H1N1 swaps genes with other pig viruses: scientists
The Canadian Press
Published Friday, June 18, 2010 8:40AM EDT
TORONTO - Scientists from Hong Kong are calling for ramped-up surveillance of pig populations after discovering a new swine flu virus that is a hybrid of the pandemic H1N1 virus and viruses previously found in pigs.
The discovery of the virus, found early this year in a pig taken to slaughter in Hong Kong, suggests what experts have feared: the H1N1 virus may reassort easily with other viruses in pigs.
That's a process that could generate new flu viruses that might have the capacity to sicken humans, they warned, noting two viruses high on the pandemic watch list -- H5N1 and H9N2 -- are occasionally found in swine in Asia.
Inherent in their report is a message flu experts know the world doesn't want to hear at this point: the mild H1N1 pandemic may be the opening act of a longer play.
"This particular paper is extremely interesting because it demonstrates for the first time what we had worried about at the very onset of the pandemic," said Dr. Nancy Cox, head of the influenza division at the U.S. Centers for Disease Control.
"And that is that this particular virus, when introduced into pigs, could reassort with the resident viruses in pigs and we would have new gene constellations. And bingo, here we are."
The discovery was reported by scientists from the University of Hong Kong and Shantou University Medical College in Guangdong province, China. It was published Friday in the journal Science.
It is the first report of a reassortment of the pandemic virus, which in humans has been slow to evolve. Cox said surveillance shows little significant variation in the viruses isolated from people and no need yet to update the virus used to make human flu vaccine.
Pigs are called the mixing vessel of flu because they can be infected both by avian flu viruses -- which rarely directly infect people -- and by human viruses. When pigs become simultaneously infected with more than one virus, the viruses can swap genes, producing new variants that can pass to humans and sometimes spread among them.
"Unlike the situation with birds and humans, we have a situation with pigs and humans where there's a two-way street of exchange of viruses," said Cox, who was not involved in the Hong Kong study. "With pigs it's very much a two-way street."
One of the senior authors of the paper said the fact the first reassortant involving pandemic H1N1 has been found in swine underscores the role the animals play.
Over the past year, especially in the early days of the pandemic, H1N1 would have had plenty of opportunities to reassort with human flu viruses in people, Dr. Malik Peiris said.
"It has been in pigs for a much shorter time and as far as we can tell to a more limited extent and already we can detect a reassortant. So it suggests -- although the numbers are very small yet -- that probably it is not difficult really for the pandemic virus to reassort with other pig viruses," said Peiris, a microbiologist at the University of Hong Kong.
The discovery was made through a surveillance system his team has operated for the past decade. Every two weeks they swab the snouts of about 250 pigs at an abattoir just after the animals have been slaughtered. The vast majority of the pigs slaughtered there come from adjacent provinces in China.
Late last October, they started finding pandemic H1N1 viruses in some of the pigs. Those viruses appeared to be the product of human-to-pig transmission, Peiris explained in an interview.
But in early January, one of the swabbing forays produced a novel finding -- a virus that had internal genes from one line of swine flu viruses, a hemagglutinin from a Eurasian avian flu lineage and the neuraminidase gene from the human pandemic H1N1.
There is no evidence the virus is continuing to spread or that it is inherently more virulent than H1N1. But if it were to find its way to humans, it would likely cause infection. Testing shows the antibodies generated by H1N1 infection or by the pandemic vaccine would not protect against this virus.
Dr. Christopher Olsen, a swine flu expert from the University of Wisconsin-Madison, said he found it striking that of 32 H1 viruses isolated from pigs from June 2009 to February 2010, 10 were pandemic viruses.
Scientists in the swine flu community have been questioning whether pandemic H1N1, which has been found in pigs in multiple countries around the world, would become established as one of the circulating viruses in herds, he said.
"And I think this really does speak to the fact that it's very likely that this virus is becoming endemic within at least this particular swine population," said Olsen, who suggested if that does happen, it will likely give rise to new hybrid viruses.
"In an area of the world where you have multiple genotypes of virus co-circulating in the swine population, you would really expect reassortment to happen."
Cox, who has long called for more influenza surveillance in swine herds, was quick to echo the authors' recommendation for more work in this area. But she, Peiris and Olsen all acknowledged there is likely to be resistance to the suggestion.
In fact, flu experts have been worried that since the emergence of H1N1, surveillance in pigs has probably declined. Pig producers have learned that finding flu viruses in their herds can have costly consequences.
Arnold Van Ginkel, a farmer who lives near Rocky Mountain House, Alta., had the great misfortune of owning the first pigs to test positive for the pandemic virus anywhere in the world. He eventually had to destroy the herd because no one would buy his animals, even after they recovered fully from their bout of flu.
Some countries closed their borders to imports of pork from countries where the virus had been found in pigs -- even though those same countries had human cases of H1N1 and even though experts insisted there was no risk of acquiring the virus from properly handled pork products.
Cox said a way needs to be found to do surveillance in a smarter way, in a way "that doesn't punish the innocent." She said the U.S. is working on a system whereby surveillance data could be anonymous so that a finding could not be traced publicly to an individual farm.