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The 'test and treat' strategy being explored by the WHO for HIV treatment may be misguided and particularly unsuited to poor countries, a disease modelling expert has said.

The strategy would involve annual HIV screening and treatment for everyone who tests positive for the virus.

This deviates from the usual method of starting treatment only once the number of CD4 white blood cells a patient has — an indicator of immune system strength — falls below a certain level.

The test and treat strategy could "lead to very serious public health problems in resource-constrained countries throughout the world," said Sally Blower, director of the Center for Biomedical Modelling at the US-based University of California, Los Angeles (UCLA).

Computer models exploring the possible outcomes of the strategy, published in The Lancet in 2009, do not take into account the behaviour of drug resistant strains, she said.

Kevin De Cock — then WHO HIV/AIDS director — and colleagues wrote that The Lancet results showed that test-and-treat could eliminate HIV transmission — defined as an incidence below one case per 1,000 people per year — within a decade.

But when resistant strains are considered, said Blower, elimination looks much less likely and the strategy could actually result in infection with drug-resistant strains.

This is because many more people would be treated for HIV, increasing the chance of an HIV strain becoming resistant and infecting others.

In research published today (14 January) in Science, Blower and colleagues have used a computer model to study the evolution of drug-resistant HIV strains and predict their future spread. They found that if resistant strains aren't treated with drugs to which they are sensitive, they are capable of creating their own epidemics as the infected person could go on to infect more than one person.

These strains are likely to have significant implications for HIV treatment programmes in developing countries where newer, second-line HIV treatments are unavailable, leaving resistance to flourish. The mass drug treatment of test and treat could exacerbate this.

Although The Lancet study did discuss the issue of drug resistance, it "did not model the spread of resistance, and made optimistic assumptions about resistance", said Geoffrey Garnett of the Department of Infectious Disease Epidemiology at UK-based Imperial College London.

"It would be helpful and important to include drug resistance in the [test and treat] model".

Blower modelled the HIVsituation among homosexual men in San Francisco, United States, but her work can be applied to other situations if information such as the types of drugs being used, how effective they are and the fraction of people taking the drugs is known.

Applying such a model to an African situation "would be very useful", said Brian Williams from the South African Centre for Epidemiological Modelling and Analysis, "but this depends first of all on getting good data".

The WHO told SciDev.Net that its commitment is to universal access to HIV drugs, and a test-and-treat strategy, is still at an exploratory stage.

Link to abstract in Science