The United Nations health agency is calling for additional studies on bird flu and other influenza viruses in pigs to assess the potential for the emergence of a human pandemic strain and determine what public health response would be needed.
The World Health Organization (WHO) has already sought details from China about the first-ever reported natural infection of pigs 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.
It is still unclear whether H5N1 has already become established in pig populations in China, according to WHO. Pigs were implicated in the emergence of previous human epidemics, including the so-called Spanish flu pandemic of 1918-20 with an estimated worldwide death toll of 20 million.
Considering the widespread nature of the current H5N1 outbreak in Asia and the ability of influenza viruses to jump the species barriers, it is inevitable the virus will be detected in some pigs, the agency said.
Pigs can be infected with both avian and human influenza A viruses. Human influenza H3N2 has already been detected in pigs in Asia, Europe and Africa. If human and bird flu viruses become adapted to pigs, co-circulation of bird, human, and pig viruses could lead to a genetic exchange of material that would potentially set off a human pandemic.
While cautioning that there is no known new pandemic strain, WHO warned that the probability is not negligible, depending on the duration of H5N1 circulation in pigs as well as the simultaneous presence of human and pig influenza A viruses.
As long as human and avian influenza viruses co-circulate, whether in humans or pigs, the possibility of an exchange of genetic material exists, the agency said, calling for laboratory experiments to shed light on the chance that pigs will act as a pathway for the emergence of a potential human pandemic strain.
These results will help national and international public health authorities not only to assess the role pigs and humans play in the emergence of a new influenza pandemic virus from H5N1, but to structure the necessary public health interventions, WHO added.