Jun 17, 2010 (CIDRAP News) Adding more details about a finding announced in February, Chinese researchers said today that a hybrid virus found in Hong Kong pigs contains elements of the 2009 human pandemic flu virus and two swine flu strains, pointing up a need for better surveillance of flu viruses in pigs.
In an article in Science, the researchers said the findings show that the pandemic virus, which originated in pigs, is capable of going back into pigs and trading genes with other flu viruses, raising the risk of generating more virulent strains. Pigs have long been regarded as mixing vessels for flu viruses, because they can be infected with human and avian as well as swine strains.
"The 2009 pandemic [virus], although mild and apparently contained at present, could undergo further reassortment in swine and gain virulence," says the report by researchers from Hong Kong University and Shantou University Medical College in China.
The virus was found in early January in the course of routine surveillance of pigs at a Hong Kong slaughterhouse, according to the report. It contained a neuraminidase gene derived from the 2009 pandemic virus, a hemagglutinin gene from a European lineage of "avian-like" swine flu viruses, and six internal genes from a swine flu group known as triple-reassortant H1N2.
The scientists said pigs that were experimentally infected with the reassortant virus became mildly ill and spread the virus to other pigs. Analysis of the virus's amino acid sequences indicates that it is likely to be resistant to the adamantane antiviral drugs but susceptible to oseltamivir, as is true of the pandemic virus, the article says.
Testing has shown that antibodies generated by the H1N1 pandemic vaccine and through infection with the pandemic virus do not cross-react with the reassortant strain, the researchers write.
The report notes that the pandemic virus has been found in pigs in several countries and that it turned up in swine at the Hong Kong slaughterhouse several times starting in October 2009. The Hong Kong finding in January marked the first report of a pandemic virus reassortant in pigs.
The scientists comment that avian H5N1 and H9N2 viruses have occasionally been found in swine, raising the possibility of reassortants combining genes from avian and mammalian-adapted viruses. Also, phylogenetic studies suggest that the pandemic viruses of 1918, 1957, and 1968 all evolved in another mammalian host for years before they surfaced in humans, they add.
Given the risk that reassortment in pigs could render the pandemic strain more dangerous, the report says, "It is therefore important that surveillance in swine is greatly heightened and that all eight gene segments are genetically characterized so that such reassortment events are rapidly identified." Relatively little swine flu surveillance is conducted in most countries, experts say.
When the reassortant was first announced in February, Malik Peiris of Hong Kong University, one of the researchers, said the finding wasn't unexpected and that such reassortments probably occur worldwide.
The earlier announcement said the hybrid virus had not been found in humans, and today's report makes no mention of any human infections.
Vijaykrishna D, Poon LLM, Zhu HC, et al. Reassortment of pandemic H1N1/2009 influenza A virus in swine. Science 2010 Jun 18;328 (early online publication) [Abstract]
Feb 26 CIDRAP News story "Hong Kong reports swine-pandemic flu reassortant"