Last summer, before anyone had ever heard of MERS-CoV (initially dubbed novel coronavirus or nCoV) and the H7N9 avian flu virus, public health officials across the nation were facing large outbreaks from two emerging disease threats; West Nile Virus (WNV) and Swine Variant Influenza.
We’ve discussed WNV several times recently (see Texas: First West Nile Case Of 2013, DVBID: 2012 Record Number Of West Nile Fatalities), but swine variant influenza has been pretty much crowded out of the headlines since late last fall.
With state and county fair season once again upon us (it runs June-November around the nation) – tens of thousands of fairgoers will once again be coming into relatively close contact with farm animals – providing fresh opportunities for swine variant flu viruses to make the jump to humans.
Although the actual number of human variant flu infections that occur each year is unknown, the CDC has reported 1 or 2 cases each year since 2005.
In 2010, that number jumped to 8, and in 2011, to 12.
These cases were generally mild, and have primarily involved children under the age of 10. They have generated enough concern, however, that in late 2011 the CDC announced their intention to work on a candidate vaccine (see H3N2v Vaccine Trials).
Last summer the floodgates opened, with more than 300 confirmed cases, nearly all linked to county or state fair attendance.
Indiana reported the most cases (n=138), followed by Ohio (n=106).
While the H3N2v virus was by far the most commonly reported strain, H1N1v and H1N2v were also detected.
As to why these swine variant viruses suddenly began jumping to humans with vigor last year?
Starting in 2011, the H1N1v virus was found to have acquired the M (matrix) gene from the 2009 H1N1 pandemic virus. Since then, this M gene has been showing up regularly in all three swine variant viruses (H1N1v, H1N2v, H3N2v).
In September of last year, the CDC published MMWR: H3N2v Related Hospitalizations In Ohio – Summer 2012 that provided an overview of 11 patients from Ohio that required hospitalization, including the nation’s only H3N2v related fatality.
While fairs have instituted inspections for any signs of illness in livestock – as we discussed last October (see Asymptomatic Pigs: Revisited) - pigs can carry this virus without showing any outward signs of infection.
The CDC offers the following advice for fairgoers and exhibitors.
Swine are highly susceptible to flu viruses - can serve as `mixing vessels’, allowing viruses to reassort into new hybrid strains – and were the originating host of the 2009 H1N1 pandemic virus.
The variant viruses that now carry the M gene from the 2009 H1N1 virus are reassortants as well.
The concern with these variant swine flu infections, as it is with any novel flu, is that it gives the virus another opportunity to better adapt to human physiology.
While humans have a long history of exposure to seasonal H3N2 flu viruses, research has shown only limited community immunity to these variant strains (see CIDRAP: Children & Middle-Aged Most Susceptible To H3N2v).
The good news is that while some limited community transmission of these variant flu strains appears to have occurred last summer, sustained and efficient human-to-human transmission did not.
Public health officials will no doubt be alert for any new cases - and more importantly, for any signs that the behavior of the virus has changed - over the coming months.
For more on the flu risks from swine reassortments, I’d heartily recommend Helen Branswell’s terrific piece in SciAm from late 2010 called Flu Factories.