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CDC Media Myths On Swine Contact By trH3N2 Cases
Recombinomics Commentary 12:45
September 12, 2011

Flu viruses occasionally jump from pigs to people, but generally there isn't ongoing transmission; rather, each infected person has had some direct contact with swine.

The above comment is from a Q&A in a media report today, which propagates another media myth: trH3N2 cases are linked to direct contact with pigs.  This myth is perpetuated by the CDC website on swine influenza in humans, and a CDC series of “Have you heard” reports for the media, which is used to perpetuate the myth.  In reality most trH3N2 testing is limited to patients with some loose linkage to swine (live in the area where swine is present or attended an agricultural fair), who develop flu-like symptoms in the off season, such as three for the four recent trH3N2 cases. 
Only one of the cases had reported contact with swine, a Schuylkill County resident (2F) in eastern Pennsylvania who visited the Washington County agricultural fair in western Pennsylvania on August 16.  This case visited the Emergency Department on August 20 and after testing positive for influenza A, her sample was PCR tested for trH3N2.  August 20 is during the off season for the northern hemisphere, so flu-like symptoms including breathing difficulties, coupled with a influenza A positive test and swine contact, led to the more specific test.

A more common scenario was reported for the case (2M) from Indiana, who did not have contact with swine.  Instead, his caretaker did have swine contact, although neither the swine nor the caretaker had flu-like symptoms and none were confirmed to be infected with any influenza (as detailed in the early release MMWR).  However, the swine linkage led to the trH3N2 test, not swine contact.

The same was true for the two Washington County cases, who attended the Washington County fair, but in contrast to the Schuylkill case, did not have swine contact.  The sequences from the two Washington County cases matched the Indiana case, not the Schuylkill case, although all four were infected with the same constellation of genes

Thus, all three cases who did not have swine contact had matching sequences, signaling human to human transmission, as was seen in the first two United States pandemic H1N1 cases reported in the April, 2009.  Neither had contact with swine or each other, but were infected with the same pandemic H1N1 indicating undetected human cases were widely transmitting the virus.

Moreover, a recent report indicated the M gene segment was critical for the aerosol human to human contact driving the spread of pandemic H1N1 and the same M gene segment has been acquired by the four recent trH3N2 cases in Indiana and Pennsylvania.

Thus, human to human transmission of trH3N2 is supported by the lack of swine contact in most cases, sequence identity between the Pennsylvania and Indiana cases, the acquisition of the M gene from pandemic H1N1, and the clustering of sequences in human trH3N2  including cases from 2010 with an H3 (and additional internal genes) closely related to the sequences from the 2011 cases.

The media myths on trH3N2 cases and swine contact continue to raise pandemic concerns. 

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