We describe an outbreak of respiratory illness, including 3 confirmed infections with a variant influenza A virus not identified in humans before August 2011. The outbreak occurred during a large agricultural fair, where humans and animals had opportunities for repeated and/or prolonged contact.
Outbreaks of variant influenza A viruses at agricultural events have been described, and these events may be key settings for zoonotic influenza transmission (17
). Triple reassortant H3N2 subtype viruses containing the pH1N1 M gene were first identified among swine in the United States in 2009 and have been detected among swine in multiple states, including Pennsylvania (25
). During July–November 2011, 13 human infections with A(H3N2)v virus containing the pH1N1 M gene were identified, and 5 were linked to agricultural fairs (24
). Although the frequency of zoonotic influenza transmission at agricultural events is unknown, these events provide opportunities for swine influenza viruses to infect humans who have contact with infected swine. Human and swine influenza viruses may circulate at these events, creating opportunities for virus reassortment and the emergence of novel strains.
This investigation suggests that swine contact during Fair A was a risk factor for illness. Persons reporting direct contact with swine were more likely to report illness. Most case-patients became ill within 4 days after the swine show or auction, suggesting a temporal relationship between human–swine contact and onset of human illness within <
4 days. The epidemic curve, which suggests that case-patients were exposed to a common infectious source that was present for several days, is consistent with the hypothesis that infected swine were present for the duration of the fair. Prior investigations of human variant influenza virus infections have documented contact with infected swine (17
), and cases have also occurred after contact with apparently healthy swine (4
). No swine were tested for influenza during this investigation because swine at Fair A had either been slaughtered or had recovered before the first human case was reported; however, triple reassortant H3N2 subtype viruses containing genetic material from pH1N1 have been detected in swine (4
Because of limited diagnostic testing, the extent and distribution of illness caused by A(H3N2)v virus among Fair A attendees are unknown; however, two thirds of children <4 years of age who were tested were seropositive for A(H3N2)v virus. This finding suggests that illness in at least some suspected case-patients can be attributed to A(H3N2)v virus infection. Suspected case-patients had illness onset dates and symptoms similar to those for probable and confirmed case-patients. Symptoms were similar to those of seasonal influenza (27
), but no seasonal influenza was circulating at the time in Pennsylvania.
Although we cannot rule out human-to-human transmission of A(H3N2)v virus at or after Fair A, enhanced surveillance after Fair A through the beginning of the typical influenza season detected no additional cases of A(H3N2)v virus infection in the community; this suggests that the virus did not exhibit efficient or sustained human-to-human transmission. However, A(H3N2)v virus infection has occurred with limited human-to-human transmission among persons who reported no swine contact (4
This investigation is subject to a number of limitations. First, interviews occurred when media sources began reporting “swine flu” linked to Fair A. Persons who became ill after attending Fair A may therefore have been more likely to report swine exposure, thus biasing toward an association between illness and swine exposure. Second, testing for influenza was not conducted for most case-patients. The timing of the investigation allowed for collection of few respiratory specimens and only convalescent-phase rather than paired serum samples. Serologic testing was further limited to young children because cross-reactive antibodies in older age groups made interpretation of test results for convalescent-phase serum samples difficult. Because only convalescent serum samples were obtained and baseline serologic studies for A(H3N2)v were conducted in a different population, it is possible that elevated HI titers among probable case-patients reflect exposure to A(H3N2)v virus before Fair A. Third, because all members of a household would often attend Fair A together, it was rare to identify ill contacts of case-patients who did not also attend Fair A. This made it difficult to evaluate human-to-human transmission in this population. Fourth, case-patients in the cohort study likely include some persons without A(H3N2)v virus infection, and some persons with mild or asymptomatic A(H3N2)v virus infection may have been considered noncase-patients; the resulting misclassification may have caused underestimation of any association between exposures and illness. One person identified during this investigation had rhinovirus infection identified by rRT-PCR testing, and it is possible that noninfluenza respiratory viruses circulated at Fair A and caused illness in some suspected case-patients. Fifth, the small sample size of the cohort limited our ability to detect statistically significant associations between exposures and case status. Last, we were unable to confirm influenza virus infection among swine at Fair A; therefore, the source of the A(H3N2)v virus cannot be confirmed.
Novel influenza A viruses will continue to emerge sporadically, but steps can be taken to reduce risks to human and animal health. Our findings suggests that swine contact increases risk for A(H3N2)v virus infection; therefore, advising fair attendees, especially those at high risk for complications from influenza, to avoid or limit swine contact may help prevent A(H3N2)v virus infections at agricultural events (28
). Agricultural club members and others with prolonged swine exposure should also be educated about the risk of zoonotic influenza transmission and actions they can take to reduce transmission risk, such as using personal protective equipment when they or their animals are ill (29
). We found simultaneous illness in humans and swine at the fair; this finding supports those from prior studies showing that transmission of influenza virus occurs from swine to humans and vice versa (30
). Preventing seasonal influenza in humans who have contact with swine (e.g., through annual influenza vaccination) can reduce reassortment opportunities in swine that become co-infected with swine and human influenza viruses. Prompt and thorough investigations should be conducted of all novel influenza virus outbreaks among humans and animals. Investigations can be more timely if patients with influenza-like symptoms inform clinicians of recent swine exposure and if clinicians consider variant influenza virus infection in patients with influenza-like symptoms and recent swine or agricultural fair exposure. Clinicians should work with public health officials to test respiratory specimens by rRT-PCR when they suspect variant influenza virus infection. This investigation was limited by the lack of influenza testing in swine. Representative and timely influenza surveillance among swine, especially during fair season in states where swine are present at agricultural events, would facilitate future investigations.
This outbreak of A(H3N2)v virus infections among persons attending an agricultural fair was likely associated with swine contact. We did not identify sustained human-to-human transmission of A(H3N2)v virus during this investigation; however, the identification of ≈300 human A(H3N2)v virus infections in multiple states during 2011 and 2012 and the occurrence of limited human-to-human transmission in small clusters (33
) demonstrate that variant influenza viruses remain a public health concern for animals and humans who may infect each other at venues such as agricultural fairs. Collaboration among public health officials with responsibilities for human and animal health is critical to determining the transmissibility and pandemic potential of variant influenza viruses, such as A(H3N2)v virus, and the epidemiologic features of illnesses caused by them.