Very early into Europe's 2016 HPAI H5N8 epizootic, it became apparent that the virus that arrived last fall was acting differently from the H5N8 virus that had struck North America (and to a far lesser extent, Europe) in 2015.
During its prior appearances, HPAI H5N8 had produced high mortality in gallinacious birds (poultry), but low mortality (and no large die offs) in wild birds and migratory waterfowl.
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 surprising dearth of wild bird mortality experienced during H5N8's first two years.
But by mid-November of last year we were seeing something new; mass die offs of wild and migratory birds (including waterfowl) due to the virus, in scores of locations, involving thousands of birds (see Europe: Unusual Mortality Among WIld Birds From H5N8).
Despite the heavy toll this virus was taking on many of its winged carriers (at last count, 73 species), H5N8 (and to a lesser extent, it's H5N5 spinoff), was still spreading with extraordinary speed across Europe, into the Middle East, and Africa.
Suggesting there must be a considerable cohort of healthy birds able to carry, and spread the virus.
The Friedrich-Loeffler-Institut (FLI) - in a January update to their Risk Assessment on HPAI in Germany - wrote:
As at present mainly wild birds which have been found dead are investigated, it is unknown which other bird species may possibly carry the virus and do not develop symptoms of disease or die. It must be concluded that there is an ongoing HPAI H5N8 epidemics among wild water bird species and that the dead birds found possibly represent no more than the tip of the iceberg.
This sudden change in virulence appears to be due to multiple reassortments that took place in China and/or Russia last summer. A reminder that all of these avian viruses are constantly evolving, and moving, targets.
All of which serves as prelude to a recent study from Veterinary Microbiology where two common avian species - Mandarin ducks and the domestic pigeon - were experimentally inoculated with the HPAI H5N8 virus.
While they were successfully infected, and shed the virus, they suffered no ill effects.
Volume 203, May 2017, Pages 95–102
Experimental infection with highly pathogenic H5N8 avian influenza viruses in the Mandarin duck (Aix galericulata) and domestic pigeon (Columba livia domestica)
Jung-Hoon Kwon, Yun Kyung Noh, Dong-Hun Lee1, Seong-Su Yuk, Tseren-Ochir Erdene-Ochir, Jin-Yong Noh, Woo-Tack Hong, Jei-Hyun Jeong, Sol Jeong, Gyeong-Bin Gwon, Chang-Seon Song, Sang-Soep Nahm,
• Mandarin ducks and pigeons can be infected with H5N8 HPAI viruses without exhibiting clinical signs.
• The titer of excreted virus was relatively high in a Mandarin duck and was detected in multiple organs.
• Mandarin ducks and pigeons may be potential healthy reservoirs of the H5N8 HPAI virus.
Wild birds play a major role in the evolution, maintenance, and dissemination of highly pathogenic avian influenza viruses (HPAIV). Sub-clinical infection with HPAI in resident wild birds could be a source of dissemination of HPAIV and continuous outbreaks. In this study, the pathogenicity and infectivity of two strains of H5N8 clade 220.127.116.11 virus were evaluated in the Mandarin duck (Aix galericulata) and domestic pigeon (Columba livia domestica).
None of the birds experimentally infected with H5N8 viruses showed clinical signs or mortality. The H5N8 viruses efficiently replicated in the virus-inoculated Mandarin ducks and transmitted to co-housed Mandarin ducks. Although relatively high levels of viral shedding were noted in pigeons, viral shedding was not detected in some of the pigeons and the shedding period was relatively short.
Furthermore, the infection was not transmitted to co-housed pigeons.
Immunohistochemical examination revealed the presence of HPAIV in multiple organs of the infected birds. Histopathological evaluation showed the presence of inflammatory responses primarily in HPAIV-positive organs. Our results indicate that Mandarin ducks and pigeons can be infected with H5N8 HPAIV without exhibiting clinical signs; thus, they may be potential healthy reservoirs of the H5N8 HPAIV.
The susceptibility of racing pigeons to HPAI H5N8 is a concern we looked at last week in DEFRA: Assessing The Risk Of Pigeon Racing In Spreading Avian Influenza. In that report they describe this year's epizootic:
The current epizootic is spreading rapidly in a wide range of migratory and non-migratory wild waterfowl in Europe (see Annex 2 for the species involved) causing mortalities in these birds. This is strikingly different to previous years and indicates a change in the virus pathogenicity for certain species of bird.
What is unknown at present, and very hard to ascertain, is whether there are species of wild waterfowl which do not show clinical signs of infection, and whether the virus can circulate in non-migratory, wild birds.
Today's report suggests that Mandarin ducks and domestic pigeons may be among those supposed healthy reservoirs of the virus.