Credit ECDC – 125 years of Pandemic History
Despite its horrific impact on West Africa, Ebola – since it is not an `airborne virus’ – is unlikely to spark the next great pandemic. None of which is to sell the impact of this virus short. If it is not contained soon, it could wreak considerable havoc around the globe - particularly in the developing world.
But respiratory viruses – particularly novel influenzas – have far greater pandemic potential, and influenza reassortments (see chart above) have a long history of doing precisely that.
For almost two decades the H5N1 virus has been on the radar screen – ever since it caused an outbreak in Hong Kong in 1997 (18 human infections, 6 fatal). Massive poultry culls seemingly sent the virus packing – but it reappeared in Vietnam in 2003 – having apparently been circulating unnoticed in wild birds and poultry in China.
For nearly a decade, the H5N1 virus captured the bulk of our attentions. Yes, there were `other’ avian strains out there of interest – H7s and H9s - but none produced the kind of dramatic high mortality in humans that this HPAI H5N1 strain did.
At least not until the spring of 2013 when – out of left field – H7N9 appeared in China, and rewrote the books on H7 avian strains.
Instead of producing mild respiratory symptoms and conjunctivitis – which is what we saw with other H7 avian viruses – this reassortant was killing up to 30% of hospitalized cases.
Either virus would be devastating were they to gain the ability to spread efficiently between humans. For now, infection comes primarily from direct exposure to infected poultry, and secondary transmission remains rare. Neither virus has evolved to the point of becoming an imminent human pandemic threat.
But these viruses – and others recently arrived on the scene – continue to mutate, reassort, and evolve. They roll the genetic dice millions of times each day - and even though successful mutations are rare - with that many opportunities the odds say they will eventually find the right combinaton.
One that makes the virus more `biologically fit’.
Reassorted viruses can result when two different flu strains inhabit the same host (human, swine, avian, or otherwise) at the same time. Under the right conditions, they can swap one or more gene segments and produce a hybrid virus.
The intermingling of wild birds, ducks, and poultry over the past two decades have produced dozens of new clades of avian flu viruses that could potentially endanger humans, including subtypes H5N1, H7N9, H10N8, and H9N2 (see EID Journal: Predicting Hotspots for Influenza Virus Reassortment).
H5N1 and H7N9 – the two avian viruses that are most widely entrenched - continue to evolve and spit into more and more clades and variants. H5N1 alone has produced more than 20 clades over the years (not all continue to circulate).
Diversity of circulating H5N1 Clades – Credit WHO
Until a couple of years ago, China was very circumspect in regards to their `avian flu problem’. Even when we heard of mass poultry die offs or culls, or when migratory birds flying out of China were often found to be infected, China rarely acknowledge human – or poultry – infections with H5N1.
In the past two years, after a decade of really only watching H5N1, we’ve seen an unprecedented explosion in new subtypes of avian flu appearing in China.
- H6N1 (in Taiwan)
The H10N8 virus emerged last winter, and infected (and killed) three people in China. H5N6 – while only infecting one person (that we know of) – has spread quickly across China and into Vietnam since it first appeared last April.
All of which brings us to 5 OIE reports filed late last week by China informing the international community of two `new’ H5 virus detections, and a significant number of H5N1, H5N2 and H5N6 detections among poultry across the country.
The `new’ viruses are 2 detections of H5N8 in Liaoning – which appeared for the first time in South Korea last January and resulted in the culling of more than 10 million birds, and H5N3 detected in a live bird market in Changsha, Hunan.
More numerous are reports of H5N1 – long known to plague China’s poultry industry, but rarely acknowledged – H5N2, and the upstart H5N6 virus which first appeared 6 months ago.
First stop, the detection through their national surveillance system of H5N1 at numerous sampling locations around the country.
Followed by 24 detections of H5N6 over the past 6 weeks.
And lastly H5N2
While culling has traditionally been the standard procedure used to stamp out H5 avian flu outbreaks – even in China where avian flu vaccines are heavily employed – the disease control measures listed in these reports indicates less drastic measures are being employed.
These recent reports only deal with the H5 family of avian viruses. H7N9 also circulates widely in Chinese poultry, as does H9N2, and an ever widening array of other reassortant viruses.
The good news is, despite an expanding array of viral components with which to play, successful reassortant viruses with genuine pandemic potential only appear very rarely.
The bad news is that nature’s laboratory is open 24/7, and it is constantly trying to produce the next `successful’ virus. And the more subtypes it has to play with, the greater the odds are it will come up with something we really don’t want to have to deal with.