After a lackluster 12 months covering the second half of 2015 and first half of 2016, Highly Pathogenic Avian Influenza (HPAI) is on the ascendant again around the globe, with both H5N8 and H5N6 making new inroads into previously unaffected regions.
Meanwhile, H5N1 remains entrenched in the Middle East and West Africa, and H7N9 is expected to return with another mini-epidemic this winter in China.Spared so far in 2016 has been North America, which was hard hit during the first half of 2015 by HPAI H5N8/H5N2, but did not see a return the following winter. While encouraging, it is far too soon to rule out a return engagement this winter.
Driving this recent expansion - particularly among clade 126.96.36.199 HPAI H5 viruses (H5N8, H5N2, H5N6, etc.) and clade 188.8.131.52c H5N1 - is an apparent enhanced ability to spread via migratory birds (see Migratory Birds & The Spread Of Highly Pathogenic Avian Flu).
Clade 184.108.40.206 viruses, in particular, have demonstrated an increased ability to reassort with other - mostly LPAI - viruses, and continue to evolve and occasionally churn out viable novel subtypes.
A recent study published in the Journal Science, called the Role for migratory wild birds in the global spread of avian influenza H5N8, stated:
Further, we found that the hemagglutinin of clade 220.127.116.11 virus was remarkably promiscuous, creating reassortants with multiple neuraminidase subtypes.We looked at one possible explanation for these changes two days ago in EID Journal: HPAI A(H5Nx) Viruses With Altered H5 Receptor-Binding Specificity, where researchers suggested:
`Altered receptor-binding properties might affect the balance between HA and NA, enable the virus to acquire different NA subtypes, and might result in altered host range and spreading.'
Regardless of the exact mechanism behind it, over the past four years we've seen an unprecedented expansion in both the number of subtypes, and range, of bird flu around the globe.
Even venerable HPAI H5N1 continues to evolve, with clade 18.104.22.168c being linked to neurological involvement in mammals, and Egypt's Clade 22.214.171.124 linked to the largest and most sustained human outbreak since the virus first emerged in China in 1996
While China's yearly H7N9 epidemic has seemingly pulled back in intensity the past two years, we've been getting substantially less information out of China over that same time period. So comparisons are difficult to make.
Despite fewer cases, we are seeing changes in that virus's behavior as well (see EID Journal: H7N9’s Evolution During China’s Third Wave – Guangdong Province).
Perhaps the biggest question mark for the 2016-17 winter and spring is whether HPAI H5N6 will continue to spread beyond Korea and Japan, and follow H5N8 into Europe or North America.
Like all of the other viruses we've discussed, H5N6 continues to evolve and reassort as well (see Novel Reassortant H5N6 Viruses In Humans, Guangdong China and J. Virology: H5N6 Receptor Cell Binding & Transmission In Ferrets).
There are others, in the `wings' so to speak, including the unusual outbreak and die off of birds in Algeria last month due to HPAI H7N1, which is normally found in poultry.
When I began this blog nearly 11 years ago, there was only 1 HPAI virus of global concern - H5N1. And until early 2013, that was the status quo. Since then we've seen H7N9, H5N8, H5N2, and H5N6 emerge and get added to that list.
And as today's ramble points out, we aren't dealing with just 1 H5N1 virus, we are dealing with multiple clades of H5N1, each with different attributes and each following their own evolutionary paths.
And the same holds true for the other viruses I've mentioned, as well.
Most of these viruses remain only (or at least primarily) a concern for poultry growers and bird populations. Even the ones that can infect humans (H5N1, H5N6, H7N9, etc.) haven't yet adapted well enough to human physiology to spread efficiently, and so their threat to human health is limited.
But if there is a common theme among all of these viruses, it is that they are always evolving and adapting in order to survive. What we can say about these viruses today may not hold true tomorrow.
Even discounting the pandemic threat, the economic, and societal impact of these viruses is considerable. In countries with low food security - including Egypt, China, and Nigeria - the loss of an important source of (relatively) cheap protein is a major concern.
While I'm not fool enough to try to predict what bird flu will do next, with so many parts in motion, I'm prepared to be surprised.