A new type of Aids virus can quickly turn resistant to one of the most effective drugs used to treat the disease. Scientists discovered the new strain among 603 HIV-positive patients who were so recently infected they had not yet even been given drugs.

A new type of Aids virus can quickly turn resistant to one of the most effective drugs used to treat the disease. Scientists discovered the new strain among 603 HIV-positive patients who were so recently infected they had not yet even been given drugs.

Their findings suggest the much-feared increase in drug-resistant forms of HIV is beginning to take root with a strain that has an in-built capacity to resist treatment with AZT, the principal anti-Aids treatment.

Although drug-resistant strains of HIV were discovered in 1992 they were almost invariably found in patients having treatment with one or more anti-viral drugs. While drug resistance has been seen in HIV-infected patients before, it has seldom been seen in untreated patients.

The latest strain was evidently transmitted among people who have never been exposed to AZT, suggesting that drug resistance is spreading rapidly within HIV-positive people who may not even know they are infected.

Gerardo Garcia-Lerma, a visiting scientist at the US Centres for Disease Control and Prevention in Atlanta, Georgia, said 20 patients – 3.3 per cent of the sample – had the HIV strain, which carries several mutations predisposing it to AZT resistance. Tests showed it could become AZT-resistant within two weeks although "normal" HIV strains took months to become immune to the drug, Dr Garcia-Lerma said. "It's important to know whether patients infected with this new class of virus respond to drugs as well as other HIV-positive patients," he added.

The latest study, in the Proceedings of the National Academy of Sciences, found several genetic mutations in the new strain of HIV made the virus perfectly poised for further mutations that conferred full AZT-resistance. Dr Garcia-Lerma said doctors would test patients for this new strain and report its prevalence so the general rise in drug-resistance could be monitored.

AZT was one of the first anti-viral drugs used for Aids patients. It also goes under the names of zidovudine or Retrovir. The drug works by blocking the replication of HIV but also can cause severe side-effects in some patients, such as anaemia, vomiting and liver problems.

Early results of clinical trials showed the drug substantially delayed the onset of Aids in HIV-positive people but, when AZT was used on its own, drug resistance developed, causing patients to fall ill with Aids.

Aids patients are being treated with a combination of three anti-viral drugs, including AZT, to minimise the risk of resistance developing. But scientists know some Aids viruses have already become resistant to up to three drugs, which forces doctors to use other treatments.

Triple-combination therapy, involving three drugs taken at once, has shown to be remarkably effective in tackling Aids. The principle is to attack the virus from several different directions at once, which makes its "escape" by mutating into drug-resistant forms almost impossible. But several studies are beginning to show HIV can become simultaneously resistant to three anti-viral drugs. If such strains become widespread, that will be a severe setback in Aids treatment.

* China's first conference on Aids and sexually transmitted diseases opens in Beijing on 13 November, for three days.

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