During the six months during which 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.
Reports from Korea during their initial outbreak during the winter and spring of 2014 indicated only limited wild bird die offs, and only a small number of infected birds were discovered in Europe the following winter.This pattern of high mortality in gallinacious birds (poultry), but low mortality in wild bird (particularly waterfowl) species, is not uncommon with HPAI viruses.
Granted, we have occasionally seen large die offs attributed to avian viruses, such as at Qinghai Lake in 2005, and Sanmenxia reservoir in 2015. Both of these high profile H5N1 events signaled a significant change in the virus, and the emergence of new clades.
In 2015's Wild bird surveillance for highly pathogenic avian influenza H5 in North America by Paul L. Flint, John M. Pearce, J. Christian Franson, and Dirk V. Derksen , the authors described the dearth of wild bird mortality experienced during H5N8's first two years (bolding mine):
Kang et al.  exposed wild caught Mallards (Anas platyrhynchos) and Baikal Teal (Anas formosa) to HPAI H5N8 in a controlled trial and noted that all birds became infected and shed virus at high rates. No mortality occurred in 41 inoculated Mallards but 1 of 2 teal died without showing clinical signs and the cause of death was not determined.
In a reported mortality event in South Korea, HPAI H5N8 was isolated from tissues of eight waterbird species and Baikal Teal, in particular, exhibited a combination of clinical signs and gross and microscopic lesions leading the authors to conclude that HPAI was a cause of mortality . Thus, Baikal Teal may be susceptible to H5N8, but Jeong et al.  found H5 antibodies in >50 % of live/wild Baikal Teal sampled, implying that the realized mortality rate in the wild must be fairly low.
Zhao et al.  exposed 6-week-old Mallards to HPAI H5N8 and similarly observed no mortality within 14 days.
Given the apparent low mortality rate for wild birds exposed to HPAI H5N8 some authors have concluded that active surveillance is the preferred approach [23, 27, 30, 31].
Further, the apparent spread of HPAI H5N8 in wild birds in North America (from Washington south within the Pacific Flyway) has been primarily detected with active surveillance and is notable for a dearth of reported mortality events.
By contrast, over the past two weeks we've seen multiple reports of wild bird die offs linked to H5N8 in Europe, including a report today of more than 1250 dead birds in Monnickendam, The Netherlands.
Bird flu spreads: all 1,250 dead birds found
Monday, November 14th, 2016 | 12:39
MONNICKENDAM In recent days there have been around the Gouwzee 1,250 dead birds were found infected with the Influenza A virus subtype H5N8 virus. Reported that the Water Board (HHNK). The fire still seems to be the Gouwzee, but the virus has surfaced now in Zeewolde.
It is a sad face on the Hemmeland at Monnickendam. Still flush there are dead birds, mostly tufted ducks and grebes. Many of the animals seem aangegeten by rats. It is unknown whether the scavengers may further spread the virus.
(Continue . . .)
While we don't yet know what is behind this apparent increase in wild bird mortality, on Friday the UK's DEFRA noted:
- The mortality events and the species of wild waterfowl affected might indicate possible changes in virus pathogenicity and/or changes in host specificity. However significant uncertainties exist at this early stage and other wild waterfowl species may be infected without mortality.
- The number, rate, mortality and geographic spread of these new cases in wild birds is significant and shares some features with the initial spread of H5N1 HPAI of the goose/Guandong lineage of viruses to Europe in late 2005/early 2006.
A week ago, in FAO On HPAI H5's 4th Intercontinental Wave, we saw the following discussion of H5N8's detection earlier this summer in Russia:
The precise origin of the Tyva 2016 H5N8 virus remains unknown although available evidence suggests it was derived from earlier H5N8 viruses detected in Eastern China (WHO 2016). The virus is readily distinguishable from strains of H5N8 virus found in Europe and North America in 2014-15.
Influenza viruses are ever changing, and over time, those changes can sometimes affect their virulence, host range, and other traits. Hopefully we'll get an updated analysis and characterization of Europe's H5N8 virus in the coming days.
But until then - whether it is seasonal flu, or H7N9 in China, H5N1 in the Middle East, or H5N8 in Europe - it is important to remember that how these viruses have performed in the past is no guarantee of how they will behave in the future.Nature's laboratory is open 24/7, and so we need to remain vigilant and prepared to deal with surprises.