Thursday, December 15, 2016

Riding The Bird Flu Rollercoaster



Although it is only mid-December, and we've yet to hear from what are traditionally the biggest players in our winter flu season (seasonal flu, H5N1, or very much from H7N9 or H5N6), the number and variety of flu stories has already reached an impressive level.

While most of the impact this fall has fallen on wildlife and poultry operations, the headline in last night's Hong Kong Standard - which carried an interview with WHO Director Margaret Chan -  read:

WHO warns of H7N9 pandemic
Mary Ann Benitez and Carain Yeung Dec 15, 2016

Perhaps a bit overstated - given H7N9's lack of sustained human-to-human transmission -  but  H7N9 is certainly at or very near the top of the list of pandemic threats. 

 It, however, is not alone. 

H5N6 continues to to burn through South Korean poultry at an alarming rate, and - like H7N9 - has shown an ability to infect - and sometimes kill - humans. 
Recent analysis (see Cell Host Microbe: Genesis, Evolution and Prevalence of HPAI H5N6 In China) shows it has become the predominant HPAI virus in Chinese poultry, and that it has expanded into at least 34 genotypes. 

H5N8 in Europe hasn't shown any inclination to infect people - yet - but it is continually evolving, and so its future threat is difficult to predict, particularly given clade's proclivity for reassortment.

And H5N1 - which infected 160 people in Egypt in 2014 - continues to spread in the Middle East and Africa, and recent studies suggest the virus may be becoming better adapted to mammalian physiology (see Nature: Risk Assessment of Recent Egyptian H5N1 Influenza Viruses).  

Added to these are a growing number second tier avian flu strains that are perceived as lesser threats (i.e.  H9N2, H7N7, H7N1, H10N8, H6N1, etc.) that are nevertheless on our radar.

We also see a growing number of swine variant viruses - many with genes from human influenza viruses - that are circulating in pigs and occasionally jumping to humans (see EID Journal: Characterization of a Novel Human Influenza A(H1N2) Virus Variant, Brazil).
Since swine variant influenza are H1,H2 and H3 subtypes - the same as have caused all the human pandemics of the past 100+ years - many researchers fear they have less of a jump to make to adapt to humans than avian viruses.

There are others, of course  - including  canine flu (H3N2 & H3N8) - which have at least some potential to pose a public health risk down the road (Virology J: Human-like H3N2 Influenza Viruses In Dogs - Guangxi, China).

But influenza's greatest weapon is its ability to reassort.  All it takes is for two compatible subtypes to co-infect a host to allow them to swap gene segments and create a new hybrid virus. 

Which is exactly how H7N9 emerged in 2013, followed by H5N8 and H5N6 in 2014.  The next reassortant virus could appear - literally - at any time.
The good news is, most reassortant viruses are evolutionary failures.  

They die out quickly because they are not as biologically `fit’ as the viruses they must compete with. Occasionally, one does hit the evolutionary lottery, and the pandemics of 2009, 1968, and 1957 are all believed to have been sparked by reassortant flu viruses. 

All of which explains why - while researchers worry about Zika, and Ebola, and Dengue and a hundred other infectious dieases - it is the ever growing panoply of influenza viruses that keeps most of them awake at night. 

Seasonal influenza - even in a mild year - kills hundreds of thousands around the globe.  In a bad year - with a novel virus - that number can be 10 to 100 times greater. 

None of this means that this winter will see another pandemic.   Only that the raw ingredients - the genetic building blocks - are out there to cobble together the next nasty flu threat. 

How and when that happens is anyone's guess. 

In the meantime, we appear to be in for a wild E-ticket ride this winter, and so those who are new to following flu will want to fasten their seat belts, and are advised keep their hands and heads inside the cabin at all times. 

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