Teen sickened with Canada's first human case of bird flu is in critical condition — and the source remains a mystery

A teenager in Canada is critically ill with the first human case of H5N1 bird flu contracted in the country, and public health officials aren't sure how the youth was exposed.

The teen had no known exposure to livestock such as cattle or poultry, where the virus is known to be circulating. The strain that sickened the teen, however, is different from the version that is infecting cows and has spread to dozens of dairy workers in the U.S.

Instead, the variant in question, a 2.3.4.4b virus of the D1.1 genotype, typically circulates in wild birds and poultry, according to StatNews.

Canada has not yet detected H5N1 in its cattle herds, though it has experienced poultry outbreaks.

The avian flu H5N1 lineage that is causing dairy outbreaks emerged in 1996 and has been circulating in wild birds and poultry ever since. A particularly aggressive variant emerged in 2020 and quickly spread throughout Asia, Africa and Europe, reaching North America in 2021 and Central and South America in 2022, according to the World Health Organization.

Since 2022, the virus has been popping up in mammals, causing devastating outbreaks in sea lions and elephant seals and spreading in dairy farms. At least 46 people in the U.S. have contracted the virus in 2024, according to the Centers for Disease Control and Prevention (CDC).

These illnesses have been mild so far, but H5N1 can cause severe disease; more than half of cases detected worldwide since 2003 were fatal, according to the WHO. This statistic does not account for mild, undetected cases that were likely circulating in this time, but does suggest a concerning mortality rate for H5N1 should it become better at spreading between people.

The unidentified teenager in British Columbia is too ill to answer questions about possible exposures, health officials said in a news conference on Wednesday (Nov. 13). It may not be possible to find the source, they said. In August, a person from Missouri fell ill with H5N1 with no known exposure to cows, poultry, or wild birds, and health officials were never able to find out how they caught the virus. (The virus was closer to the version circulating in cows than in wild birds, however. The patient ultimately recovered.)

A recent CDC study has found that H5N1 is evolving to better spread between mammals. It still does not spread as easily as the seasonal flu, however, researchers reported Oct. 28 in the journal Nature.

The case could be a one-off or a signal that H5N1 is spreading undetected, according to StatNews. Canadian health officials said that they were alert to both possibilities.

"We’ve not found anybody else who is sick. And we’ve been testing,” British Columbia provincial health officer Bonnie Henry told StatNews. She said that people close to the teen have not fallen sick. "So I’m more comfortable that it was a single exposure and a more rare event. … More cases would have come to light by now if this was an exposure event that exposed multiple numbers of people, or there had been person-to-person transmission, which we know is rare with H5N1."