I hadn’t planned on writing another swine variant influenza-related blog again so soon (see yesterday’s blog CDC: Measures to Minimize Influenza Transmission at Swine Exhibitions, 2014), but today the CDC’s EID Journal has posted an analysis of the 2012 outbreak of H3N2v associated with attendance of agricultural fairs in Ohio in 2012.
State & local fairs have instituted inspections for any signs of illness in livestock – but as we’ve discussed previously (see Asymptomatic Pigs: Revisited) - pigs can sometimes carry these viruses without showing any outward signs of infection.
The concern with these variant swine flu infections, as it is with any animal flu that jumps to humans, is that it gives the virus another opportunity to better adapt to human physiology.
While humans have a long history of exposure to seasonal H3N2 flu viruses, research has shown only limited community immunity to these variant strains (see CIDRAP: Children & Middle-Aged Most Susceptible To H3N2v). The good news is that while several hundred infections were recorded in 2012, sustained and efficient community transmission was not observed, and for the most part, the virus only caused mild to moderate illness.
Today’s study confirms the link between fairs with H3N2v infected pigs, and human cases, and confirms that the strains detected in humans, and those detected in pigs, were > 99.7% identical. Follow the link below to read:.
Andrew S. Bowman , Sarah W. Nelson, Shannon L. Page, Jacqueline M. Nolting, Mary L. Killian, Srinand Sreevatsan, and Richard D. Slemons
Author affiliations: The Ohio State University, Columbus, Ohio, USA (A.S. Bowman, S.W. Nelson, J.M. Nolting, R.D. Slemons); Ohio Department of Health, Columbus (S.L. Page); US Department of Agriculture National Veterinary Services Laboratories, Ames, Iowa, USA (M.L. Killian); University of Minnesota, Saint Paul, Minnesota, USA (S. Sreevatsan)
Agricultural fairs provide an opportunity for bidirectional transmission of influenza A viruses. We sought to determine influenza A virus activity among swine at fairs in the United States. As part of an ongoing active influenza A virus surveillance project, nasal swab samples were collected from exhibition swine at 40 selected Ohio agricultural fairs during 2012.
Influenza A(H3N2) virus was isolated from swine at 10 of the fairs. According to a concurrent public health investigation, 7 of the 10 fairs were epidemiologically linked to confirmed human infections with influenza A(H3N2) variant virus. Comparison of genome sequences of the subtype H3N2 isolates recovered from humans and swine from each fair revealed nucleotide identities of >99.7%, confirming zoonotic transmission between swine and humans.
All influenza A(H3N2) viruses isolated in this study, regardless of host species or fair, were >99.5% identical, indicating that 1 virus strain was widely circulating among exhibition swine in Ohio during 2012.
(Continue . . . .)
Swine are highly susceptible to a variety of flu viruses (human, swine, avian) - and are viewed as excellent `mixing vessels’, allowing viruses to reassort into new hybrid strains.
While a handful of novel swine variant flu infections that don’t appear to transmit efficiently may not sound like a big deal, as the authors of this paper point out, Agricultural Fairs provide a favorable environment for multiple swine flu viruses to get together. They write:
Swine-to-human transmission and human-to-swine transmission of influenza A virus are known to occur at fairs (28), highlighting the fact that swine in this setting are potentially exposed to multiple lineages of influenza A viruses simultaneously, making fairs ideal locations for genomic reassortment and novel virus formation.
And not only can swine pass flu viruses on to people, people can pass flu viruses on to swine. Something we saw happen around the world after the 2009 H1N1 pandemic virus emerged and began transmitting in humans.
This bi-directional transfer of influenza viruses allows for even greater mixing and matching of genes, and explains how the H3N2v virus picked up the M (matrix) gene from the 2009 H1N1pdm virus in 2012.
The authors write:
The results of this study support previous calls for enhanced surveillance of influenza A viruses among swine, especially at high-risk swine–human interfaces
For more on swine variant influenza viruses, and why they matter, you may wish to revisit: