May 20, 2015
2 min read

Swine exposure at agricultural fair source of H3N2 outbreak

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Several cases among a multistate outbreak of variant influenza A(H3N2) may have resulted from swine exposure during an agricultural fair, according to recently published data.

“Variant influenza viruses pose a public health threat because pre-existing immunity in the general population may be low, and current seasonal influenza vaccination may not confer protection against them,” the researchers wrote. “From January 2005 to June 2012 only 36 cases of variant influenza virus infections were identified in the United States. However, during July 2012 through September 2012, an outbreak of variant influenza A(H3N2) virus involving 10 states resulted in 306 confirmed cases.”

Researchers examined H3N2 cases at an Ohio agricultural fair, one of the earliest identified sources of infection during the 2012 outbreak. Cases of influenza-like illnesses (ILI) within the county were surveyed and lab-tested for H3N2 virus, while the households of swine exhibitors at the fair were surveyed for ILI and H3N2 risk factors. Confirmed cases were defined as those with positive reverse-transcription PCR test results; probable cases met clinical and exposure criteria but either did not have testing performed or demonstrated inconclusive results. Possible cases of human-to-human transmission were determined through contact tracing.

The researchers identified 20 confirmed and 94 probable cases associated with the fair. Of these, 85% had either direct or indirect exposure to swine, 13% attended the fair but did not report swine exposure, 3% did not attend the fair, and 3% had an unknown source of exposure. The median age of these 114 cases was 10 years, 51% were female, and the median length of illness was 3 days.

Six confirmed and 40 probable cases were identified among the 359 people surveyed within the exhibitor household cohort; 52% of whom were swine exhibitors. In addition, eight potential cases of human-to-human transmission were investigated, but only one instance between siblings was considered to be likely.

“This investigation in Ohio provided data for public health decision making in response to the larger [H3N2 virus] outbreak in 10 states,” the researchers wrote. “The low severity, relatively low attack rate and lack of sustained human-to-human transmission helped determine that this virus had low pandemic potential, an insight that helped shape a focused public health response to this outbreak.

“Vigilant surveillance during future agricultural fair seasons will be essential to determine if ongoing transmission of [H3N2 viruses] between swine and humans continues or if efficient human-to-human transmission of variant viruses emerges.” – by Dave Muoio

Disclosure: The researchers report no relevant financial disclosures.