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Influenza H3N2v Infection

H3N2v influenza caused a twelve-state outbreak during the summer of 2012, totaling 309 cases resulting in 16 hospitalizations and 1 fatality. Most cases were associated with exposure to pigs at agricultural fairs, although limited person-to-person spread of this virus has been documented. In 2013, the first cases of influenza A H3N2v virus infection were reported in June 2013 in Indiana. Genetic sequencing by CDC confirmed that these H3N2v viruses were nearly identical to those detected during summer 2012. 

Infected pigs may spread influenza viruses even if they are not symptomatic. Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine, but are called variant viruses when isolated from humans. Influenza A H3N2 variant (H3N2v) viruses contain the matrix (M) gene from the 2009 H1N1 pandemic virus. This M gene appears to confer increased transmissibility to and among humans, compared to other variant influenza viruses. While the viruses identified in these cases are genetically nearly identical, separate swine exposure events in each state were associated with human infections.

Clinically, H3N2v resembles seasonal influenza, with fever, cough, pharyngitis, myalgia, and headache. Clinicians should consider antiviral treatment with oral oseltamivir or inhaled znamivir in patients with suspected or confirmed HCN2v virus infection.

Although rapid influenza antigen tests may detect H3N2v virus, a negative result does not exclude infection with H3N2v or any influenza virus. Additionally, a positive result for influenza A by rapid antigen does not confirm H3N2v virus infection, because these tests cannot differentiate subtypes. The BioFire Respiratory PCR panel is more sensitive than rapid antigen tests and differentiates the H3 subtype, but does not distinguish the swine variant from seasonal H3 influenza.

CDC recommends that patients with suspected influenza and recent exposure to swine should have respiratory specimens collected for sub-type specific real-time polymerase chain reaction (RT-PCR). Specimen requirement is a nasopharyngeal swab or aspirate in viral transport medium. Testing will be forwarded to a state public health laboratory.

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