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Source – CDC
# 5954
While no details have been released yet (we’ll probably get something in tomorrow’s FluView ), today’s MMWR from the CDC lists an 8th trH3N2 human infection, once again from the state of Maine.
This makes the third case from Maine (or possibly not . . .see the update below), which - along with 3 others in Pennsylvania, and 2 out of Indiana – have been detected since late summer.
UPDATE: Based on comments offered by Alert on FluTrackers, I’m now not certain whether this 8th case reported in today’s MMWR represents a 3rd case in Maine, or is simply a delayed reporting of the 2nd case from late October.
We’ll need to wait for clarification from the CDC in order to know for sure. In any event . . .
Triple Reassortant (tr) influenza viruses contain genetic material from avian, swine, and human flu strains. In the case of the 7 reported over the past few months, the H3N2 swine virus has recently picked up the M gene segment from the 2009 H1N1 pandemic virus.
We’ll have to wait for confirmation from the CDC as to whether this latest trH3N2 virus matches the earlier isolates genetically (again, see the update above)
Epidemiological investigations are ongoing in all three states, and so far it appears that most of the cases have had recent opportunities for exposure to pigs. However, the exact route of transmission has not been firmly established.
For some background on all of this, the CDC updated their Swine Flu In Humans webpage three days ago:
Reported Human Infections with Swine Origin Influenza Viruses (SOIV) in the United States since 2005
As of October 31, 2011, 28 cases of human infection with swine origin influenza viruses have been reported in the United States. These are viruses that normally infect pigs. Like human influenza viruses, there are different subtypes and strains of swine-origin influenza viruses. The main swine influenza viruses circulating in U.S. pigs in recent years are swine triple reassortant (tr) H1N1 influenza virus, trH3N2 virus and trH1N2 virus.
Of the 28 human cases reported since 2005, 12 have been trH1N1 viruses, 15 have been trH3N2 viruses and one has been a trH1N2 virus. All 28 persons infected with swine viruses recovered from their illness. Twenty cases occurred in children (persons 18 or younger) and 8 cases occurred in adults. In 24 cases, direct or indirect exposure to swine prior to onset of illness has been identified.
Likely transmission of swine-origin influenza virus from close contact with an infected person has been observed in investigations of human infections with swine-origin influenza A virus, but has not resulted in sustained human-to-human transmission.
The concern here is that given enough opportunities, this virus might evolve into a more human-adapted virus.
Over the past couple of decades we’ve seen a small number of avian, swine, and triple reassortant viruses sporadically jumping from the barnyard to infect humans – but only one has managed to spark a pandemic.
Given limited testing and surveillance around the globe, how often these sorts of human infections really happen is unknown. But it is probably safe to assume we only learn about a fraction of them.
Like the H5N1, H9N2, H7N7, and other swine and avian flu strains, we watch these sporadic human trH3N2 infections with considerable interest - not because of the public health threat they pose today - but because of the threat they may potentially pose tomorrow.
Earlier blogs on these emerging triple reassortant viruses include:
CDC Update On trH3N2 Swine Infections
Maine Confirms A 2nd trH3N2 Case
CDC Update On trH3N2 Cases
A 5th trH3N2 SOIV Report
CDC Update On Recent Novel Swine Flu Cases
MMWR: Swine-Origin Influenza A (H3N2) Virus Infection in Two Children
