The H1N1 swine flu virus has been spreading quietly in pigs in Hong Kong and swapping genes with other viruses, and researchers said the findings support calls for tighter disease surveillance in pigs before new bugs can emerge and infect people.
The finding, published in Science June 18, supports the theory that flu viruses infecting swine can swap genes with other viruses that are in pigs, including more dangerous bugs like the H5N1 or H9N2 bird flu viruses.
Malik Peiris, an influenza expert who worked on the study, said the discovery underlines the importance of disease surveillance in pigs.
“It demonstrates the pandemic virus can easily go back to pigs. Once it does so, it can reassort with other pig viruses and give rise to potentially unexpected consequences,” said Peiris, a microbiology professor at the University of Hong Kong.
Peiris and colleagues, including Guan Yi at the University of Hong Kong, have found pandemic H1N1 viruses in nasal swabs taken from apparently healthy pigs at a Hong Kong abattoir during routine checks since October 2009.
“From genetic analysis, what it suggests is each of those viruses we found in pigs all came from humans,” Peiris said in a telephone interview.
“It is not surprising because the pandemic virus emerged from pigs, so it is not surprising that it goes back to pigs.”
A sample isolated from Hong Kong pigs in January 2010 carried genes from three viruses – the pandemic H1N1, a European “avian-like” H1N1 and a so-called “triple reassortant” virus containing bits of human, pig and bird flu viruses which was first discovered in North America in 1998.
“This suggests that the pig is a place where the pandemic virus might actually change and reassort and get new properties possibly,” Peiris said.
“The pandemic virus in humans has been extremely stable. It hasn’t changed at all even though people were concerned it might reassort and mix with human viruses … but it seems that it can mix with other flu viruses (in a pig).”
Genetic research has suggested that H1N1, first identified in people in April 2009, had in fact been circulating for at least a decade and probably in pigs. Despite tight controls on herds to protect them from people, little checking is done globally to see whether food herds are infected and if so, with what viruses.
Studies in the past year have turned up pigs in Canada and other countries infected with the pandemic H1N1 virus, evidently carried to the animals by people.
“I must emphasize the point that it doesn’t mean that pork is dangerous to eat at all (if well cooked). What it means is it is important to carry out systematic surveillance in pigs so we know what is going on in pigs in regard to influenza viruses in general and the pandemic virus in particular,” Peiris said.
Pigs are the reservoir of many human, bird and swine viruses and experts often refer to them as an ideal mixing vessel for new, and possibly more dangerous pathogens.
Asked if there was a possibility of the H1N1 getting mixed up with the H5N1, Peiris said: “That is certainly a possibility, that’s why we need to keep track.
“If it is quite able to readily reassort and pick up genes from pig viruses, you might have other combinations of genes that can arise. Unless we are alert to it, we potentially could have a virus that is … more virulent coming back to humans.”
Although H5N1 is a mostly avian virus, it causes more severe illness in people than seasonal flu and kills 60 per cent of the people it infects. It has infected 499 people and killed 295 of them since reemerging in 2003.
The World Health Organization said early in June that the H1N1 pandemic was not yet over although its most intense activity has passed in many parts of the world.