Epidemiological update: H3N2v cases in Ohio, US, 4 September 2014

Epidemiological update

​In August 2014, the US Centers for Disease Control and Prevention (CDC) reported two laboratory-confirmed human infections with a A(H3N2) variant influenza virus of swine origin A(H3N2)v in Ohio, USA.

No epidemiologic links were found between the two cases; however both had close contact with pigs at a county agricultural fair prior to disease onset.

Human cases associated with visits to pig fairs and close contact with pigs have been previously reported in the USA in 2011 (12 cases), 2012 (309 cases) and 2013 (19 cases).  

The nucleoprotein (NP) and matrix (M) genes originating from the 2009 influenza A(H1N1) pdm09 virus have been detected in the A(H3N2)v virus isolates from August 2014. This is a slightly different combination of genes compared to what has been observed previously in human cases of A(H3N2)v.

A(H3N2)v isolates from previous seasons had one of the following three combinations of genes from the pandemic human A(H1N1)pdm09 virus: • the “M” gene by itself;• both the “M” and “PA” (polymerase acidic) genes;• or the “M,” “PA” and “NP” genes” together.

The integration of the nucleoprotein deriving from A(H1N1)pdm09 in addition alone to the matrix protein is a new combination of genes, although the previous combinations already included both these genes. So far no information is available if the A(H3N2)v virus isolated in the 2 cases in Ohio is currently predominantly circulating in the U.S. pig population and replacing the variants detected in the previous years. 

Most of the human infections with A(H3N2)v influenza viruses of swine origin occurred in persons with previous contact to live pigs at county fairs or agricultural events.
The recent human infections with A(H3N2)v containing the M and NP genes from the A(H1N1)pdm09 were mild. Sustained human-to-human transmission was not documented for these influenza viruses of swine origin.