Outbreak of Influenza A (H3N2) Variant Virus Infection among Attendees of an Agricultural Fair, Pennsylvania, USA, 2011

Karen K. Wong

, Adena Greenbaum, Maria E. Moll, James Lando, Erin L. Moore, Rahul Ganatra, Matthew Biggerstaff, Eugene Lam, Erica E. Smith, Aaron D. Storms, Jeffrey R. Miller, Virginia Dato, Kumar Nalluswami, Atmaram Nambiar, Sharon A. Silvestri, James R. Lute, Stephen Ostroff, Kathy Hancock, Alicia Branch, Susan C. Trock, Alexander Klimov, Bo Shu, Lynnette Brammer, Scott Epperson, Lyn Finelli, and Michael A. Jhung

Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (K.K. Wong, A. Greenbaum, R. Ganatra, M. Biggerstaff, E. Lam, A.D. Storms, J.R. Miller, K. Hancock, A. Branch, S.C. Trock, A. Klimov, B. Shu, L. Brammer, S. Epperson, L. Finelli, M.A. Jhung); Pennsylvania Department of Health, Harrisburg, Pennsylvania, USA (M.E. Moll, E.E. Smith, J.R. Miller, V. Dato, K. Nalluswami, A. Nambiar, J.R. Lute, S. Ostroff); Allegheny County Health Department, Pittsburgh, Pennsylvania (J. Lando, S.A. Silvestri); and Pennsylvania Department of Agriculture, Harrisburg (E.L. Moore)

Abstract

During August 2011, influenza A (H3N2) variant [A(H3N2)v] virus infection developed in a child who attended an agricultural fair in Pennsylvania, USA; the virus resulted from reassortment of a swine influenza virus with influenza A(H1N1)pdm09. We interviewed fair attendees and conducted a retrospective cohort study among members of an agricultural club who attended the fair. Probable and confirmed cases of A(H3N2)v virus infection were defined by serology and genomic sequencing results, respectively. We identified 82 suspected, 4 probable, and 3 confirmed case-patients who attended the fair. Among 127 cohort study members, the risk for suspected case status increased as swine exposure increased from none (4%; referent) to visiting swine exhibits (8%; relative risk 2.1; 95% CI 0.2–53.4) to touching swine (16%; relative risk 4.4; 95% CI 0.8–116.3). Fairs may be venues for zoonotic transmission of viruses with epidemic potential; thus, health officials should investigate respiratory illness outbreaks associated with agricultural events.

Triple reassortant swine influenza A viruses have circulated in swine herds in North America since 1998 (1–3). On the rare occasions that these viruses infect humans, they are called influenza A variant viruses (4). Viruses resulting from reassortment of swine influenza A (H3N2) virus and influenza A(H1N1)pdm09 (pH1N1) virus have emerged among US swine (4–6), and similar viruses have been identified among swine outside the United States (7,8). During August 2011, the first known human infection with influenza A (H3N2) variant [A(H3N2)v] virus containing the pH1N1 matrix (M) gene was reported in the United States (9). The pH1N1 M gene is implicated in increasing influenza transmissibility in animal models (10,11), and there was concern that this new A(H3N2)v virus could be efficiently transmitted among humans. Because these viruses contain a novel combination of genes, little is known about the epidemiologic and clinical characteristics of human infections.
During August 2011, a child who had attended an agricultural fair in Pennsylvania (Fair A) attended by ≈70,000 persons became ill; the Centers for Disease Control and Prevention (CDC) confirmed infection with A(H3N2)v virus in the child 6 days after Fair A closed and immediately began an investigation with the Pennsylvania Department of Health (PA DOH), the Allegheny County Health Department, and the Pennsylvania Department of Agriculture (PDA) to determine the extent of A(H3N2)v virus transmission and to identify illness risk factors among Fair A attendees.