Indonesian scientists have found the H5N1 bird flu virus in a pig. The strain has infected poultry across east Asia, and killed at least 51 people so far.
Scientists fear pigs could act as a “mixing vessel” in which a human pandemic strain could evolve, because the animals can harbour both human and avian flu viruses.
But while suspected, such hybridisation has never been proven. Furthermore, New Scientist has learned of preliminary results from scientists in the US that suggest pigs might not be able to transmit H5N1 flu to one another.
Java was the worst-affected part of Indonesia during 2004’s H5N1 bird flu outbreak. But by slaughtering and vaccinating poultry, Indonesia stopped the outbreak in October – with no reported human cases. But in April 2005, bird flu struck poultry throughout Java once again, mainly in village-based “backyard” flocks.
Indonesia offers free vaccination for such flocks. But vaccinated birds can still harbour and spread the virus unless they are strictly monitored for infection. Amin Soebandrio, a microbiologist and Indonesia’s assistant deputy minister for health sciences, told New Scientist that while larger farms do such monitoring, smallholders cannot.
Now C A Nidom, of Airlangga University in Surabaya, in east Java, has found the H5N1 virus in throat swabs and blood samples from a pig in Surabaya. It is the same “highly pathogenic” variant that causes severe disease in chickens. And, says Soebandrio, the isolated virus’s gene for a crucial surface protein, haemagglutinin, was more than 98% identical to samples taken from infected Indonesian chickens and quail.
Traces of H5N1 infection had previously been reported in one pig in Vietnam and two in China. Pigs are of concern because they can be infected by both bird flu viruses and flu viruses from mammals. A hybrid bird-human virus could have a haemagglutinin from birds that human immune systems will not recognise, and genes from human-adapted flu that will make the hybrid highly contagious in people.
“It is almost certainly true that viruses of different species can mix in pigs – pigs have numerous “hybrid” viruses,” says Richard Webby of St Jude’s Children’s Research Hospital in Memphis, US. “But the pig as a mixing vessel for human flu pandemics is just a hypothesis.”
The pandemics of 1957 and 1968, which killed millions, did seem to result from the mixing of bird flu strains with mammalian flu, although where this occurred is not known. However, the even greater 1918 pandemic did not.
Robert Webster of St. Jude’s told a meeting on biosecurity in Lyon, France in March that his lab has found that H5N1 grows well in pigs, making hybridisation theoretically possible. But it might not get far – infected pigs, Webster told New Scientist, do not pass H5N1 to each other. This means relatively few pigs will catch it.
Poultry may be more important for the virus’s evolution. And this week Vietnam announced that it had found H5N1 infection in 71% of farmed ducks in the crowded Mekong Delta, where there have been many cases of human infection.
Ducks seem able to harbour the virus without showing symptoms, and in Vietnam many live in the open where they can spread the virus. So far Vietnam has not vaccinated poultry, but the Vietnamese agriculture ministry announced in March that it would start vaccinating ducks in the Mekong delta for H5N1 flu in April.