The bird flu situation in North America continues to worry epidemiologists. Three people across the continent have now been infected with the H5N1 virus, with no known sources of contamination. This may not seem like much, given the 53 cases of farm workers who have also contracted the virus on dairy and poultry farms, the scene of an epizootic – an animal epidemic – that is spreading further and further across the US.

But these three atypical cases are leading specialists to increasingly fear a scenario similar to the early days of the H1N1 flu epidemic in 2009. Back then, two sporadic cases of swine flu infection among children in California who had had no contact with pigs or each other were the first signs of a pandemic that caused 280,000 deaths worldwide.

"During epidemics, it's important to understand where and how transmission occurs," said British epidemiologist Adam Kucharski. "If we don't know the source of infections, we can't be sure of the threat we're facing, or whether the situation is under control."

A first case with no known source was identified in the US state of Missouri on September 6. The epidemiological investigation, which was completed at the end of October, eventually concluded that it was probably not one, but two people from the same household who had contracted the H5N1 virus at the same time but it is still not known where or how.

On November 13, a second worrying signal came from Canada, where a teenager fell seriously ill in British Columbia, in the west of the country. Samples taken from his family and close environment revealed no presence of the H5N1 virus, and it would appear that he had not infected anyone else. In the absence of any new information, the provincial authorities declared the epidemiological investigation closed. On November 26, the patient was still in hospital, on a respirator. This is the first very serious case of avian influenza since the beginning of the year, the others presenting more conjunctivitis or mild flu-like symptoms.

This case is all the more worrying as it appears that the virus has acquired additional mutations. Scott Hensley, Professor of Microbiology at the University of Pennsylvania's Perelman School of Medicine, analyzed the sequence published in open-access databases by the Public Health Agency of Canada and reported on social media that two key mutations had appeared in hemagglutinin, a protein on the surface of the virus that binds to cells it attempts to invade. These mutations are known to help the virus better attach to human lung cells, a significant change for this virus, which normally binds more readily to bird cells.

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