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Scientists from the USGS analyzed the genes, or genome, of the avian flu viruses that spread in the United States during 2014-2015. This outbreak resulted in more than $3 billion in losses to the United States poultry industry. The study found that even though the viruses likely evolved in Asia, they easily infected and spread among North American wild birds. The viruses were also able to spread between domestic and wild birds, in a process called spillover. However, this study found that the rate of spillover was minor, and the poultry outbreak was able to persist without further transmission from wild birds.

“Results from our study are important because they can help managers enhance biosecurity and guard against the most likely sources of avian flu outbreaks in the United States,” said Dan Grear, a disease ecologist with the USGS National Wildlife Health Center and the lead author of the study.

The scientists studied avian flu viruses of the highly pathogenic variety. The designation of low or highly pathogenic avian flu refers to the potential for these viruses to kill domestic chickens. Strains of highly pathogenic avian influenza, or HPAI, cause contagious and severe illness and often death in poultry and wildlife, and some strains can infect humans. The 2014-2015 HPAI outbreak in the U.S. did not infect people.

Infection and transmission of HPAI in wild birds are difficult to measure during a fast-moving outbreak. The new study used cutting-edge techniques, known as phylodynamics, to analyze the genetic similarity between viruses infecting wild birds and poultry that were collected during the outbreak. This new method of analysis can answer questions about transmission within waterfowl populations and between waterfowl and poultry.

“Our analyses showed that HPAI viruses can adapt easily to novel species and environments,” Grear said.

Like most people, the genetic make-ups of influenza viruses are different from one another. Influenza viruses are constantly evolving and can develop into different strains with varying risks to animal and human health. Viruses from afar can also mix with local ones to create completely new strains of flu. Influenza viruses are classified by a combination of two groups of proteins: "H" proteins, of which there are 18 – H1 to H18 – and "N" proteins, of which there are 11 – N1 to N11. HPAI viruses in poultry are usually H5 or H7 subtypes.

In 2014 and 2015, three different subtypes of HPAI were detected in the United States and Canada: H5N1, H5N2 and H5N8. Although those viruses did not infect humans, they were related to the Asian H5N1 subtype that infected people in other countries.

The new USGS study is published in the journal Evolutionary Applications.

For more information about avian flu and other wildlife diseases, please visit the USGS National Wildlife Health Center website. For more information about avian influenza and poultry, please visit the U.S. Department of Agriculture website.