During the first two years after it emerged in South Korea in January of 2014, HPAI H5N8 spread rapidly around the globe - turning up in Europe and North America in less than a year - presumably carried in by migratory birds.
Somewhat remarkably, during the six months when H5N8 (and its reassorted offspring H5N2) decimated the US poultry industry (Dec 2014-June 2015), fewer than 100 dead or dying birds were found infected across the United States.Even more remarkable, by the summer of 2015, the virus had virtually disappeared in both North American poultry and wild birds (see PNAS: The Enigma Of Disappearing HPAI H5 In North American Migratory Waterfowl).
While still causing losses in Korea and China, to everyone's surprise H5N8 did not return the following winter to Europe or North America.When H5N8 did return to Europe in the fall of 2016, it was quickly apparent that something had changed. In November of 2016, in Europe: Unusual Mortality Among WIld Birds From H5N8, I wrote about thousands of wild birds dying from the virus in the Netherlands.
In January of 2017, we would learn that the H5N8 virus had reassorted - either in Russia or China (see EID Journal: Reassorted HPAI H5N8 Clade 188.8.131.52. - Germany 2016) - picking up virulence in wild birds, an expanded avian host range, and unusual environmental persistence.
Despite this increased mortality in wild birds, the virus has continued to spread - first into the Middle East - and then into central and southern Africa, although it was largely supplanted by a reassorted HPAI H5N6 virus in Europe and Asia this past winter.All of which brings us to a new research article, published this week in Emerging Microbes and Infections, which looks at the carriage of the (pre-2016-reassortant) HPAI H5N8 virus in wild ducks and finds that it produced far fewer clinical symptoms than did HPAI H5N1.
I've only excerpted the abstract, so follow the link to read it in its entirety.
Wild ducks excrete highly pathogenic avian influenza virus H5N8 (2014–2015) without clinical or pathological evidence of disease
Judith M. A. van den Brand, Josanne H. Verhagen, Edwin J. B. Veldhuis Kroeze, Marco W. G. van de Bildt, Rogier Bodewes, Sander Herfst, Mathilde Richard, Pascal Lexmond, Theo M. Bestebroer, Ron A. M. Fouchier & Thijs Kuiken(Continue . . . )
Emerging Microbes & Infectionsvolume 7, Article number: 67 (2018)
Published online: 18 April 2018
Highly pathogenic avian influenza (HPAI) is essentially a poultry disease. Wild birds have traditionally not been involved in its spread, but the epidemiology of HPAI has changed in recent years. After its emergence in southeastern Asia in 1996, H5 HPAI virus of the Goose/Guangdong lineage has evolved into several sub-lineages, some of which have spread over thousands of kilometers via long-distance migration of wild waterbirds.
In order to determine whether the virus is adapting to wild waterbirds, we experimentally inoculated the HPAI H5N8 virus clade 184.108.40.206 group A from 2014 into four key waterbird species—Eurasian wigeon (Anas penelope), common teal (Anas crecca), mallard (Anas platyrhynchos), and common pochard (Aythya ferina)—and compared virus excretion and disease severity with historical data of the HPAI H5N1 virus infection from 2005 in the same four species.
Our results showed that excretion was highest in Eurasian wigeons for the 2014 virus, whereas excretion was highest in common pochards and mallards for the 2005 virus. The 2014 virus infection was subclinical in all four waterbird species, while the 2005 virus caused clinical disease and pathological changes in over 50% of the common pochards. In chickens, the 2014 virus infection caused systemic disease and high mortality, similar to the 2005 virus.
In conclusion, the evidence was strongest for Eurasian wigeons as long-distance vectors for HPAI H5N8 virus from 2014. The implications of the switch in species-specific virus excretion and decreased disease severity may be that the HPAI H5 virus more easily spreads in the wild-waterbird population.
HPAI H5N8 did in just over 3 years what HPAI H5N1 has yet to do in more than 15 years; it made the leap to North America, sparked record setting epizootics there and in Europe, and has set recently taken up residence in the Southern Hemisphere (Zimbabwe & South Africa).
Whatever barriers that limited H5N1's spread via migratory birds have been largely overcome by HPAI H5N8.A reminder that avian flu viruses - like all viruses - are constantly changing; seeking an evolutionary advantage that will give them greater access to a ready supply of susceptible hosts.
And that we can't necessarily assume that the novel viruses we will face tomorrow will continue to behave as they have in the past.