A Canadian teenager who is in critical condition after contracting H5N1 bird flu was infected with a version of the virus that is different from the one circulating in dairy cattle in the United States, Canadian authorities announced Wednesday.

The National Microbiology Laboratory in Winnipeg confirmed the infection was indeed caused by the H5N1 virus. But genetic sequencing showed that it is of a genotype that has been found in wild birds, not the version that has been circulating in dairy cattle in the U.S.

Canada has been doing surveillance in dairy cows looking for the virus, but to date has not detected it in any herds.

Bonnie Henry, British Columbia's provincial health officer, told STAT in an interview that she'd been expecting these genetic sequencing results. "That's what we've been seeing consistently," she said.

The unidentified teenager lives in British Columbia.

The H5N1 virus has evolved into a variety of strains over the nearly 30 years that it has circulated globally. The version that is spreading in cows is known as a 2.3.4.4b virus, of a genotype called D3.13. There have been about 46 human cases confirmed in the U.S. this year, all involving very mild illness.

Historically, H5N1 has been associated with severe illness. Of the roughly 950 human cases recorded to date, just under half have died.

The virus that infected the Canadian teenager was a 2.3.4.4b virus of the D1.1 genotype. This version of the virus, which is spread by wild birds, has caused poultry outbreaks in a variety of places, including recently in Washington state.

British Columbia currently has 26 H5N1 outbreaks in poultry operations, many in the Fraser Valley in the southwestern part of the province. The infected teenager lives in that part of the province.

The individual is the first confirmed person to have contracted H5N1 in Canada. In 2014, a person in the neighboring province, Alberta, was diagnosed with H5N1, but that individual had recently returned to Canada from China and was believed to have been infected there. That earlier H5N1 case was fatal.

Health officials in British Columbia have been unable so far to determine how the teenager contracted bird flu. He or she is too ill to answer questions, but information provided by family members suggests there was no exposure to infected poultry, Henry said. The teenager did have contact with a number of pets -- cats, dogs, and reptiles -- both at home and the homes of friends. But none of those animals tested positive for the virus.

During a press conference on Tuesday, Henry warned that while the search for the source of the infection continues, it's possible it will never be known how the teenager contracted the virus. STAT asked if that was an attempt to be prudent or an effort to set expectations. "A bit of both," Henry said.

However, comparing the genetic sequence of the virus from the teenager to those of known H5N1 poultry outbreaks could point to further clues, she said. "I'm more confident today than I was yesterday that we might actually find the [source], but we may not."

The existence of an unexplained H5N1 case is unsettling for experts who watch this virus, which has for years been high on the list of possible pandemic flu viruses. Such a case could be a one-off, like the recent unexplained case in Missouri, in a person with no known exposure to cows, wild birds or poultry. But likewise, at the start of the 2009 H1N1 flu pandemic, two sporadic cases of swine flu infection in children in California who had had no contact with pigs or with each other were the first signs that a pandemic had started.

Henry is aware of both scenarios. "Obviously my spidey senses went off when we first heard about this," she said. "But I'm more reassured in that we followed up with anybody that they had close contact with during the ... infectious period. We've not found anybody else who is sick. And we've been testing."

"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."

Allison McGeer, an infection control expert at Toronto's Sinai Health, and an infectious diseases professor at the University of Toronto, shares Henry's sense of optimism that this may be a one-off event.

"You would think that if this was the beginning of something that we would have seen other cases by now," McGeer said. "Everyday that we don't hear anything from B.C. is a good day."

Henry said British Columbia is drawing blood samples from people who were in close contact with the teen to see if any developed antibodies to the virus. But conducting that kind of work will take some time, because it takes about two weeks for people to develop antibodies to a pathogen after an exposure. She suggested some results may become available next week.