WEDNESDAY, Nov. 11, 2009 (HealthDay News) -- Researchers report that a father in a Canadian family developed a strain of the H1N1 swine flu that was resistant to the antiviral Tamiflu, after being given the drug to prevent the disease.
To date, the World Health Organization has reported some 45 cases of H1N1 swine flu that were resistant to Tamiflu, which is why the U.S. Centers for Disease Control and Prevention and others caution against using Tamiflu to prevent the disease in people who are symptomless.
"We have a person who was taking post-exposure prophylaxis with Tamiflu, probably at the time he was already incubating the virus, but before symptoms developed," said lead researcher Dr. Guy Boivin, of the Centre Hospitalier Universitaire de Quebec, in Quebec.
According to the report, when the father's 13-year-old asthmatic son was hospitalized with H1N1 swine flu, other family members were given a reduced dose of Tamiflu to prevent them from getting the disease. These included the 59-year-old father, who suffered from chronic obstructive pulmonary disease (COPD), his 50-year-old wife and two teenage daughters.
Within a day the father developed flu symptoms, but he did not see a doctor for eight days. A test of his virus showed that it was resistant to Tamiflu. None of the other family members developed flu. The father recovered from the flu, according to the report in the Nov. 11 online edition of the New England Journal of Medicine.
The mutated strain of H1N1 swine flu remained susceptible to another antiviral, Relenza, Boivin added.
The family's experience was a good setting for resistance to emerge, Boivin said. When someone has the H1N1 swine flu, Tamiflu is given twice a day for five days. But when the drug is used to prevent the disease, it is given only once a day for 10 days, he explained.
Since the father was most likely already infected with swine flu, the dose of Tamiflu he received was ineffective at stopping the virus and low enough to allow the virus to develop resistance to the drug, Boivin said.
"In that context, Tamiflu is a risk factor for the emergence of resistance," he explained.
Boivin said that one-third of the known cases of H1N1 virus resistant to Tamiflu have been among people already exposed to the pandemic flu. "Another third emerged in imuno-compromised [weakened immune system] patients who received prolonged therapy with Tamiflu," he said.
The mutation of the H1N1 virus that caused its resistance to Tamiflu was the same one that was found in seasonal flu resistant to the drug. Last year, one strain of the seasonal flu was resistant to Tamiflu, Boivin said.
"You should be careful when you use post-exposure prophylaxis," Boivin said. In most cases, Tamiflu should be reserved for early treatment of flu when symptoms develop, he said.
Dr. Marc Siegel, an internist and associate professor of medicine at New York University Langone Medical Center in New York City, said that "Tamiflu prophylaxis given to the wrong people can increase your risk of a resistant strain developing -- and that's the last thing we need."
Siegel thinks, however, that Tamiflu used correctly can help prevent people from getting the flu, especially when the vaccine is in short supply.
"In the absence of available vaccine, one of the ways you can cut down on the spread of H1N1, to people you feel are at risk, is with the proper use of Tamiflu prophylaxis," he said.
Another expert, Dr. Pascal James Imperato, dean and Distinguished Service Professor of the School of Public Health at SUNY Downstate Medical Center in Brooklyn, N.Y., said that "based on our current knowledge, it appears that resistance of influenza viruses to the anti-influenza drug Tamiflu occurs in a setting of drug overuse."
This reported case from Canada underscores the need to limit the use of Tamiflu in people who aren't displaying flu symptoms, Imperato said.
"It also points out the need for increasing the dose of the drug to therapeutic levels in anyone who develops symptoms of influenza while on lower post-exposure prophylactic doses," he said.
For more on H1N1 swine flu, visit the U.S. Department of Health and Human Services.