JOHANNESBURG, 18 September 2009 (IRIN) - The number of HIV-infected children in southern Africa who survive into adolescence without treatment is greater than previously thought, and will keep increasing for years to come.

These are the findings of a study estimating the scale and likely course of the HIV epidemic among older children in southern Africa, published in the September 24 issue of AIDS, the journal of the International AIDS Society.

Previously, it was assumed that few children who contracted HIV from their mothers would survive beyond the age of five without antiretroviral (ARV) treatment, but Prof Robin Wood, director of the Desmond Tutu HIV Centre at the University of Cape Town and one of the co-authors of the study, noticed unexpectedly high rates of HIV infection among older children in large sets of prevalence data he was analyzing.

"I was surprised; my first assumption was that there was some sexual molestation going on here, but when I looked more closely I saw that the number was increasing as every year went by, and it was equally distributed among males and females," he told IRIN/PlusNews. "Then we talked to people in Zimbabwe who'd seen the same thing and we decided to put together a model."

Most HIV-prevalence surveys exclude children aged between five and 15, but recent surveys in Zimbabwe, Botswana, South Africa and Swaziland included them.

The authors compared figures from these surveys with data on mother-to-child transmission (MTCT) and child survival to make projections about the prevalence of HIV among older children in South Africa and Zimbabwe over the next 20 years. They estimated that about one-third of HIV-infected infants in Africa were "slow progressors", with an average life expectancy of 16 years.

No one knows precisely why some children succumb rapidly to AIDS-related illnesses and others much more slowly, but the authors of the study published in AIDS speculated that infants infected post-natally, through breast feeding, were more likely to be slow progressors than those infected during pregnancy or delivery.

Few children aged between five and 15 are tested for HIV, as most healthcare providers are unaware that some infected children can survive for many years. As a result, there is often a significant time lag before slow progressors are diagnosed.

The growing number of HIV-infected older children now being diagnosed in South Africa is the result of high rates of MTCT 10 years ago. "It highlights the fact that if we don't get on top of MTCT now, we'll be seeing the consequences of it for decades afterwards," Wood said.

In Zimbabwe, where adult HIV prevalence peaked in the late 1990s but has since declined, the infection rate among 10-year-olds is expected to fall from 3.2 percent in 2008 to 1.6 percent in 2020.

In South Africa, HIV prevalence among 10-year-olds is expected to increase from 2.1 percent in 2008 to 3.3 percent in 2020 if prevention of MTCT is not provided.

Wood warned that health care providers in South Africa should be prepared to deal with rising numbers of HIV-infected adolescents - not only slow progressors, but also those who have been on ARV treatment since infancy, and those who have contracted HIV through sex.

"They're a tricky group to deal with for many reasons - adherence to therapy is not as good as in younger children," he said. "They have special needs and we need to design more adolescent-friendly clinics for them." The Desmond Tutu HIV Foundation has set up one such clinic in Gugulethu, a township outside Cape Town.

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