Outbreak of Variant Influenza A(H3N2) Virus in the United States

Abstract

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 multistate outbreak of an influenza A(H3N2) variant virus (H3N2v) 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 transmission if swine contact was not identified within 4 days prior to illness onset. Results From 9 July to 7 September 2012, we identified 306 cases of H3N2v in 10 states. The median age of all patients was 7 years. Commonly reported signs and symptoms included fever (98%), cough (85%), and fatigue (83%). Sixteen patients (5.2%) were hospitalized, and 1 fatal case was identified. The majority of those infected 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. Viruses recovered from patients were 93%–100% identical and similar to viruses recovered from previous cases of H3N2v. All H3N2v viruses examined were susceptible to oseltamivir and zanamivir and resistant to adamantane antiviral medications. Conclusions In a large outbreak of variant influenza, the majority of infected persons reported exposures, suggesting that swine contact at an agricultural fair was a risk for H3N2v 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.

Outbreak of Influenza A(H3N2) Variant Virus Infections Among Persons Attending Agricultural Fairs Housing Infected Swine — Michigan and Ohio, July–August 2016

“…This HA gene was likely introduced from humans into swine in 2010 or 2011, and has since circulated and evolved in swine to be genetically and antigenically different from both previous and currently circulating human seasonal influenza A(H3N2) viruses (3). The viruses in the remaining two specimens had HA genes similar to those of swine-origin H3N2 influenza viruses circulating in the U.S. swine population since 1998, and previously identified in human H3N2v virus infections in the United States since 2009 (4,5). Preliminary analysis suggests that the viruses identified in all human specimens were nearly identical to H3N2 viruses detected in swine at agricultural fairs in Michigan and Ohio.…”

Increased risk of A(H1N1)pdm09 influenza infection in UK pig industry workers compared to a general population cohort

“…Pigs are a particularly important species in this regard as the occurrence of both types of SA receptors permits binding of human and avian influenza viruses making them an efficient ‘mixing vessel' 11, 12, 13, 14. Interspecies transmission (in both directions) of swine and human influenza viruses is well recognised, evidenced by the isolation of human influenza virus in swine15, 16 and evidence of swine influenza virus (SIV) infection in people with close occupational17, 18, 19 and/or residential proximity to pigs20, 21; or prolonged exposure at an agricultural fair22. Transmission between pigs and bird species is exemplified by various reports of isolation of SIV from turkeys 23, 24, 25.…”

Nasal Wipes for Influenza A Virus Detection and Isolation from Swine

“…12 The pandemic virus (A (H1N1)pdm09) has since reassorted with endemic swine IAV strains 13,14 and some of these newly reassorted strains have been transmitted back to humans. 15 The frequency of reassortment events and emergence of new IAV strains with pandemic potential makes active surveillance of circulating IAV viruses in swine imperative, especially at the swine-human interface.…”

“…Agricultural fairs are the largest settings for the comingling of people and swine in the United States and are known sites for the zoonotic transmission of IAV. [15][16][17][18][19][20][21] In 2012, during the outbreak of a variant H3N2 IAV, 93% of cases reported attendance at an agricultural fair in the days prior to onset of disease. 15 Genomic analysis of viral isolates from exhibition pigs compared with human isolates confirmed zoonotic transmission.…”

“…[15][16][17][18][19][20][21] In 2012, during the outbreak of a variant H3N2 IAV, 93% of cases reported attendance at an agricultural fair in the days prior to onset of disease. 15 Genomic analysis of viral isolates from exhibition pigs compared with human isolates confirmed zoonotic transmission. 21 Exhibition swine infected with IAV often do not show clinical signs of illness, [21][22][23] indicating the need for direct diagnostic tests.…”