An investigation into the case of a teenager infected with avian flu in British Columbia has found no new cases or evidence of human-to-human transmission, while failing to identify how the teen caught the virus, the province's top doctor said.
Provincial health officer Dr. Bonnie Henry said the teen remains in critical condition with H5N1 bird flu but is stable and has made "some progress" toward recovery.
"Our detailed public health investigation is closed for now, unless anything new arises. I am very confident that there are no additional cases out there right now, but we still need to be careful," Henry told a news conference on Tuesday.
The teen's infection, announced earlier this month, was the first human case of H5N1 avian flu acquired in Canada.
Henry said the investigation aimed to identify any contacts who may have had exposure to the teenager.
It found 34 health-care workers had some exposure and they were each monitored for symptoms through a 10-day incubation period.
Sixteen close friends and family members were also monitored, in addition to testing on birds, rodents, dogs, cats and reptiles and environmental samples.
Henry said the investigation found the infection was not directly related to outbreaks of bird flu at B.C. poultry farms, and genome sequencing of the virus showed it most closely matched that of wild birds found in the Fraser Valley in early October.
"There may have been an intermediary, either another bird or an animal, between the geese that were detected and this young person," she said.
"But very importantly, it also tells us that this genotype was quite different from what is causing the devastating outbreaks in the poultry industry. So we had no evidence from speaking to family and friends that this young person had been near poultry farms."
The Canadian Food Inspection Agency lists 62 premises where poultry flocks are infected with highly pathogenic avian flu across the country, with 54 of those in B.C.
The figures show that nearly 6.9 million birds in B.C. have died or been culled as a result of the virus.
Officials have released few details about the teenager who caught the illness, other than saying they were in the region covered by Fraser Health and were receiving treatment at BC Children's Hospital.
Henry said the teen is not breathing on their own and officials have not been able to speak to the patient directly as part of their investigation.
She said the teen has no underlying conditions that could contribute to why they are so ill.
"There's about 900 cases globally of H5N1, and we do see that younger people tend to have more severe illness. And it may be that as we get older we have some exposure to different influenza viruses, with particularly the N1 part, that might give us some protection from that severe illness," she said.
Researchers are also looking into whether mutations may have made it easier for the virus to bind to receptors deep in the lung, Henry said.
Human cases of the virus continue to be rare, she said, and people can protect themselves by washing their hands, avoiding touching dead or sick animals, and staying up to date on immunizations, especially the seasonal flu vaccine.
