Virus update: The World Health Organisation (WHO) is expecting clarification today from the Chinese ministry of health of a report that a deadly strain of avian influenza, H5N1, has been found among pigs for the first time.
If the report is confirmed, it brings ever closer the prospect of a new influenza virus to which humans would have no resistance.
"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," the WHO said.
Pigs have been implicated in the emergence of new influenza viruses responsible for two of the previous century's influenza pandemics. These included the great influenza pandemic of 1918-1919, when a new influenza virus spread around the world and killed 40 to 50 million people in several waves of infection over a two-year period.
Pigs are susceptible to infection with avian influenza viruses but, until now, there have been no recorded cases of pigs being infected with the highly pathogenic H5N1 virus. WHO said it had requested official confirmation and details of the study, which was presented on Friday at an international symposium in Beijing.
The meeting was also attended by Dr Karl Stohr, director of the WHO Global Influenza Programme. He said that, following the presentation, there was some confusion as to whether the data presented referred to 2003 alone or included findings from 2004.
The H5N1 virus has been detected in bird life in at least eight countries across Asia this year and killed 23 people in Thailand and Vietnam, who caught the disease directly from poultry. A concern in all these outbreaks has been to determine whether the virus has mutated or remains entirely of avian origin.
Public health experts say the extent of the threat can only be known once a comparison has been made between the H5N1 strain isolated in pigs with strains recently circulating in poultry populations. This will help determine if the virus is being passed directly from poultry to pigs.
"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' between the two viruses in a process known as 'reassortment'," the WHO said. "The resulting new virus, which will not be recognised 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."
WHO spokesman Dick Thompson said there were many unanswered questions about the Chinese report. Nonetheless, he said, "it would not be a big surprise if these reports are confirmed. We are advising countries to pay attention to their pandemic preparedness plans."