Mallard ducks (Anas platyrhynchos) are the green-headed ducks commonly recognized by the public. In North America, mallards are the principal host of low pathogenic avian influenza strains -- not to be confused with highly pathogenic avian influenza such as H5N1, which has not yet been detected in North America. “Low path” avian influenza viruses rarely cause mortality in domestic poultry or wild waterfowl populations, and they serve as important models to study how viruses are transmitted and spread to new geographic areas.

“We knew that California mallards have a mix of migration strategies -- some breed in Alaska, some breed in Oregon, Washington or British Columbia, while others breed locally and reside here year-round,” says USGS Western Ecological Research Center and MIT ecologist Nichola Hill, the lead study author. “This study confirms that mallards bring viral strains from their northern breeding grounds to California, while resident mallards appear to be serving as reservoirs for virus that likely persists year-round.”

The study was published online in the journal Molecular Ecology last month.

Researchers sampled hunter-killed mallards from the Sacramento National Wildlife Refuge in California’s Central Valley, a major wintering ground for wild birds. Swab samples were analyzed for virus type. Stable isotope analysis of feathers identified each mallard’s migration strategy by comparing their isotopic signatures with mallards breeding along the Pacific Rim.

Next, Hill and colleagues compiled a genetic family tree out of the viral strains detected and compared that against the geographic origin of each mallard sampled.

These tests revealed where each duck flew in from, and the influenza strains it brought to California.

“All mallards are not the same, and our results clearly show that pathogen transmission is strongly associated with migratory versus resident status of these mallards,” says UC Davis veterinarian Walter Boyce, a study coauthor. “Isotope analysis coupled with genetic sequence analysis gave us a unique perspective on the complexity of host-pathogen dynamics.”

While the study confirms that viral strains are being imported via mallard migrations, it also found limited circulation and persistence of these imported strains within mallards wintering in California.

However, resident mallards acted as a source of locally-circulating virus throughout the year, suggesting they may act as a reservoir for hardier, warmth-tolerant strains.

The persistence of viral strains -- combined with the importation of novel strains from afar -- offers researchers new ideas in their investigation on avian influenza transmission.

“We’re trying to learn about wildlife disease transmission,” says WERC ecologist John Takekawa, also a coauthor. “There is more to learn about how waterfowl migration relates to disease spread and about the environmental or human factors that shape these migration strategies.”

The study was funded by the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, and U.S. Fish and Wildlife Service and USGS.

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Frequently Asked Questions

Q: Has the highly pathogenic avian influenza H5N1 been detected in the United States?

A: No, the highly pathogenic avian influenza strain H5N1 has not yet been detected in North America. The Hill and colleagues study examined “low pathogenic avian influenza” strains, many of which already exist in the United States, and to date, they primarily affect wild birds. Transmission from wild birds directly to humans has never been documented, and because of the species barriers, it is anticipated to be an unlikely event. Mutation to highly pathogenic forms typically occurs in domestic poultry.

Q: Should people be worried about migrating birds carrying H5N1 from Asia and other parts of the world?

A: So far, researchers have been unable to show that waterfowl migrating from Asia are carrying highly pathogenic avian influenza to Alaskan breeding grounds. However, research like Hill and colleagues’ study sheds light on how virus strains move within North America by wild birds. Additionally, the study evaluates the role of migrant and resident birds in spread of avian influenza virus, helping to inform surveillance efforts in wild birds.

Q: How does avian influenza research fall under the USGS mission?

A: USGS is charged with surveying the natural resources of our Nation -- including our interconnected ecosystems and how they can shape the well-being of our society. Studying diseases in wildlife is obviously important work for the health and welfare of wildlife, but it is also important for the health of humans and domestic animals -- 70 percent of recent emerging human diseases originated in wildlife or domestic animals, including West Nile virus, plague, AIDS, SARS and avian influenza. The health of humans, animals -- wild and domestic -- and ecosystems are all inter-related; this is the concept of the “One Health” program, which advocates understanding and appreciating the links among human, animal and ecosystem health, and the importance of and commitment to working together to address health challenges.

• Visit the WERC global avian influenza research webpage
• Visit the USGS National Wildlife Health Center webpage

-- Ben Young Landis