WEDNESDAY, July 8, 2009 (HealthDay News) -- Health experts say they can't predict at this point how widely a new strain of swine flu resistant to the drug Tamiflu will spread, or how dangerous it might become.
"This is not unexpected, but it's very unpredictable whether this will end up spreading," said Dr. John J. Treanor, professor of medicine and of microbiology and immunology at the University of Rochester Medical Center in New York.
"There is certainly a potential for the novel H1 viruses to develop resistance to oseltamivir [Tamiflu] and for those resistant viruses to become widespread," he added. "Everyone recognizes that. That is the Achilles' heel of antiviral therapy and it's completely possible that we will see this with the novel H1N1."
"This is expected to a certain extent," agreed Dr. Manjusha Gaglani, associate professor of pediatrics at Texas A&M Health Science Center College of Medicine and a pediatric infectious diseases specialist at Scott & White Hospital.
Strains of the "regular" seasonal flu often become resistant to antiviral drugs but, generally speaking, antiviral resistance is less common with Tamiflu and a related antiviral drug, Relenza, than it is with two other drugs, amantadine and rimantadine, which the U.S. Centers for Disease Control and Prevention has recommended against using for seasonal flu.
Tamiflu is preferred over Relenza in general because it is easier to administer, Gaglani said.
So far, three people have been stricken in recent weeks by a resistant strain of the new H1N1 swine flu virus -- one in the United States, one in Denmark and one in Japan, according to published reports.
In all three cases, the illnesses were mild and all of the patients recovered.
Tamiflu is the main weapon available in the effort to prevent and treat H1N1 infections. As of yet, there is no vaccine for the H1N1 swine flu.
There is one troubling aspect to the resistance developments however: One of the people who fell ill had had no known contact with Tamiflu.
"This is a little bit more disturbing," Treanor said. "It seems to be a spontaneously arising virus. How did that happen?"
More widespread resistance could come from one of two scenarios, Treanor said.
The first would be if the virus develops mutations in people who are being treated with Tamiflu and then spreads to others.
The second would involve a random genetic reassortment, in which the new swine flu and the seasonal flu exchange genes. "The current seasonal viruses are resistant to Tamiflu," Treanor said. "We could see a get-together resulting in a hybrid which potentially could be able to infect people more easily."
But it's also possible that changes in the genetic structure of the H1N1 swine flu virus could reduce the likelihood of transmission, Treanor said.
Gaglani stressed that, at this point, there is no evidence that the seasonal flu has mixed with the swine flu. That would be a concern because last year's seasonal flu was resistant to Tamiflu, she said.
"We need to monitor now what is circulating and what proportion are resistant to Tamiflu," Gaglani said. That will guide future treatment decisions.
There has been speculation that overuse of the drug has contributed to the new drug resistance but, at this point, that is another unknown, and people who fall ill do need to be treated.
"I don't know how much that's being driven by use of the drug," Treanor said. "I think the drug should be used where indicated. If resistance happens, it's going to happen, but we have to use medicine to treat sick people appropriately."
Dr. Len Horovitz, a pulmonary specialist at Lenox Hill Hospital in New York City, thinks overuse of Tamiflu might be to blame for the limited resistance.
"It's my observation that Tamiflu is being overused, particularly in mild cases that don't have an indication for using Tamiflu," he said. "I'm also observing that, particularly among pediatricians and maybe doctors not reading CDC recommendations, that they are prophylaxing household contacts very often unnecessarily. Because this is a mild flu, most of these cases don't need to be treated with Tamiflu unless there's a concern about complications. Otherwise, for a mild case, where there is no underlying illness or chronic illness, you don't treat the index case and you certainly don't prophylax people around the index case."
On June 11, the World Health Organization declared the first flu pandemic since 1968, triggered by the rapid spread of the H1N1 swine flu virus across North America, Australia, South America, Europe and regions beyond. Two weeks ago, U.S. health officials said they were considering a swine flu immunization campaign that could involve an unprecedented 600 million doses of vaccine. But a vaccine does not yet exist.
Find out the latest on the swine flu outbreak at the CDC.