A 3D model of the influenza virus. Scientists discover that the H7 viruses can adapt to humans more easily now Graphic: M Eickmer/Wikipedia

Some North American avian influenza viruses of the H7 subtype have adapted to infect humans more easily, a US study has found. While there is no immediate threat to human health as a result of thechange, the findings highlight the need to watch andstudy the viruses closely, say researchers.

The study by scientists at the US Centers for Disease Control and Prevention examined North American and Eurasian H7 viruses isolated between 2002 and 2004, comparing their preference for human or avian receptors, and their transmissibility in ferrets, a standard animal model for human influenza.

Human influenza viruses bind preferentially to alpha 2-6-linked sialic acids (SA), the receptors that predominate in the human respiratory tract.Avian influenza viruses tend to bind to the alpha 2-3-linked SA receptors found in birds.

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The researchers found that Eurasian lineage viruses displayed a binding preference for avian receptors, and were not readily transmissible in ferrets. However, certain North American viruses showed an increased affinity for the human receptors, and one was capable of infecting healthy ferrets that came into contact with infected ones.

The findings show that H7 influenza viruses from the North American subtype have acquired binding properties that more closely resemble those of human influenza viruses, and have the potential to spread between animals through direct contact.

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