Researchers from the US have found a flu virus that is prevalent in pig populations that is easily spread and highly lethal in ferrets, a recognized model of human influenza infection and transmissibility.

The researchers used the ferret model to assess the pathogenicity and transmissibility of H1N2 and H3N2 influenza viruses. The researchers found that one virus in particular [A/Swine/Korea/1204/2009; Sw/1204 (H1N2)] was particularly deadly in ferrets, causing death within 10 days of inoculation, and was efficiently transmitted to other ferrets via respiratory droplets. The same virus also demonstrated enhanced infectivity and growth in human lung tissues and replicated better than other swine derived flu viruses under conditions simulating the human airways.

The authors suggest the findings demonstrate the pandemic potential of influenza viruses circulating in pigs, and indicate the need for efficient global surveillance of influenza viruses in swine populations.

Dr Ian Barr, Deputy Director of the WHO Collaborating Centre for Reference and Research on Influenza, Melbourne Australia, says:

“The paper by influenza scientists in the USA and Korea, reports on findings from swine influenza A-type viruses collected from pigs in Korea in 2007 and 2009.  Their results highlight a potential threat to man as many similar (and different) influenza A viruses continue to circulate in the pig herds around the world.  Whilst the influenza viruses described in this study had little or no health effects when they were used to infect pigs, one of the viruses caused serious infections and deaths when given to experimental animals such as mice and ferrets and also transmitted easily via respiratory droplets.  In 2009 an influenza virus of swine origin emerged and infected humans in Mexico before rapidly spreading and causing a global pandemic.  Fortunately the A(H1N1) 2009 virus was relatively mild compared to some previous influenza pandemics, although it was still responsible for tens of thousands of fatalities.  There is a need to better understand the extent and variety of swine influenza viruses currently infecting the global swine herds and to determine their ability to become serious human pathogens in the future.  This study is one small step into the large and complex race to better understand zoonotic diseases and how best to intervene to prevent their spread to man.”

Professor Peter Doherty from the Department of Microbiology and Immunology at the University of Melbourne and a winner of the Nobel prize in Medicine in 1996 for his work on T cells, is currently Chief Investigator on an NHMRC Program grant to study T cell responses to the influenza A viruses, with the aim of developing a more effective influenza vaccine. He says:

“The influenza A viruses have an 8-segmented genome which allows them to ‘re-assort’ (or repackage) if, for example, an epithelial cell in the lung of a pig is infected simultaneously with,  say, different viruses of human and pig origin. Of late, this has been occurring at a much faster rate than observed previously for North American swine. One consequence was the emergence of the 2009 H1N1 swine ‘flu, which arose from the re-assortment of different European and North American H1N1 viruses (both of swine origin). The 2009 pandemic strain also has at least some genetic elements that are very similar to those found in the ‘reconstructed’ 1918/19 pandemic virus, reflecting that ‘flu viruses can go back and forth between humans and pigs and are generally more stable in pigs.

“Currently, there are human cases (including one death) caused by a swine H3N2 virus that is infecting those who get close to pigs at US agricultural fairs. To date this virus does not seem to be transmitting effectively between people. The emergence of a novel H1N2 human pandemic strain would be a first since we’ve had the technology to look at such events, but virologists have long been speculating about the possible re-emergence of something like the 1957 H2N2 Asian ‘flu that caused more than a million deaths worldwide. That was in turn displaced by the 1968 H3N2 Hong Kong ‘flu, variants of which are still circulating today and caused many of the severe influenza cases in Australia this year.

Influenza epidemiologists have long been watching for the re-emergence of a virulent H2N2 strain that spreads to cause severe disease in humans. But it hasn’t happened yet.”

Reference: smc.org.au.

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