For the second time in two months, authorities in Hong Kong have detected a human infection with the H9 avian flu virus. This is only the 7th known case of H9 jumping to humans, and all of those cases have been reported in either Hong Kong or China.
While considered rare, H9 infection in humans may be more common than we know. According to CIDRAP’s excellent overview Avian Influenza (Bird Flu): Implications for Human Disease:
H9 is only known to include LPAI strains. H9N2 viruses had been isolated in multiple avian species throughout Asia, the Middle East, Europe, and Africa.
Most of these places are areas where, quite frankly, testing and surveillance for H9 human flu infections are unlikely to occur.
Hong Kong appears to be a hotbed for H9 reports simply because they actively test for avian viruses. A legacy of having narrowly dodged the H5N1 bullet in 1997.
Last year, in a PLoS ONE journal article:
Wan H, Sorrell EM, Song H, Hossain MJ, Ramirez-Nieto G, et al. 2008 Replication and Transmission of H9N2 Influenza Viruses in Ferrets: Evaluation of Pandemic Potential. PLoS ONE 3(8): e2923. doi:10.1371/journal.pone.0002923
Researchers discussed the pandemic potential of the H9 viruses, and they conclude that `results suggest that the establishment and prevalence of H9N2 viruses in poultry pose a significant threat for humans.’
They go on to say:
However, considering the widespread prevalence of H9N2 viruses in poultry, the human virus-like receptor specificity of some avian and swine H9N2 isolates, co-circulation of H9N2 with H3N2 viruses in Asian swine, and the repeated direct transmission to humans, the public health threat of H9N2 viruses cannot be overemphasized.
If all of this sounds a bit ominous, it may help to know that all of the H9 human infections detected to date have been relatively mild, that human-to-human transmission has not been detected, and that the H9 virus has managed to circulate in birds for many years without sparking a pandemic.
Of course, past performance is no guarantee of future results.
The H9N2 virus has reassorted into many different clades, some having the ability to bind to α2-6 receptor cells found in the upper airway of humans. The authors write:
A significant proportion of H9N2 field isolates have acquired human virus-like receptor specificity, preferentially binding α2-6 linked sialic acid (SAα2-6) receptors, in contrast to the classic avian virus-like receptor specificity that preferentially binds α2-3 linked sialic acid (SAα2-3) receptors.
Interestingly, a few of the H9N2 viruses that recognize SAα2-6 receptors have transmitted directly to humans, causing mild flu-like illness and the consequent fear that they may become pandemic.
In ferret testing, H9 replicated well but aerosol transmission to contact ferrets did not occur, strongly suggesting the virus must undergo further changes before it could spread efficiently among humans.
Which brings us to today’s story.
The Centre for Health Protection is investigating a mild form of avian influenza A H9N2 found in a 35-month-old girl.
The toddler lives in Sha Tin. Since late November, she had developed a cough, fever and runny nose and was admitted to Prince of Wales Hospital. She has since recovered, and was discharged on December 11.
Lab tests found the bird flu virus in samples from her respiratory system. The World Health Organisation will be alerted. Such H9N2 infections in people are rare, and are usually mild.
This is the seventh time that H9 viruses were found in people in Hong Kong since 1999.
As a precautionary measure, people are reminded to avoid contact with live poultry to minimise the chance of being infected. For more information about bird flu, visit the centre's website.
As it exists now, H9 poses a low threat to humans.
Sporadic reports of human infections – particularly when there is no evidence of ongoing transmission – are interesting, but not alarming.
But H9, like a number of other avian viruses (H5’s, H7’s) have some pandemic potential, particularly if they can co-circulate with – and pick up genetic material from – other viruses. This `reassortment’ can conceivably create a new, hybrid strain of influenza.
How likely is this to happen?
Well, that’s the big question. No one really knows.
We just know that it is possible.
It obviously doesn’t happen often, otherwise we’d be hip deep in new, hybrid viruses all of the time. But this is essentially the route that the 2009 H1N1 virus took to become a pandemic, and is likely the way the 1957 and 1968 pandemics came about.
And so we watch these cases with great interest. A dangerous reassortment may never occur with the H9 virus, or it could happen tomorrow.
Influenza viruses, as they say, are unpredictable.