As a Canadian teen remains in critical care with an H5N1 avian flu infection in a British Columbia hospital, health investigators said today they haven't identified the source, but have found other clues such as virus changes that may have led to a more severe lower respiratory infection.

B.C. health officials say they still don't know how a teenager became infected with the H5N1 strain of avian flu but say no further cases have been identified.Earlier this month, health officials announced they were investigating what was believed to be the first human case of H5N1 avian influenza caught in Canada.

BC's top doctor Bonnie Henry says that a potential source of the exposure to the teen is a pet dog that was sick at the time the young person was infected.But she said all the testing that was done on the dog have come back negative for the virus.

The B.C. teen who was infected with H5N1 avian influenza remains in critical care in hospital, said provincial health officer Dr. Bonnie Henry. The unidentified teen, who tested positive for avian influenza nearly three weeks ago,