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Anti-HIV drugs could be useless for nearly half of patients within four years, according to a computer model prediction.
The new study, using data from the San Francisco gay community, suggests that by 2005, at least 42 percent of HIV cases could be drug resistant. In 1997, the figure was less than five percent.
The researchers found that unprotected sex is unlikely to be the cause of an epidemic of drug resistant strains. Instead, resistance appears primarily to be caused by patients who are taking the drug cocktails not sticking to a strict pill-popping regime. Any deviation from the treatment programme can allow the virus the opportunity to replicate and develop resistance.
This presents a serious problem as the health of people infected with HIV has been vastly improved by the drugs. Resistant strains can multiply unchecked, suppressing their host's immune system so that illness and death follow.
"The emphasis needs to be on stopping drug resistance in the first place,'' rather than preventing transmission of resistant strains, said lead researcher Sally Blower, from the AIDS Institute at the University of California, Los Angeles. She pointed out that doctors need to be as vigilant as patients in making sure correct treatment programmes are followed. She recommends creating specialist treatment centres for the disease.
The results have serious implications for developing countries, where it may be even more difficult to ensure medication is taken correctly.
"For resistance to occur you need the virus to evolve," says HIV specialist Andrew Phillips from the Royal Free Hospital, London. He told New Scientist that if patients follow an intensive regime that includes three different classes of drugs, the virus cannot replicate enough to evolve.
Resistant strains take hold when patients continue to take drugs that are unsuccessful at lowering the viral load. "If it evolves in the presence of drugs then it gets a chance to out compete the drug-sensitive virus," says Phillips. This can be prevented if patients quickly switch to different drugs.
The study predicted the transmission of drug-sensitive and drug-resistant cases of HIV between 1996 to 2005. The rate depended on the number of infections, the level of drug treatment and the rate at which resistant strains evolve within a person.
The study showed that if fewer people are treated for HIV, resistance will not become such a problem. However, not only is it unethical to withhold treatment, but treatment reduces the chance that the infection will be passed to a sexual partner. The researchers estimate that for every case of resistance caused by treatment, one other infection will be prevented.
Computer modelling tools are not perfect, says Phillips, because they rely on uncertain assumptions. But he adds that they are the only real tools we have for predicting the future of HIV.
The model predicted that 28.5 percent of HIV cases would be drug resistant in 1999, and this was almost exactly correct.
Journal reference: Nature Medicine (vol 7, p 1016)