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A new study suggests some North American avian influenza A H7 virus strains have properties that might enhance their potential to infect humans, as well as to spread from human to human.

The study by the Centers for Disease Control and Prevention (CDC) was published in the Proceedings of the National Academy of Sciences USA. Avian influenza A H7 viruses are fairly common in birds but rarely infect humans. Most cases of avian-influenza infection in humans have resulted from contact with infected poultry or contaminated surfaces.

Influenza viruses infect humans by attaching to certain sugar receptor molecules found on cells in the human respiratory tract. Influenza viruses can have differing degrees of ability to bind to these receptors. The greater an influenza virus’s ability to do so, the greater the likelihood the virus can cause illness in humans and possibly be passed from human to human.

Three recent H7N2 strains and two H7N3 strains from North America were tested and found to bind to varying degrees to both avian and human receptors. One virus, an H7N2 virus strain isolated from an immune-compromised man in New York in 2003, was found to have the greatest binding to the human sugar receptors.

This study’s findings suggest these North American avian influenza A H7 viruses are partially adapted to recognize sugar receptors preferred by human influenza viruses, which are found in the human upper respiratory tract.