Avian influenza: H5N1 detected in pigs in China

20 August 2004

A researcher from China’s Harbin Veterinary Research Institute has today presented initial evidence that pigs from farms in parts of China have been infected with the H5N1 strain of avian influenza. The findings, set out in a table and without further supporting data, were presented today at an international symposium on SARS and avian influenza held in Beijing.

WHO has requested confirmation and further details about this study.

Pigs are known to be susceptible to infection with avian influenza viruses. However, natural infection of pigs with the H5N1 strain has not been previously reported.

In order to assess the implications for human health, it is important to know whether the reported infections in pigs are rare events, possibly caused by contact between pigs and wild birds. Wild aquatic birds, which are the natural reservoir of all influenza A viruses, can carry the H5N1 strain without developing symptoms, and are known to excrete large quantities of the virus in their faeces.

A comparison of the H5N1 strain isolated in pigs with strains recently circulating in poultry populations in parts of Asia is needed to determine whether the virus is being passed directly from poultry to pigs. Evidence of direct transmission of H5N1 from poultry to large numbers of pigs would be of particular concern, as this would increase opportunities for a new influenza virus with pandemic potential to emerge.

Pigs have been implicated in the emergence of new influenza viruses responsible for two of the previous century’s influenza pandemics. 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.

Confirmation of H5N1 infection in pigs would add complexity to the epidemiology of this disease, but needs to be viewed in perspective. During the peak of the poultry outbreak of H5N1 in Viet Nam earlier this year, extensive testing of pigs on farms where poultry were heavily infected failed to find evidence of infection in pigs. In addition, Hong Kong authorities regularly perform random testing for the H5 avian influenza virus subtype in pigs imported from mainland China. No infection in pigs has been detected to date.

WHO, in collaboration with FAO and OIE, will be assessing the implications of reported H5N1 infection in pigs as further details become available.