CDC Comments On Swine H3N2 Cases In United States
Recombinomics Commentary 21:07
November 12, 2010

Two cases of human infection with a novel influenza A virus were reported: one case by the Wisconsin Department of Health Services and one case by the Pennsylvania Department of Health. Both patients were infected with a swine origin influenza A (H3N2) virus similar to the three other swine origin influenza A (H3N2) viruses previously identified in 2009 and 2010. The Wisconsin case reported contact with pigs in the week preceding symptom onset on September 8, 2010 and required hospitalization. No contact with pigs has been identified in the Pennsylvania case in the week before symptom onset on October 24, 2010; however the case lives in an area close to pig farms. Both patients have fully recovered from their illness. The cases are not related and the viruses from these two cases have some genetic differences, indicating that they did not come from the same source.

The above comments are from the CDC’s week 44 FluView and provide detail on the two cases which were in this week’s pager alert.  The sequence differences cited above would eliminate clonal expansion of a recent swine H3N2 jump to humans.  However, the two cases increase the number of reported swine H3N2 jumps to humans in the United States, and all cases have been infected since May 2009. 

Moreover four of the five cases were reported in 2010, raising concerns that such jumps have increased to frequencies seen just before swine H1N1 jumped to humans, producing the 2009 pandemic.

All of these H3N2 cases involved swine triple reassortants, with a constellation of internal genes that are similar to pandemic H1N1.  However, only sequences from the earliest case (22M) have been made public (A/Kansas/13/2009 released at GISAID by the CDC).  As expected the H3 is human and most closely related to H3 sequences circulating in the mid-90’s (see list here).  The N2 is also human and most closely related to H1N2 sequences from 2003 (see list here).  The CDC update indicated all five swine H3N2 sequences in the human cases were similar, raising concerns that minor changes in these sequences has led to the increased frequency of reported swine H3N2 cases.

Release of full sequences from the four cases reported this year would be useful.

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