Examining the Role of Wild Birds in Midwestern US H5N2 Outbreaks
In early December, 2014 Highly Pathogenic Avian Influenza (HPAI) H5N2 was discovered in several poultry operations in the Fraser valley in southern British Columbia. By mid-December H5N2 and H5N8 viruses had been identified in Whatcom County, Washington, across the border from British Columbia. By early January, H5N8 was found in backyard poultry flocks in southeastern Washington. The geographic range of virus identifications in wild birds and poultry has continued to expand and by the end of January included sites in the states of Oregon, California, Idaho, Nevada and Utah, with H5N2, H5N8 and a new strain of H5N1 all being identified. All 3 strains have been found in wild birds, but only H5N2 and H5N8 have been identified in poultry.
Genetic evidence indicated that the H5N2 and H5N8 viruses contained a mixture of Eurasian and North American virus lineages, suggesting that they were likely the product of viral mixing in wild birds, probably in the western Arctic during the summer months. The fall southward migration of waterfowl could have brought these new viral variants into southern British Columbia and the northwestern USA.
In early March, H5N2 virus was identified in a turkey flock in Minnesota and since then, there have been subsequent identifications in turkey flocks in Missouri and Arkansas and a backyard chicken and duck flock in Kansas. These have been unexpected developments and news reports have highlighted the fact that these were the first identifications of these viruses in the Mississippi Flyway. The use of the flyway as a geographic identifier puts the blame, by implication, on wild birds as the source of the virus and many commentators make this connection, even going so far in one case as to suggest that the virus was moving south from Canada in March with wild birds.
Ever since the identification of HPAI H5N1 in wild birds in 2005, there has been concern over the potential for wild birds, particularly waterfowl, to serve both as mixing vessels for HPAI and as vectors to bring new virus variants into contact with poultry. At that time, it was recognised that the migration patterns of waterfowl could potentiate the transmission of virus from Asian or European birds to North American populations and allow the spread of the virus into new geographic regions.
The flyway is a critical concept for understanding these risks. Each species of bird has a preferred or typical migration route which it tends to follow year after year. These routes have a degree of commonality and come together broadly into geographic patterns that are referred to as flyways. A look at the map of world flyways reveals the potential for different species of birds to mix together in certain parts of the world, where the sharing of viruses becomes possible. The western Arctic and the seas off the coast of Alaska are areas in which birds from Asia potentially mix with birds from the Americas.
While the genetic makeup of these viruses found in poultry and wild birds in North America this winter strongly suggests an origin in wild birds and a likely movement from the Arctic into the mid-continent, through either the Pacific or Mississippi flyways, the timing of the Midwestern outbreaks highlights how poorly we understand the epidemiological links between wild birds and poultry.
It is reasonable to believe that these strains of HPAI arrived in the Pacific Northwest and in the Midwest after being transported south with migrating waterfowl in the fall. However, waterfowl typically do not stop and overwinter in the areas in the Midwest where the virus was initially found in turkeys. They continue their southward migration to their overwintering grounds.
The obvious question is: where have these viruses been since their postulated arrival in the fall with migrating waterfowl and the later outbreaks that have occurred in poultry. Unlike the occurrences in southern British Columbia and the northwestern states, there is no significant resident population of wild waterfowl in Minnesota in which the virus might be sustained. An attempt to identify the presence of virus in wild birds following the poultry outbreak was hampered by a shortage of open water and relatively few birds from which to obtain samples.
If wild birds are the source of the Minnesota, Missouri and Arkansas outbreaks, somehow the virus has persisted in the environment or in some population of wild birds, but presumably not waterfowl. It has then found a way of moving from this population into poultry barns, where it has become the source of significant mortality. Presumably, epidemiological investigations of these outbreaks in turkey flocks will examine all of the possible sources and routes of entry of HPAI virus into these farms and consider other means (not associated with wild birds) by which the virus could have moved from the Pacific to the Mississippi flyway.
The identification of new strains of HPAI in apparently healthy wild birds and the spread of these strains over large distances raise important questions regarding how and where surveillance efforts in wild birds should be targeted. The emphasis since 2006 has been on detection of HPAI in dead birds, based upon the findings of wild birds dead of infection with the H5N1 Asian strain of virus. It now appears that there are HPAI strains circulating in wild birds in North America, without causing harm to those birds. This suggests that surveillance efforts now need to include apparently healthy as well as dead birds. Although it still seems reasonable to target waterfowl as the most likely carriers of virus, perhaps there are other species of birds, ones with closer links to poultry barns, which should also be included.
There are large gaps in our understanding of how these viruses are moving from wild birds into poultry barns. Ongoing surveillance for these viruses in wild bird populations will be a critical part of these investigations.
Submitted by Doug Campbell, Claire Jardine (CWHC Ontario/Nunavut), & Jane Parmley (CWHC National Office)
