Health officials in Hong Kong reported Friday a single instance in which a strain of the swine flu was found to be resistant to Tamiflu, the main antiviral drug used to treat the virus.

The case involved a 16-year-old girl who came from the United States in mid-June, had mild symptoms, and was eventually discharged.

The current strain of H1N1 influenza A virus has been circulating in several countries since it first appeared in Mexico in April.

The Tamiflu-resistant strain in Hong Kong was found during routine sensitivity testing of the swine flu virus to both oseltamivir and zanamivir, the Hong Kong Department of Health said in a statement on its website.

Oseltamivir (Tamiflu) and zanamivir (Relenza) are the two antiviral drugs that appear to be effective in treating the H1N1 virus, Health Canada says.

First time in Hong Kong

"This is the first time Tamiflu resistance in HSI virus found in Hong Kong," the statement said, also noting that there have only been two other cases where this has happened, in Denmark and Japan.

The young woman came to Hong Kong on June 11 from San Francisco. A specimen was taken from her when she arrived at the Hong Kong International Airport and she was then intercepted by health officials and taken to hospital.

Although she tested positive for the H1N1 virus, the teenager, who had mild symptoms, opted not to take Tamiflu and left hospital on June 18, the Department of Health said.

In 200 cases of testing, this was the only instance that a Tamiflu-resistant strain was found, the Department of Health said.

Further tests are underway, it said and the case will be reported to the World Health Organization.

Like any other flu, swine flu is a respiratory infection. It exploits a weakened immune system to attack major organs, especially the lungs. Most cases are mild and require no medical treatment.

The common flu sends about 20,000 Canadians to hospital each year. Canada's Public Health Agency's website says between 4,000 and 8,000 Canadians can die of influenza and its complications annually, depending on the severity of the season.