to be susceptible to infection with avian influenza viruses but natural infection with H5N1 has not been previously reported.

Pigs have receptors in their respiratory tract that make them susceptible to infection with human and avian influenza viruses. If a pig is simultaneously infected with both a human and an avian influenza virus, it can serve as a “mixing vessel,” facilitating the exchange of genetic material between the two viruses in a process known as “reassortment.” The resulting new virus, which will not be recognized by the human immune system, will have pandemic potential if it retains sufficient human genes to allow efficient human-to-human transmission, and if it causes severe disease in humans.

Thus confirmation of such H5N1 infection would add complexity to the disease’s epidemiology but needs to be viewed in perspective, WHO cautioned. During the peak of the H5N1 poultry outbreak in Viet Nam earlier this year, extensive testing of pigs on farms where poultry were heavily infected failed to find evidence of pig infection. Moreover, Hong Kong authorities regularly perform random testing for the virus subtype in pigs imported from mainland China. No such infection has been detected to date.

The latest development comes as a WHO research team is at work in Viet Nam where a fresh bird flu outbreak killed three more people in July and August.