It’s been awhile since I wrote anything on influenza. It’s certainly not that nothing interesting has happened recently–far from it, there are new stories on influenza out every day. Rather, there are just a lot of people out there covering it, and covering it well. However, it’s been an unusually busy few days on the influenza front, so I thought I’d update after the jump.
First, though much of the mainstream media has lost interest in avian influenza, scientists are still busy keeping an eye on things–and H5N1 is still spreading. Human cases have now been reported in Pakistan, Benin, and Myanmar. The year is winding down, and there have been fewer human H5N1 cases reported in 2007 than 2006 (77 versus 115), but mortality remains high (51 deaths this year, versus 79 last year, as tallied by WHO).
Perhaps more interesting is a new paper out in PNAS, which showed that a novel influenza virus has been found in swine–a serotype H2N3.
Why is this a concern?
First, H2N3 viruses haven’t been previously reported in swine–and these were found in ill animals on Missouri farms, where they still appear to be circulating.
Second, H2 viruses haven’t circulated in the human population since the strain which caused the 1957 pandemic (an H2N2 serotype virus) was replaced by the 1968 pandemic strain (serotype H3N2). Therefore, those who were born after 1968 don’t have any immunity to serotype H2 viruses.
Third, this virus is already adapted to mammals, and was successfully transmitted among both pigs and ferrets. This is concerning as far as transmission among humans: to have pandemic potential, a virus must successfully spread from human to human. H5N1 seems to have done this occasionally, but most cases have been in humans who have had contact with the reservoir host: birds. With a virus that can already spread in mammal populations, the chance of it doing the same in a human population is likely greater.
The press has died down a bit regarding H5N1, but as Robert Webster reminded us, we’ve no guarantee that would be the next pandemic virus, and we need to keep an eye out for other emerging influenza viruses as well. In the PNAS paper, they noted that many swine were seropositive for the H2N3 virus (suggesting prior infection), but one thing they didn’t do was see if any of the swine workers had evidence of infection. Previous work by my colleagues has shown evidence of infections with swine influenza viruses in humans–is this strain of H2N3 crossing over as well?
Ma et al. 2007. Identification of H2N3 influenza A viruses from swine in the United States. PNAS. Link.
More information on the H2N3 paper can be found at this CIDRAP article.