# 5559
Pigs, as everyone probably knows by now, are highly susceptible to many types of flu; human, avian, and swine.
It is this susceptibility, and the fact that they can be infected by more than one strain at the same time, that makes them ideal `mixing vessels’ for influenza.
The 2009 H1N1 `swine flu’ apparently kicked around the world’s pig population for many years – picking up bits and pieces of genetic material from other strains – until it eventually evolved into a virus that was easily transmitted among humans.
Shift, or reassortment, happens abruptly when two different influenza viruses co-infect the same host and swap genetic material.
Influenza A viruses have 8 gene segments (PB2, PB1, PA, HA, NP, NA, M1, M2, NS1, NS2).
Which means that any two compatible influenza viruses could conceivably – and under the right conditions – generate more than 250 different combinations by swapping one or more of their 8 (potentially) interchangeable gene segments.
The key words being “under the right conditions”.
If it were easy, or a common occurrence, we’d be up to our hip boots in new, reassorted viruses all the time.
But it happens often enough that we recognize it as a real threat. Shift is how new pandemic strains are born, and it was precisely the mechanism that created the 2009 `swine’ flu strain.
While `humanized’, the resultant 2009 H1N1 virus has also continued to circulate in pigs, and has been detected in domestic cats, turkeys, ferrets, and even a badger (see Companion Animals And Novel H1N1).
So scientists continue to monitor pigs – at least in a few places around the globe – hoping to detect any changes in the H1N1 virus that might signify a new public health threat.
Which brings us to the most recent Hong Kong surveillance report, covering slaughterhouse testing between February and April of this year. Not unexpectedly, a couple of pigs tested positive for the H1N1 virus.
None were reported to have any significant genetic mutations.
First the report, then I’ll return with more.
Results of regular influenza virus surveillance in pigs from February to April released
The Centre for Food Safety (CFS) of the Food and Environmental Hygiene Department (FEHD) today (May 18) announced results for February to April under a regular influenza virus surveillance programme for pigs conducted by the University of Hong Kong (HKU) at the Sheung Shui Slaughterhouse. Two samples taken in February tested positive for the human swine influenza (pandemic H1N1) virus, but no significant genetic reassortment of viruses was found.
A spokesman for the CFS said that given the wide transmission of the pandemic H1N1 virus in humans, detection of the virus in pigs would not be a surprise. It was expected that positive findings might appear from time to time in HKU's surveillance programme in future.
Under the regular influenza virus surveillance programme for pigs, the CFS has been helping HKU researchers by collecting blood, tracheal and nasal swabs from pigs at the Sheung Shui Slaughterhouse twice a month.
The spokesman said, "The CFS would continue to monitor reports of the HKU surveillance programme and make announcements on a regular basis. Results will be announced immediately if there are significant public health impacts such as genetic reassortment of viruses."
Although bird flu captured the pandemic headlines up until a couple of years ago, since the outbreak of H1N1 in 2009 - which circulated under-the-radar in pigs for about 10 years – scientists are looking at pigs, and swine influenzas with keener interest today.
With global pig production growing rapidly to meet the demands of a hungry world, each year we add millions more `mixing vessels’ to natures laboratory. And the bulk of the predicted growth in hog farming is expected in developing countries.
Source: FAO
While extremely limited viral surveillance of pigs may be found in a few parts of the developed world - for much of the globe - that simply doesn’t happen.
Unfortunately, even in the U.S. some hog farmers are reluctant to allow their herds to be tested (see Swine Flu: Don’t Test, Don’t Tell) out of fears that the discovery of a new swine flu virus would depress pork sales.
Some will only consent to anonymous testing, which limits the CDC’s ability to investigate cases.
Every once in awhile we get a report from somewhere in the world that a human has been infected with a novel influenza virus, usually of swine, avian - or quite often - mixed origin.
Rarely, as in the case of the 2009 swine flu virus, a novel strain adapts well enough to humans to spread efficiently through the population.
Most of the time, however, these are one-off type infections that are directly transmitted from an animal (pig, bird) to a single human. Secondary transmission of the virus to others probably occurs, but only rarely and in a limited fashion.
This is the pattern we’ve seen with the H5N1 bird flu virus, various other avian strains (H7s, H9s, H11s), and with 20 reported novel (H1N1, H3N2) swine flu strains detected in the United States since 2005.
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).
And in February the CDC reported:
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 occur occasionally (and 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.
It’s probably more often than we think.
These rare SOIV infections are important to monitor and analyze, of course, and may give us an early warning about the next pandemic threat.
But for now they pose a very low public health threat.
If you’ve not already read Helen Branswell’s excellent article on Flu Factories, or listened to the SciAm Podcast, you really should.
While it is always possible that the next pandemic will emerge from the wild, the odds say it will come from a farm – where large numbers of animals intermingle, swap viruses, and come in daily contact with humans.
Which is why increasing our surveillance of livestock for zoonotic diseases must become a priority.
Over the past 5 years, we’ve revisited the subject of influenza reassortment dozens of times. For more on this topic, you may wish to visit:
Virology Journal: Receptor Cells In Minor Poultry Species
mBio: A/H1N1 Potential For Mutation
Study: The Continuing Evolution Of Avian H9N2
EID Journal: Co-Infection By Influenza Strains
EID Journal: Swine Flu Reassortants In Pigs
If You’ve Seen One Triple Reassortant Swine Flu Virus . . .
