Infected Swine Transmit H3N2 Variant Virus to State Agricultural Fair Attendees


In a recent CDC Morbidity and Mortality Weekly Report, authors reported on an outbreak of influenza A(H3N2) variant (H3N2v) virus at agricultural fairs in Ohio and Michigan during the summer of 2016 due to infected swine.

In a recent Morbidity and Mortality Weekly Report, authors reported on an outbreak of influenza A(H3N2) variant (H3N2v) virus at agricultural fairs in Ohio and Michigan during the summer of 2016.

Novel influenza viruses contain gene segments from human and swine influenza viruses; these gene segments resort to form new viral strains. Novel influenza viruses raise the risk of a pandemic if they are able to efficiently spread from person to person. In the United States, human infections with a novel influenza A virus are reported to the Centers for Disease Control and Prevention (CDC).

The H3N2 virus is one of the four influenza A virus subtypes found in swine. It is referred to as ‘variant’ if it is transmitted from infected swine to people. Most H3N2v infections occur in children under 18 years of age who come in direct contact with infected swine. From 2011 to 2014, 343 people were infected with H3N2v in the United States.

In the current outbreak, initial identification of H3N2v virus occurred in two children who developed respiratory illnesses in late July 2016. In Ohio, a 13-year-old became ill after attending an Ohio agricultural fair. The Ohio Department of Health laboratory tested a respiratory specimen from the child using real-time reverse transcriptase PCR (rRT-PCR); the specimen tested positive for H3N2v virus. In Michigan, a 9-year-old child became ill after attending a Michigan agricultural fair; the Michigan Department of Health and Human Services laboratory determined that the respiratory specimen from this child also tested positive for H3N2v virus. Both infections were reported to the CDC in early August 2016.

Respiratory specimens were then collected from persons with respiratory illness who had either swine exposure or attended agricultural fairs in Michigan and Ohio. Specimens were also collected from swine that were at the fairs. All specimens were tested using rRT-PCR.In total, 18 human H3N2v virus infections were reported to the CDC, which verified the infections and performed genomic sequence analysis on the specimens. Six cases were from Ohio and twelve were from Michigan. Infected persons attended one or more of seven agricultural fairs.

Nearly all infected persons were children. Sixteen were under 18 years of age, with seven being younger than five-years-old. All eighteen persons reported exposure to swine at the fairs, with thirteen having direct swine contact (touching or handling). Of the five persons with indirect swine contact, four reported passing through swine barns and one did not specify the type of indirect contact. None of the infected persons reported having come in contact with others known to be infected with H3N2v virus. In addition, there was no identification of person-to-person transmission of the virus.

Eight of the infected persons were considered ‘high risk’ due to underlying disease or young age; one of these eight persons required a two-day hospitalization. Fortunately, all infected persons fully recovered from the infection, with six being treated with influenza antiviral medication.

Seventeen persons had a known vaccination history. Of these seventeen, three received a seasonal flu vaccine in the 12 months preceding agricultural fair attendance.

Sixteen of the eighteen variant viruses (one variant virus per person) were reassortants, meaning they contained gene segments from human and swine influenza viruses. These 16 variant viruses contained a hemagglutinin (HA) gene that was similar to HA genes detected in the 2010 and 2011 human seasonal H3N2 viruses. Authors believed this HA gene spread from humans to swine in 2010 or 2011, then evolved within the swine population until it was genetically and antigenically different from human seasonal H3N2 viruses.

Genomic analysis identified the matrix (M) gene in all fully sequenced viruses from the human specimens. H3N2v virus outbreaks since 2011 have contained this gene. Resistance to several antiviral medications (amantadine, rimantadine) is associated with the M gene. Genomic analysis also revealed a genetic relation between the H3N2 viruses identified in the infected persons and those identified in the swine at the same agricultural fairs that the infected persons attended.

Results of this outbreak emphasize the importance of using protective measures to prevent the spread of variant influenza virus from infected swine to people at agricultural fairs; these measures are especially important for those most-susceptible to influenza complications, such as children under five years old and adults 65 years of age or older. The authors suggested several preventive measures for agricultural fair organizers:

  • Keeping swine on the fairgrounds for ≤ 72 hours
  • Immediately isolating all sick swine
  • Keeping a veterinarian on call while swine are on exhibit
  • Placing prominent handwashing stations in or near animal barns
  • Posting signage prohibiting or discouraging eating and drinking in animal barns
  • Discouraging ‘high-risk’ persons from entering animal barns

Although the human seasonal flu vaccine does not protect against swine influenza viruses, authors suggested the vaccine could lessen the likelihood of continued reassortment of gene segments between swine and human seasonal influenza viruses.

Dr. JoAnna Pendergrass received her doctorate in veterinary medicine from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, LLC.

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