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2012 Media Myth On H3N2v Transmission
Recombinomics Commentary 02:00
April 13, 2012

After a lull of several months, an influenza virus that is sporadically jumping from pigs to people in the United States has made yet another appearance.

A department spokesperson said via email Thursday that an investigation is ongoing and that information is still being gathered, “but there is no indication that there were ill pigs at the plant.”

The above comments are from a Helen Branswell piece on the Utah girl infected with H3N2v which is said to be similar to the 12 human H3N2v cases in 2011, which had an H1N1pdm09 M gene, which is also true for the 2012 Utah case described above.  6 of the 12 cases in 2011 were linked to a swine exposure, but none have been linked to swine with confirmed H3N2v.  Moreover, like the above case, 5 of the 6 swine exposure cases were only linked to asymptomatic swine, and testing of the symptomatic swine associated with the one case, A/Maine/06/2011, was negative.  The above comments from the USDA indicate the Utah swine are also asymptomatic.

The USDA has increased surveillance after the spread of H1N1pdm09 in humans and swine in 2009.  Although 100’s of swine H1N1v, H1N2v, and H3N2v have been detected, including increasing numbers with an H1N1pdm09 M gene, only two of the isolates match the lineage found in the first 10 human cases in 2011, and neither of the two swine isolates were linked to human cases.

The association of swine exposure to H3N2v detection is driven by testing, which relies upon a PCR test that is based on cross reactivity between swine H3 and seasonal H3, as well as cross reactivity between H3N2v NP and H1N1pdm09 NP.  Consequently, the sensitivity and specificity of the test is far from ideal and confirmation is based on sequence data by the CDC, which is exclusively in samples collected in the off season or at the beginning of end of the flu season.  None of the confirmed H3N2v cases have been due to infections in January or February, when the seasonal flu season peaks.

In 2011 the first case was in late July and cases continued through early December.  The Utah isolates signals the start of the 2012 season, and the reported similarity between the 2012 case and the 2011 cases again supports human to human transmission.  The only two matching swine isolates were in September, 2011 (New York and Iowa), which followed the human cases in July and August (Indiana and Pennsylvania).

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