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Researchers have developed the first quick way to measure and monitor 'hotspots' of HIV infection involving the strains of the disease that predominate in developing countries.

The new test — developed by scientists at the US Centres for Disease Control (CDC) in collaboration with colleagues in Thailand — can provide a measure of the incidence of the virus in a population in a matter of days, as it is based on an assessment of how long a particular individual has been infected. In comparison, the traditional approach takes about three years, as it requires tracking individuals through a population to monitor for new infections.

By allowing HIV 'hotspots' to be quickly and easily identified, the test could help decision makers assess which groups of people should be the top priority for AIDS prevention strategies, such as promoting condoms or AIDS education. It could also give a rapid indication of the success of these strategies, as well as of the effectiveness of vaccines and microbicides undergoing clinical trials.

CDC researcher Timothy Granade announced the development yesterday (30 March) in London at the Microbicides 2004 conference on behalf of Bharat Parekh, a CDC colleague and instigator of the work. Granade said that there was already "a high demand from people who would like to use this assay".

It has taken six years to adapt the test to strains of HIV responsible for the epidemics in Africa, Asia and South America. Although researchers at CDC are still refining the technique, they are already creating prototype kits, and training scientists from around the world — including from Cambodia, Ivory Coast, South Africa, Thailand, Vietnam and Zimbabwe — in how to use them.

Kevin de Cock, epidemiologist and director of the CDC's division in Kenya, welcomed the findings as an important breakthrough. "It's as close as you can get, epidemiologically, to walking on water," he said.

The new test, called the 'BED-capture assay', involves just a single test of individuals already diagnosed as HIV-positive, and measures how recently they became infected. This in turn gives an indication of how many new infections are appearing in the population each year.

The technique is based on an assay that has been in routine use in the United States for around a decade to monitor epidemics with the subtype B strain of HIV, particularly in men who have sex with men. It works by using a fragment of HIV protein to stick to — and reveal the presence of — antibodies against HIV in a patient's blood.

But the new test is different in that it can distinguish between people who have low levels of the antibody — and were therefore only recently infected — and those who have been infected for much longer, perhaps years. This is possible because a person's antibody levels against HIV continue to rise with time, as the body constantly attempts to fight off the virus. 

The CDC researchers began devising their BED-Capture version of the assay in 1998, using a fragment of subtype B. But when they attempted to perform the test on people infected with the subtypes typically circulating in Thailand, a subtype E and a different variant of subtype B, they got less reliable results. They realised that this was because antibodies are specifically tailored to different strains of the HIV virus.

The next step was to adapt the test to a variety of HIV strains by adding extra protein fragments representing subtypes A to E. The team have showed that the technique works for measuring HIV incidence in Thailand (subtypes B and E), Ethiopia (subtype C), Kenya (subtypes A and D) and Zimbabwe (subtype C).

In Cambodia, the researchers have discovered different trends in the HIV epidemic for different regions. In central and western Cambodia, the incidence of infection is decreasing, while in the east, there is no change. This means that in the east of the country — which borders on Thailand, and contains a high proportion of migrant workers and a thriving commercial sex industry — the rapid spread of HIV infection is still occurring.

These results — due to be published soon — are a clear indication that "it's difficult to make inroads in that region [of Cambodia]", said Granade.

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