UPDATED: The CDC released a lengthy report Friday afternoon (9/2/11) on these cases and you can read about it at MMWR: Swine-Origin Influenza A (H3N2) Virus Infection in Two Children.
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Adding to our constantly evolving pool of human influenzas viruses are the rare introductions of new, `novel’ flu viruses that jump from other species (usually avian or porcine) to humans.
This week the media’s focus has been primarily centered on the H5N1 bird flu virus (see FAO Warns On Bird Flu) - which continues to be poorly adapted to human physiology - but there remain concerns over novel swine viruses as well.
In The (Swine) Influenza Reassortment Puzzle last December, I wrote about the then 19 (and now 23) human infections by novel swine viruses that have been detected in the United states since 2005 (excluding, of course, the 2009 H1N1 Swine flu).
The H1N1 virus that sparked the 2009 pandemic was a descendent of a triple reassorted H1N1 (trH1N1) swine flu virus that first appeared in North American swine herds in 1998.
That strain bounced around in swine herds for at least a decade before acquiring the right genetic changes to enable it to spread efficiently among humans.
And once that happened, the virus spread quickly as a pandemic.
While successful adaptation of a novel swine flu virus to humans happens only rarely, we do see occasional limited transmission of reassorted swine flu viruses to humans, mostly among people in direct contact with infected livestock.
For the most part, these viruses don’t appear to transmit well between people - and so far - only rarely are these infections passed on to others.
Over the past few years influenza surveillance systems in the United States have detected nearly 2 dozen such infections, but it is highly likely that others have occurred without our knowledge.
This week, in the CDC’s MMWR we learn of two recent novel flu infections – both triple reassorted H3N2 swine viruses – that have been detected in Pennsylvania (1) and Indiana (1). The MMWR summarizes saying:
During 2009, four cases of human infection with novel influenza A viruses, different from the 2009 pandemic influenza A (H1N1) strain, were reported to CDC.
The four cases of novel influenza A virus infection reported to CDC during 2010, and the four cases reported during 2011, were identified as swine influenza A (H3N2) virus and are unrelated to the 2009 pandemic influenza A (H1N1) virus.
We have very little information on the epidemiology of these latest cases, but hopefully we’ll learn more in time. We do have this update from earlier this year on the CDC’s SOIV (Swine Origin Influenza Virus) page:
Reported Human Infections with Swine Origin Influenza Viruses (SOIV) in the United States since 2005
As of January 25, 2011, 20 cases of human infection with swine origin influenza viruses (SOIV) 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 viruses circulating in U.S. pigs in recent years are swine triple reassortant (tr) H1N1 influenza virus, trH3N2 virus and trH1N2 virus.
Of the 20 human cases reported since 2005, 12 have been trH1N1 viruses, seven have been trH3N2 viruses and one has been a trH1N2 virus. All 20 persons infected with swine viruses recovered from their illness.
Thirteen cases occurred in children (persons younger than 19) and 7 cases occurred in adults. In 16 cases, direct or indirect exposure to swine prior to onset of illness has been identified.
Although no person-to-person transmission of swine influenza viruses has been laboratory confirmed in the investigation of these cases to date, some cases reported only exposure to ill persons and no exposure to live pigs.
Thus, limited person-to-person is likely to have occurred.
It is likely that limited infection by novel influenza reassortments such as these happen largely unnoticed all over the world. Since only a tiny fraction of influenza flu viruses are ever sequenced, we really don’t know how often these types of novel infections occur.
Last January Hong Kong’s Centre For Health Protection (CHP) released a statement regarding a single human infection by a novel European avian-like H1N1 swine influenza A in Jiangsu Province (see China: Single Novel Swine Flu Infection Reported).
While not particularly alarming, we watch reports of these infrequent viral escapees from the farm with considerable interest. Until one adapts well enough to transmit efficiently among humans, they pose a very low public health threat.
But, as saw in 2009, given the right circumstances
. . . that can happen.
All of which highlights the importance of establishing better global surveillance of humans, and of farm animals, for the next emerging influenza virus.
While it is always possible that the next pandemic will emerge 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 background on all of this, I would heartily recommend Helen Branswell’s excellent article in Scientific American last December called Flu Factories, and her SciAm Podcast.
You may also wish to revisit some of these earlier blogs.
Two Reassortment Studies To Ponder
Hong Kong: Influenza Surveillance In Pigs
EID Journal: Swine Flu Reassortants In Pigs
If You’ve Seen One Triple Reassortant Swine Flu Virus . . .
