Swine are notoriously prone to influenza infections, and are viewed as potential `mixing vessels’ for flu viruses since they can be infected by swine, avian, and human flu strains. The most common swine flu strains are H1N1, H1N2, H3N1, H3N2, and H2N3.
Although predominantly affecting swine, on the rare occasions when they jump to humans, these strains are called variant viruses (e.g. H3N2v).
During 2012, a record number (300+) of swine variant infections were recorded in humans, mostly associated with swine exposure at county and state fairs across the Midwest. The concern with these variant flu infections, as with any novel flu, is that each new human exposure provides the virus another opportunity to better adapt to human physiology.
While sporadic human-to-human transmission of these viruses was strongly suspected last year, sustained community transmission did not appear to be occurring. It is likely, however (see CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012)), that the number of confirmed cases under-represents the total number of variant flu infections that occur each year.
Today, another study appearing in the CID Journal, conducted by CDC and local Health department researchers, looks at 306 human variant flu infections reported across 10 states last year, and finds the vast majority appear to have contracted the virus from direct exposure to infected pigs, while only about 5% appeared to be the result of H-2-H transmission.
Michael A. Jhung1, Scott Epperson1, Matthew Biggerstaff1, Donna Allen9, Amanda Balish1, Nathelia Barnes1, Amanda Beaudoin10, LaShondra Berman1, Sally Bidol7, Lenee Blanton1, David Blythe16, Lynnette Brammer1, Tiffany D'Mello1, Richard Danila8, William Davis1, Sietske de Fijter13, Mary DiOrio13, Lizette O. Durand2, Shannon Emery1, Brian Fowler13, Rebecca Garten1, Yoran Grant6, Adena Greenbaum2, Larisa Gubareva1, Fiona Havers2, Thomas Haupt14, Jennifer House9, Sherif Ibrahim15, Victoria Jiang1, Seema Jain1, Daniel Jernigan1, James Kazmierczak14, Alexander Klimov1, Stephen Lindstrom1, Allison Longenberger11, Paul Lucas4, Ruth Lynfield8, Meredith McMorrow1, Maria Moll11, Craig Morin8, Stephen Ostroff11, Shannon L. Page13, Sarah Y. Park12, Susan Peters7, Celia Quinn3, Carrie Reed1, Shawn Richards9, Joni Scheftel8, Owen Simwale11, Bo Shu1, Kenneth Soyemi4, Jill Stauffer9, Craig Steffens1, Su Su1, Lauren Torso11, Timothy M. Uyeki1, Sara Vetter8, Julie Villanueva1, Karen K. Wong2, Michael Shaw1, Joseph S. Bresee1, Nancy Cox1, and Lyn Finelli1
Background. Variant influenza virus infections are rare but may have pandemic potential if person-to-person transmission is efficient. We describe the epidemiology of a multi-state outbreak of an influenza A H3N2v virus first identified in 2011.
Methods. We identified laboratory-confirmed cases of H3N2v and used a standard case report form to characterize illness and exposures. We considered illness to result from person-to-person H3N2v virus transmission if swine contact was not identified within 4 days prior to illness onset.
Results. From July 9—September 7, 2012, we identified 306 cases of H3N2v in ten states. The median age of all cases was 6 years. Commonly reported signs and symptoms included fever (98%), cough (84%), and fatigue (83%). Sixteen cases (5.2%) were hospitalized, and one fatal case was identified. The majority of cases reported agricultural fair attendance (93%) and/or contact with swine (95%) prior to illness. We identified 15 cases of possible person-to-person transmission of H3N2v virus. Viruses recovered from cases were 93% to 100% identical and similar to viruses recovered from previous cases of H3N2v. All H3N2v viruses examined were susceptible to the oseltamivir and zanamivir and resistant to adamantane antiviral medications.
Conclusion. In a large outbreak of variant influenza, the majority of cases reported exposures suggesting swine contact at an agricultural fair was a risk for H3N2v virus infection. We identified limited person-to-person H3N2v virus transmission, but found no evidence of efficient or sustained person-to-person transmission. Fair managers and attendees should be aware of the risk of swine-to-human transmission of influenza viruses in these settings.
Compared to last year, reports of swine variant infections this summer are greatly reduced. Thus far, only 20 cases have been reported to the CDC (see Arkansas Reports Two H1N1v Infections for the latest reports). While we watch these flu strains for any signs they may be changing, for now, the threat to the public from these swine variant viruses appears low.
The CDC, in a statement on June 28th of 2013, offered this assessment on H3N2v:
CDC will continue to watch this virus closely to make sure there are no changes in the epidemiology of related human infections. That means watching for any changes in the severity of illness caused by infection with this virus and any signs that the virus is becoming more adept at spreading from person-to-person. Like all influenza viruses, it’s possible that mutations could occur that would allow this virus to become more severe or to spread more easily between people.
The risk of this virus triggering a full-blown pandemic is considered relatively low, however, because serology studies have suggested that significant numbers of adults have some existing immunity against this virus. Children younger than about 10 years old, however, have little to no immunity against H3N2v virus. Given this, a more likely scenario if H3N2v were to become more transmissible among people would be localized outbreaks in pockets of the population that do not have immunity against this virus, for example, in day care or school settings.
A few of my recent blogs on Swine variant influenza include:
And for more information, you may wish to visit the CDC’s Variant (Swine Origin) Influenza Viruses in Humans webpage, or visit these related links.
- Key Facts about Human Infections with Variant Viruses (Swine Origin Influenza Viruses in Humans)
- Reported Infections with Variant Influenza Viruses in the United States since 2005
- Influenza A (H3N2) Variant Virus
- What People Who Raise Pigs Need To Know About Influenza (Flu)
- Reports of Human Infections with Variant Viruses