A Possible Way for Wild Birds to Spread Influenza

Northern Pintail fitted with a transmitter / USGS

One of the great questions during the outbreak of the H5N1 influenza virus a few years ago was how the virus spread so quickly. The virus outbreak was most severe in the domestic poultry industry, but it was also detected among waterfowl and spread to humans, many of whom died. New research from the USGS suggests a possible route the virus could have spread via wild birds.

In the study, scientists from the USGS Alaska Science Center and the University of Tokyo attached satellite transmitters to 92 northern pintail ducks several months before the H5N1 virus was discovered in dead and dying whooper swans at wetlands in Japan.

They found that 12 percent of marked pintails used the same wetlands as infected swans and that pintails were present at those sites on dates the virus was discovered in swans. During the first week after they become infected with H5N1, ducks such as pintails can shed the virus orally or in their feces, potentially contributing to the virus’ spread.

Researchers found that some of the marked pintails migrated 700 miles within four days of leaving the outbreak sites; marked pintails ultimately migrated more than 2,000 miles to nesting areas in eastern Russia.  The study’s discovery that northern pintails made long-distance migrations during the period when an infected duck would likely shed the virus offers insight on how H5N1 could be spread by wild birds across large areas.

The research, published in the journal Ibis, does not prove the marked pintails were actually infected with the H5N1 virus or that they definitively contributed to its spread.  However, it does demonstrate that pintails satisfied two requirements necessary for migratory birds to spread the virus: they used outbreak sites at times when the virus was present and some birds migrated long distances within a week of using the sites.

Jerry Hupp, Ph.D., a U. S. Geological Survey scientist and one of the lead authors of the study, noted that the H5N1 virus has been found in wild birds, including northern pintails, which show no visible signs of illness. Also, laboratory studies have shown that pintails and some other wild birds remain healthy when infected with H5N1.

While the study does not prove that wild birds spread H5N1, it does show that they are at least a possible vector. Because waterfowl migrate between Asia and North America and birds have carried less pathogenic viruses from Asia to Alaska in the past, infected Northern Pintails might spread H5N1 into North America. However, this highly pathogenic strain still has not been detected in North America despite its rapid spread in Eurasia and Africa several years ago. Whether this will happen in the future remains to be seen. 

Northern Pintails wintering in Japan / USGS