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As a Canadian teenager’s condition worsened amid the first presumptive positive test for bird flu in that country in over a decade, health officials were struggling to answer a broader and potentially worrisome question: how the infection occurred.

None of the obvious connections to an H5N1 influenza case were in play, according to Dr. Bonnie Henry, the provincial health officer in British Columbia, where the teen was admitted to B.C. Children’s Hospital in Vancouver over the weekend with conditions that left them in critical condition. The patient was experiencing acute respiratory distress syndrome, or ARDS.

Henry said at a media briefing Tuesday that the teen, who has not been identified by name or gender, was not on a farm and didn’t have any connection or contact with a poultry operation. She noted that the teen was likely infected by contact with a diseased animal or something else in the environment, but did not have any contact with birds. The person did interact with other pets in and outside the home, including dogs, cats and reptiles, in the days before they began reporting symptoms. Testing done on the animals so far has been negative for influenza.

“Having said that, I must caution that there is a very real possibility we may not ever determine a source,” Henry added.

For months, virologists and experts in the United States have urged government health officials to more aggressively push for testing and treatment of workers at dairy farms and poultry operations at which bird flu has been present. The British Columbia case, with its unknown origin, is likely to accelerate those requests both in the U.S. and Canada.

“This case is very concerning for a number of reasons,” says Rick Bright, an American immunologist and vaccine researcher and a U.S. government health official from 2016 to 2020. “It’s in an otherwise healthy teenager. Like a similar case in Missouri, there’s no clear source for infection and no direct farm connection.”

Bright also notes that testing for influenza was missed when the teen first reported symptoms, “and it’s taking far too long to confirm details about the virus, to understand any changes or mutations that could indicate increased virulence or antiviral drug resistance.”

Henry said the teen, who lives in the Fraser Valley region of southwest B.C., reported initial symptoms of conjunctivitis, fever and cough, visited an emergency room on Nov. 2 and was sent home. Six days later, with their condition worsening, the patient was admitted to the hospital.

At that time, the patient was tested for flu and other respiratory viruses. Further testing by Vancouver’s public health reference lab yielded a presumptive positive result for H5 virus or avian influenza. Samples have been sent to a Canadian federal laboratory in Winnipeg for confirmation of the teen’s case. 

Contact tracing amongst 35 to 40 individuals, including medical workers who came into contact with the teen, has so far revealed no other cases. A human case of bird flu in Canada is a rare event: The only other confirmed human case of bird flu in the country came in 2014. The person in that case, who most likely contracted the virus while traveling in China, died, health officials said.

Acute respiratory distress syndrome (ARDS), from which the teen is suffering, is a life-threatening lung condition, says Gabriel Wardi, a critical care and emergency physician at the University of California San Diego. Survival rates, Wardi says, “vary based on age of the patient, cause of the lung inflammation, and severity of lung failure.” On average, between 55% and 65% of patients with ARDS survive, though they may experience long-lasting effects.

“We are looking very, very carefully at all potential animal exposures, bird exposures,” Henry said, referring to the teen’s case. “There were other pets in the house, and there was contact with pets in other houses. Right now, we have no specific source identified, but the testing is ongoing in partnership with our veterinary colleagues, and we’ll be continuing that investigation very thoroughly.” (H5N1 has not yet been identified in Canada’s dairy herds or dairy products.)

Sources tell Fortune that the U.S. Centers for Disease Control and Prevention(CDC) has been in touch with Canadian health authorities, providing clinical consultation. The CDC offered additional assistance if needed.

In the U.S. this year, 46 cases of bird flu in humans have been confirmed, nearly all of them farm workers who were exposed to infected poultry or cattle. That total includes the Missouri case, which, like the one in British Columbia, does not appear to have any obvious connection to either farm or commercial flocks.

“It is incredibly important for us to investigate this really thoroughly to understand if anybody else is infected to try and understand the transmission and the exposure patterns,” Henry says.

“Sadly, we’ve seen an uptick in the number of H5-infected animals and people in the U.S.,” Bright says. “It’s no surprise to see the virus spreading beyond our borders, or the introduction of additional variants of the virus from wild birds to poultry, cows, pigs and people.”

The Canadian teenager, who is receiving intravenous antiviral treatment, did not cross the U.S. border or have contact with farms or ill animals in Washington state, Henry said. Nor did the teen or any of their contacts recently travel to southeast Asia or Vietnam, where fatal H5N1 outbreaks have occurred.Beyond those certainties, though, there are many unanswered questions about how the infection occurred.

According to the World Health Organization, the risk of sustained human-to-human transmission is considered “unlikely” at this time, but continued vigilance is warranted. In the United Kingdom and France, though, government officials have already raised the risk level of the bird flu disease from medium to high. And in the U.S., human H5N1 cases in California and Washington are on the rise, though so far the reported symptoms have been mild.

The Canadian teen was not in school during their infectious period, Henry said. The health official added that one of the keys “is preventing infection in the first place” by staying up to date on vaccinations, as well as staying away from school, work, etc. if flu symptoms occur.

In the U.S., “We need to accelerate our readiness posture for a pandemic influenza response” during the weeks left before President-elect Donald Trump assumes office, Bright says. Trump’s first term included a botched response to the COVID-19 pandemic, according to a congressional report.

This time, “The (H5N1) virus seems to be gearing up for wider impact,” Bright says—all the more reason, he adds, for both U.S. and Canadian officials to begin pushing harder for more personal protective equipment, testing and available antiviral treatment options in the weeks and months to come.

This article originally appeared on https://fortune.com/well/2024/11/13/bird-flu-canadian-teen-wider-spread/