# 6466
It’s been nearly a year since we first heard about a new strain of swine influenza - originally dubbed trH3N2 but now called H3N2v – that began jumping from pigs to humans last summer.
Since this swine flu strain is once again making headlines, a review of this upstart virus’s brief history seems appropriate.
While uncommon, human infection by swine-origin flu viruses are not unheard of, and probably occur more often than official numbers would suggest.
Between 2005 and the end of 2010, the CDC documented 19 human infections by swine origin influenza viruses (SOIV) across the United States, 12 of which were trH1N1 viruses, 6 were trH3N2, and 1 was trH1N2.
During the summer of 2011 a new swine flu virus was detected in the Midwest, and It has been popping up sporadically across the country ever since. We first heard about in on Sept. 2nd, 2011, via an early release from the CDC’s MMWR.
That report described two young children – one in Indiana and another in Pennsylvania - who were infected by a new swine-origin H3N2 virus (see Swine-Origin Influenza A (H3N2) Virus Infection in Two Children --- Indiana and Pennsylvania, July--August 2011).
What made this virus different from the earlier trH3N2 novel strains was that it was a reassortant swine H3N2 which had acquired the matrix (M) gene from the 2009 H1N1 pandemic virus.
Reassortant H3N2 virus detected in Pennsylvania & Indiana – Source CDC
While these first two cases first appeared to be one-off, dead end transmissions, it was only a few days later that Pennsylvania Reported 2 More Novel Flu Cases.
All three cases in Pennsylvania were linked to the Washington County Agricultural Fair during the week of Aug. 13-20, 2011.
On September 9th, the CDC updated their SOIV (Swine Origin Influenza Virus) page (see CDC Update On Recent Novel Swine Flu Cases), acknowledging the possibility that limited human-to-human transmission of this trH3N2 virus might be occurring.
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. – CDC
In late October we saw A 5th trH3N2 SOIV Report, this time out of the state of Maine. On November 1st, we learned that Maine Confirmed A 2nd trH3N2 Case.
Three weeks later, the Iowa Department of Public Health announced that they have detected three cases of novel trH3N2 SOIV as well. The following day, in More Details On The trH3N2 Story we learned that the virus had likely been passed from one child to two others.
The CDC’s MMWR on November 23rd detailed the Iowa cases in a dispatch called Limited Human-to-Human Transmission of Novel Influenza A (H3N2) Virus — Iowa, November 2011.
By now, it was pretty apparent that this swine H3N2 virus had a greater affinity to human hosts than most of the other SOIVs we’d seen in the past (excluding the 2009 H1N1pdm virus).
Near the end of November the USDA reported they had detected a number of reassortant H3N2 viruses in swine, including 8 that displayed the 2009 H1N1 matrix (M) gene (see CIDRAP article New details emerge in novel H3N2 reports).
And in early December we learned that the CDC was Developing A trH3N2 Seed Vaccine . . . just in case this virus continued to spread.
December also saw a few more scattered cases in Minnesota and West Virginia, and the virus was given a new name WHO/FAO/OIE: Call It A(H3N2)v. By the end of the year, there were an even dozen cases reported around the country.
Reports of new infections died off after the first of the year, however. With just one case reported out of Utah in March (see CDC Update On Utah H3N2v Infection).
That is, until last week.
Four new cases were detected in Indiana (see Reports Of H3N2v In Indiana) in mid July. In rapid succession, another case was reported this week in Hawaii, and there are additional suspected cases in Indiana and Ohio.
While the `official’ number now sits at 18 cases, that figure may well increase over the coming days as new test results are confirmed.
But the truth is, the number of cases documented by the CDC has likely never fully represented the number of cases from around the country. As the pyramid chart below indicates, only a fraction of diseases can be picked up by surveillance.
Credit CDC
Any time a novel virus emerges in another species and jumps to humans – even in limited numbers – it gets our attention. This has been the route by which pandemics have started in the past.
But before anyone decides to head down to the bunker, a few points to consider.
Novel flu viruses - like H3N2v - have probably always circulated in the human population at very low levels, but it is only recently that our surveillance systems have been able to detect and identify them.
Most of these viral contenders likely withered on the vine, never adapting well enough to human physiology to spread into a genuine public health threat.
The handful of human trH1N1, trH3N2, and trH1N2 infections detected between 2005-2010 are good examples of swine flu viruses that tried, and failed, to thrive in the human arena.
H3N2v appears better poised than its predecessors, but the jury is out as to whether, and when, that will ever happen.
And so far at least – reports suggest that this H3N2v strain is not particularly virulent. All known cases have recovered, and most did not require any specialized medical care.
The caveat being that flu viruses are constantly changing, and their behavior today may not hold true tomorrow.
In the short term, it looks as if the H3N2v virus is becoming better entrenched in swine herds, and that should afford it more opportunities to jump to humans.
Which leaves us with a fascinating viral interloper to observe against the backdrop of our upcoming flu season, but no clue really as to how any of this will turn out.
H3N2v might sputter and die out over time, or it might eventually adapt well enough to humans to spread efficiently. And depending on its virulence, and the degree of existing community immunity, it could even represent a serious public health threat.
For now, there are a lot of possibilities.
Stay tuned.
