![Reassortant pig[6]](http://lh6.ggpht.com/-en0-Nrsvi98/Td5GUe36_rI/AAAAAAAAFSc/jk9JNRBkea4/Reassortant%252520pig%25255B6%25255D_thumb%25255B1%25255D.jpg?imgmax=800 "Reassortant pig[6]")
Since pigs can be infected by more than one flu virus at the same time, it is possible for two viruses to swap genetic material (reassort), resulting in a new hybrid strain.
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Although the potential exists for viral reassortment in just about any host susceptible to influenza, swine – which can carry a wide variety of human, swine & avian strains – are considered to be excellent potential `mixing vessels’.
Swine possess both avian-like (SAα2,3Gal) and human-like (SAα2,6Gal) receptor cells in their respiratory tract, which many researchers believe can facilitate a `bridging’ between avian and human strains.
The pandemic virus that emerged in the spring of 2009 was the end product of several influenza strains that had kicked around the world’s swine population for many years, trading bits of genetic material back and forth, until they produced a version capable of jumping to humans.
The H3N2v virus is likewise a reassortant virus; swine H3N2 combined with the M (matrix) gene from the 2009 H1N1 virus. While the odds of seeing a pandemic emerge from this H3N2v virus appear low right now, they are not zero.
Of particular concern, due to its severity when it infects humans, is the H5N1 avian flu virus (and now, H7N9). The worry is - that given enough opportunities – one of these avian strains could reassort (swap genes) with a more `humanized’ flu virus, and produce a pandemic strain.
Despite the risks, surveillance of swine in the United States, and around the world, is dismally low. A topic Helen Branswell addressed in her terrific piece in SciAm from late 2010 called Flu Factories.
Flu Factories
The next pandemic virus may be circulating on U.S. pig farms, but health officials are struggling to see past the front gate
By Helen Branswell | December 27, 2010 |
Today we’ve a new study appearing in the Archives of Virology (h/t Tetano on FluTrackers), that reinforces the concern that the H5N1 virus can, and does, infect pigs (I’ll have more on that, after the abstract).
While the body of the article is behind a pay wall, we can get a pretty good idea of the findings from the abstract.
Isolation and characterization of two H5N1 influenza viruses from swine in Jiangsu Province of China
Liang He, Guo Zhao, Lei Zhong, Qingtao Liu, zhiqiang Duan, Min Gu, Xiaoquan Wang, Xiaowen Liu, Xiufan Liu
Abstract
Pigs are susceptible to infection with both human and avian influenza A viruses and are considered intermediate hosts that facilitate virus reassortment. Although H5N1 virus has spread to a wide range of avian and mammalian species, data about swine H5N1 isolates are scarce.
To determine whether Asian H5N1 influenza viruses had been transmitted to pigs, a total of 1,107 nasal swab samples from healthy swine were collected from 2008 to 2009 in Jiangsu province of eastern China.
In this survey, two H5N1 viruses A/swine/Jiangsu/1/2008 (JS/08) and A/swine/Jiangsu/2/2009 (JS/09) were isolated and identified.
Phylogenetic analysis showed that JS/08 and JS/09 belonged to clade 7 and clade 2.3.4, respectively, and shared over 99.0 % sequence identity with poultry H5N1 isolates of the same clade in China. Receptor specificity analysis also showed that both of the swine H5N1 isolates bound preferentially to avian-type receptors. However, experiments in mammals indicated that JS/09 was moderately pathogenic to mice without prior adaption, whereas JS/08 had limited ability to replicate.
Our findings suggest that pigs are naturally infected with avian H5N1 virus and highlight the potential threat to public health due to adaption or reassortment of H5N1 virus in this species.
Obviously viral reassortment that results in a biologically `fit’ pandemic virus is a rare event, else we’d be hip deep in novel viruses all of the time. But history has shown – given enough opportunities – these types of reassortments can happen.
While it has been documented previously, the evidence for how well the H5N1 virus infects pigs has been both limited, and mixed.
In previous experiments, domestic pigs were shown to have low susceptibility to the H5N1 virus, producing asymptomatic to mildly symptomatic infection of the respiratory tract and tonsils.
Viral titers were lower than normally seen with swine influenzas, suggesting a limited ability to adapt and spread among pigs.
Late last year, in Seroprevalence Study: Avian Flu In Chinese Pigs, we looked at a study that found low levels of H3, H4, and H6 subtypes of avian influenza in Chinese pigs. Somewhat reassuringly, no evidence of previous infection with the H5N1 virus was found.
But an earlier study out of Indonesia (see 2010’s EID Journal: Asymptomatic H5N1 In Pigs) found during the period of 2005-2007 that 7.4% of pigs surveyed in Indonesia carried the H5N1 virus, and that phylogenic analysis showed at least 3 separate introductions into the pig population.
More recent surveillance, during 2008-2009 did not turn up any active infections, but 1% of pigs tested carried antibodies to the H5N1 virus.
Of note, in today’s study (and the others mentioned above) infected pigs almost always appear healthy, or asymptomatic.
The ability of influenza viruses to evolve, mutate, or reassort in swine hosts has been a frequent topic of discussion in this blog. For more on this, you may wish to revisit:
