# 5917
On Thursday of this week I wrote about A 5th trH3N2 SOIV Report, where the latest human infection from a recently identified swine flu virus was reported in the state of Maine. Over the past three months four other similar cases have been detected, 1 in Indiana and 3 in Pennsylvania.
While not spreading like the A(H1N1)pdm09 virus did in the spring of 2009, this new SOIV (swine origin Influenza virus) is nonetheless of some concern, since over time it might better adapt to human physiology and become easier to transmit.
How often viruses like this one make the jump from pigs, birds, or other hosts to humans is unknown, but it probably happens more often than we know. Many viruses may prove only marginally adapted to humans and die out quickly.
With limited testing and surveillance, such one-off, dead-end infections would likely go unnoticed.
However, since 2005 improved testing and surveillance in the United States has picked up 26 novel swine flu infections (excluding the thousands of novel H1N1 infections that sparked the 2009 pandemic).
Most (but not all) had some exposure to pigs. As the CDC has previously indicated:
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.
In these five most recent cases, the virus has been a swine H3N2 virus with the M segment gene borrowed from the 2009 H1N1 virus. Essentially a hybrid – a new reassortant virus - that until this summer had not been seen before.
Reassorted viruses can result when two different flu strains inhabit the same host (human or otherwise) at the same time. Under the right conditions, they can swap one or more gene segments and produce a hybrid virus.
Five cases in three states over a few months does not a pandemic make, but this pattern does suggest that this new reassortant virus may be circulating (likely in pigs, but possibly in people) at low levels.
Which leaves us essentially in a mode of watchful waiting.
This novel virus may sputter and die out, finding too few susceptible hosts to keep going, or it could – over time – better adapt to humans and begin to spread.
The good news here is that so far, this new virus does not appear to be exceptionally virulent. The bad news is that this year’s vaccine is not expected to offer protection against it.
The CDC, along with many local health departments, continue to monitor the situation, and epidemiological investigations on how it is transmitted are ongoing.
While the current threat to public health appears low, the Sept. 2nd MMWR report on this virus reminds us:
What are the implications for public health practice?
Non-human influenza virus infections rarely result in human-to-human transmission, but the implications of sustained ongoing transmission between humans is potentially severe; therefore, prompt and thorough identification and investigation of these sporadic human infections with non-human influenza viruses are needed to reduce the risk for sustained transmission.
Yesterday, the CDC’s HAVE YOU HEARD website updated the situation with the latest case information from Maine. Follow the link to read it in its entirety, and I’ll return with a postscript.
CDC Confirms 5th Case of New Swine-Origin H3N2 Reassortant in Maine
October 21, 2011 – – CDC has confirmed the fifth case of human infection with a swine–origin influenza A (H3N2) virus that carries the M gene from the 2009 H1N1 virus. This virus was first detected in a child in Indiana in July. Subsequently three additional cases of human infection with swine–origin influenza A (H3N2) viruses carrying the same genetic change were detected in Pennsylvania. Though rare, human infections with swine–origin influenza viruses can occur, usually after close contact with infected swine. The patient in Maine was reported to have attended an agricultural fair where animals, including pigs, were present. The child was not hospitalized. He received antiviral treatment and is recovering from his illness. Though an investigation is ongoing and surveillance for influenza–like–illness is being enhanced, no human–to–human transmission of this virus has been identified in Maine.
The patient had not received a seasonal influenza vaccination. However it should be noted that seasonal influenza vaccine would NOT be expected to protect against this swine–origin influenza virus because it is so different from the human viruses in the seasonal vaccine. Seasonal influenza vaccination is important to decrease the risk of getting infected with human seasonal influenza viruses, which are beginning to circulate in the United States for the 2011–2012 season.
While there is not a vaccine to protect humans against these swine–origin influenza viruses, there are two FDA–cleared drugs that can be used to treat illness with these viruses. The antiviral drugs oseltamivir and zanamivir – which are used to treat infection with human seasonal viruses – also have shown activity against swine–origin viruses.
Anytime a novel influenza virus jumps species and demonstrates the ability to infect humans, it is a matter of both scientific curiosity and concern.
We’ve seen a number of novel flu viruses emerge over the past few years (H9N2, H7N7, H10N7, H5N1, etc.), and with the notable exception of the 2009 H1N1 pandemic virus, all have turned out to transmit poorly in humans.
But of course, influenza viruses are notoriously unstable, constantly changing and evolving. What is true today may not hold true tomorrow.
While it is always possible that the next pandemic will spring directly from the wild, the odds favor that it will come from a farm – where large numbers of animals intermingle, swap viruses, and come in daily contact with humans.
For more on the reassortment potential of avian, swine, and human flu viruses, you can’t do better than Helen Branswell’s excellent Scientific American article from last December called Flu Factories, or her SciAm Podcast interview.
For earlier posts on this newly discovered trH3N2 swine flu virus, you may wish to revisit:
CDC Update On Recent Novel Swine Flu Cases
MMWR: Swine-Origin Influenza A (H3N2) Virus Infection in Two Children
