Researchers have confirmed the ability of HIV to mutate inside the body faster than any known infective agent - enabling drug-resistant strains to develop within weeks or months.
A separate research team has demonstrated conclusively that the virus infects a far greater proportion of blood cells than previously supposed and does so by 'hiding' away in the body's own genetic material - making an effective vaccine extremely unlikely.
Resistance to the drug AZT was noted three years ago, but the problem seems to be more severe than first thought, according to David Ho, professor of medicine and microbiology at New York University School of Medicine.
Using a new technique for analysing different strains of virus growing in one patient, Professor Ho and colleagues have shown that drug-resistant HIV is even present in people who have not been treated with AZT.
In some patients who have been given the drug for six months or more, practically all strains of virus growing in their body are resistant, they report in the Proceedings of the National Academy of Sciences. 'These findings serve to emphasise the magnitude of the AZT-resistance problem,' the researchers say.
Professor Ho says the results confirm the belief that HIV mutates so rapidly inside the patient that strains able to survive treatment with AZT quickly multiply.
The results emphasise the importance of tackling Aids with a variety of weapons, he says.
Separate research also published in the journal shows that HIV infects a high proportion of the body's cells and yet does so in a dormant form that has made its presence difficult to detect.
Scientists had detected actively replicating HIV in only about one in every 10,000 white blood cells of the immune system. They found such a low level of infection perplexing as it seemingly could not explain how the virus could damage enough cells to cause disease. However, Janet Embretson, of the University of Minnesota Medical School, and her colleagues have discovered that as many as 30 or 40 per cent of the cells attacked by HIV are latently infected with the virus, providing a reservoir 'to perpetuate and disseminate infection'. They stressed that even these figures 'may be low'.
Such latent virus can covertly spread infection within an individual and between people, the researchers say. Latent virus, hidden within the cell's own genetic material, escapes attack from vaccines and the body's immune defences. 'It's pretty scary,' Dr Embretson says. 'Looking under the microscope it's quite frightening to see how many cells are infected.'
The researchers used a technique that amplifies the minute quantities of viral DNA hidden in the human cell. They used the process on tissue samples taken from people who died of causes other than Aids. 'We've been looking at patients who didn't even know they were infected. It shows the virus replicates quite well early on, before there are any symptoms,' Dr Embretson says.Reuse content