Devastating viruses disappeared in wild birds

Published on August 31, 2016
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The influenza viruses that killed 50 million chickens and turkeys during 2014 and 2015 have disappeared from wild birds, according to a recent scientific article.

Their disappearance, along with an absence of highly pathogenic avian influenza viruses in samples collected from wild birds over previous decades, could indicate the viruses are ill-suited to perpetuating in the wild.

A highly pathogenic H5N2 avian influenza virus killed most of those chickens and turkeys in the U.S., and a highly pathogenic H5N8 avian influenza virus killed about 200,000. Those viruses were discovered in wild waterfowl along with a highly pathogenic H5N1 influenza virus that remained unseen in commercial and domesticated flocks, according to information from the Department of Agriculture’s Animal and Plant Health Inspection Service.

An article published in July in the Proceedings of the National Academy of Sciences of the United States of America indicates animal health authorities in the U.S. and Canada eradicated the viruses from poultry, but those actions do not explain the disappearance from wild waterfowl. The article is “The enigma of the apparent disappearance of Eurasian highly pathogenic H5 clade influenza A viruses in North American waterfowl.”

A mallard drake photographed in Anchorage, Alaska (Courtesy of the U.S. Fish and Wildlife Service)

After tests on 46,000 wild birds between July 2015 and June 2016, animal health authorities found H5 influenza viruses in only two mallards in Utah and Oregon, according to APHIS. The Utah bird was captured and released in July 2015, and the Oregon bird was killed by a hunter in November 2015.

Avian influenza surveillance in North America over the 43 years before the outbreak also found zero highly pathogenic avian influenza viruses among more than 100,000 birds tested, according to the article. The authors indicate that those findings support a premise that “unresolved mechanisms” prevent wild birds from perpetuating highly pathogenic avian influenza viruses, abbreviated in the article as HPAIVs.

“The significance of these findings is that timely and efficient strategies used to successfully prevent and eradicate HPAIVs infecting poultry, without the use of vaccines, appear to complement natural biological mechanisms in disrupting the perpetuation and possible spread of HPAIVs by wild aquatic birds,” the article states.

The article concludes that the highly pathogenic avian influenza viruses may be ill-fitted to persistence in wild waterfowl and likely to disappear without an endemic poultry source, infections from which could spill over into wild birds and other host species. As a result, the eradication efforts in North America may have combined with natural resistances among wild birds to prevent the highly pathogenic viruses from becoming incorporated into the pool of circulating avian influenzas.

Related JAVMA content:

Devastating flu, ongoing harm (Sept. 1, 2015)