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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 likely occurs, but only rarely.
This is the pattern we’ve seen with the H5N1 bird flu virus, various other avian strains (H7s, H9s, H11s), and with 19 reported novel (H1N1, H3N2) swine flu strains detected in the United States since 2005.
Last month, the CDC reported:
Reported Human Infections with Swine Origin Influenza Viruses (SOIV) in the United States since 2005
As of December 17, 2010, 19 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 19 human cases reported since 2005, 12 have been trH1N1 viruses, six have been trH3N2 viruses and one has been a trH1N2 virus. All 19 persons infected with swine viruses recovered from their illness. Twelve cases occurred in children (persons younger than 19) and 7 cases occurred in adults. In 15 cases, exposure to swine 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.
In other words, these rare infections are important to monitor, but for now, pose a very low public health threat.
That could change, of course, if one of these viruses becomes better adapted to human physiology.
Yesterday, we began to hear reports of a single novel H1N1 swine flu case out of Jiangsu, China. Today, Hong Kong’s Centre For Health Protection (CHP) has released the following statement.
Notification of a human case of swine influenza in Jiangsu
The Centre for Health Protection (CHP) of the Department of Health received notification from the Ministry of Health (MoH) concerning a confirmed human case of European avian-like H1N1 swine influenza A in Jiangsu Province.
A CHP spokesman said the patient was a three-year-old boy living in Jiangsu, who had chronic renal disease and on long term steroid treatment. He developed symptoms on December 31 last year.
Laboratory tests on the patient's specimen by the Chinese Centre for Diseases Control and Prevention yielded positive result for European avian-like H1N1 swine influenza A.
His close contacts were asymptomatic.
The CHP is maintaining close liaison with the MoH.
The spokesman reminded members of the public to strictly observe the following measures to prevent influenza infection.
- Keep hands clean and wash hands properly. Alcohol-based hand rub is also effective when hands are not visibly soiled;
- Avoid touching eyes, nose or mouth;
- Wash hands with liquid soap promptly if they are contaminated by respiratory tract secretions, e.g. after sneezing or coughing;
- Cover nose and mouth while sneezing or coughing and dispose of nasal and mouth discharge properly;
- Always wrap nasal and mouth discharges with tissue paper, and dispose of the tissue paper properly in a lidded rubbish bin;
- Wear surgical mask when symptoms of respiratory tract infection or fever develop. Seek medical advice promptly;
- Refrain from work or school if you develop symptoms of influenza.
It is likely that limited infection by novel influenza reassortments such as this one, occur occasionally (and largely unnoticed) all over the world. Only rarely are these novel viruses `biologically fit’ enough to be passed on to others.
But each novel virus that jumps to a human host is another opportunity for the virus to adapt to human physiology. And as 2009 showed us, once that happens, a virus can spread around the world in a matter of months.
If you’ve not already read Helen Branswell’s excellent article on Flu Factories, or listened to the SciAm Podcast, you really should.
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 . . .
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