Since pigs can be infected by more than one flu virus at the same time, the potential exists for two viruses to swap genetic material (reassort), resulting in a new hybrid strain.
Although we tend to think of H5N1 whenever someone mentions `avian flu’, in truth there are many different avian influenza viruses. If you go back far enough, all influenza A strains – even those thought of today as primarily adapted to humans, equines, or swine – appear to have an avian origin.
Influenza A viruses have 8 gene segments (PB2, PB1, PA, HA, NP, NA, M1, M2, NS1, NS2) and are broadly categorized by their HA (hemagglutinin) and NA (neuraminidase) genes.
With the recent discovery of H17N10 (see A New Flu Comes Up To Bat), scientists have now identified 17 different serotypes of Hemagglutinin (HA) and 10 serotypes of Neuraminidase (NA), which make many different combinations of HA and NA proteins possible.
Thus far, only about 100 of these combinations have been isolated in nature.
But the flu universe is far more diverse than that might lead to you believe, as influenza’s 8 gene segments are largely interchangeable parts. If you were to take two different influenza A viruses (say H3N2 and H5N1) and reshuffle them using reverse genetics, you can come up with 254 possible reassortments.
And within each of these HA/NA combinations you can have multiple clades (genetically distinct families), and within each clade, many minor variants – all of which makes for tremendous variety in these viruses.
Many of the the H1, H2, and H3 viruses have adapted to human physiology, but we occasionally see infection by other `novel’ strains. Most worrisome, due to its high fatality rate, is H5N1. But we occasionally see (usually mild) infections by other avian strains.
Last year scores of dead seals were discovered along the shoreline of New England, predominantly from the North Shore of Massachusetts to the southern coast of Maine. Investigations showed (see mBio: A Mammalian Adapted H3N8 In Seals) their deaths to be due to a variant of the H3N8 avian flu strain, versions of which are known to also infect horses and dogs.
With the propensity of these avian strains to evolve and adapt, scientists are understandably very interested in any avian flu virus that appears to be moving towards `humanization’.
While it is possible for a flu virus to jump directly from birds to humans, often it requires an intermediate host to aid in its development. The graphic below illustrates that swine – which are generally susceptible to human and avian flu viruses - are considered excellent mixing vessels’ for influenza viruses.
The H1N1 `swine flu’ virus of 2009 kicked around in pigs for up to a decade before it adapted well enough to human physiology to spark a human pandemic. Over the past few years we’ve watched attempts by three other swine `variant’ viruses (H1N1v, H1N2v, H3N2v) to jump to mankind.
Which makes the active surveillance of swine a potentially important tool for the early detection of emerging influenza viruses.
Unfortunately, while there are some surveillance systems in place, only a tiny fraction of the world’s swine population is actively monitored (see Helen Branswell’s SciAm Article from late 2010 called Flu Factories).
All of which serves as prelude to a new study, recently published in the Journal of Clinical Microbiology, that found low levels of H3, H4, and H6 subtypes of avian influenza in Chinese pigs. Somewhat reassuringly, evidence of infection with the H5N1 virus was not found.
These H3, H4, and H6 avian viruses were not seen in a similar study done in 2001, suggesting their arrival into the swine population may be a recent development.
Shuo Su, Wenbao Qi, Jidang Chen, Wanjun Zhu, Zhen Huang, Jiexiong Xie, and Guihong Zhang
J. Clin. Microbiol. published ahead of print 21 November 2012 doi:10.1128/JCM.02625-12
Recently, three novel avian-origin swine influenza viruses (SIVs) were first isolated from pigs in Guangdong Province, southern China, yet little is known about the seroprevalence of avian influenza among pigs in southern China. Here, we report for the first time the seroprevalence of avian H3, H4, and H6 influenza viruses in swine populations and the lack of seroepidemiological evidence of avian H5 influenza transmission to pigs in China.
A copy of the study can be downloaded here.
The American Society for Microbiology has published a press release with more information, excerpts of which you’ll find below:
WASHINGTON, DC – December 19, 2012 -- Researchers report for the first time the seroprevalence of three strains of avian influenza viruses in pigs in southern China, but not the H5N1 avian influenza virus. Their research, published online ahead of print in the Journal of Clinical Microbiology, has implications for efforts to protect the public health from pandemics.
Influenza A virus is responsible both for pandemics that have killed millions worldwide, and for the much less severe annual outbreaks of influenza. Because pigs can be infected with both human and avian influenza viruses, they are thought to serve as “mixing vessels” for genetic reassortment that could lead to pandemics, and pigs have been infected experimentally by all avian H1-H13 subtypes. But natural transmission of avian influenza to pigs has been documented only rarely.
In the study, from 2010-2012, Guihong Zhang and colleagues of the College of Veterinary Medicine, South China Agricultural University, Guangzhou, People’s Republic of China, tested 1080 21-25 week old pigs for H3, H4, H5, and H6 subtypes of avian influenza virus, and H1 and H3 subtypes of swine influenza virus. Thirty-five percent of the serum samples were positive for H1N1, and 19.7 percent were positive for H3N2 swine flu virus, and 0.93 percent, 1.6 percent, and 1.8 percent were positive, respectively, for the H3, H4, and H6 subtypes of avian influenza A virus. However, no serum samples collected in 2001 were positive for any of these viruses, indicating that transmission into swine was recent.
Given the recent transmission of avian influenzas into swine, “We recommend strongly that the pork industry worldwide should monitor the prevalence of influenza in pigs, considering their important role in transmitting this virus to humans,” says Zhang.
Previously, novel reassortant H2N3 influenza viruses were isolated from US pigs, which “were infectious and highly transmissible in swine and ferrets without prior adaptation,” according to a 2009 paper in the Journal of Molecular and Genetic Medicine by Wenjun Ma et al. Those viruses resembled, but were not identical to the H2N2 human pandemic virus of 1957.
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: