A new study may shed some light on why sub-Saharan Africa is the hardest-hit region in the world when it comes to HIV/AIDS. Researchers say people who are infected with parasitic worms are much more vulnerable to infection with HIV, the AIDS virus.

The findings are based on a study of monkeys infected with a worm that causes schistosomiasis. In many countries, parasites released by freshwater snails penetrate the skin, eventually causing internal organ damage. Others become trapped in tissue, causing the body's immune system to react. Up to 300 million people worldwide may be infected with the parasites, according to the World Health Organization.

One of those studying the link between schistosomiasis and HIV infection is Dr. Ruth Ruprecht of the Dana-Farber Cancer Institute at Harvard Medical School. Dr. Agnes Chenine, formerly with Harvard Medical School, and Evan Secor of the US Centers for Disease Control, joined her in the research.

Dr. Ruprecht spoke to VOA English to Africa Service reporter Joe De Capua about the link, which she says has been suspected in field research for a while. She and her colleagues concluded that "what really was needed in the field was a primate model for a co-infection."

She explains further. "We were able to ask the question how much more virus it takes to get a chronic infection in a monkey that has no parasites compared to a monkey that has parasites? So what we did was we had two groups of animals. One group had no parasites, completely healthy, completely normal. The second group of animals had a new, acute parasite infection. So we continuously took lower and lower virus doses in one animal after the next until we found a dose of virus that no longer led to chronic infection. So that gave us the minimal dose of virus that was required to still achieve infection," she says.

The findings found that viral infection could be achieved in some animals with parasites using doses 17-fold lower than for animals with no parasitic infections. She says, "This is not a small difference. So in other words, a normal monkey can get exposed to a low dose of virus and not have chronic AIDS virus infection and a monkey that had the parasites could get exposed to that same low dose of virus but, because of the parasite infection, it now also became chronically infected with the AIDS virus."

She says the worms caused an imbalance in the monkeys' immune systems. She says, "It kind of provides a more fertile ground for the virus to grow in."

Dr. Ruprecht says it's beneficial to not only treat people with schistosomiasis to help protect them from HIV, but also those people who already have both parasites and HIV.

"There are data from a group in Israel that looked at Ethiopian migrants and some of them were HIV-infected. But when they were treated for their parasite infections, some of the abnormalities in the immune system subsided and their virus burden actually also decreased," she says.

However, those findings have not always been duplicated in other studies in other countries. Dr. Ruprecht has a theory as to why. She says, "I believe it's difficult in developing countries to come to clear-cut conclusions because you treat the parasite, but then the people get re-infected. And it's a chronic cycle of infection and treatment and re-infection."

Re-infection can occur so easily because some much of the freshwater may be contaminated with the flukes or worm-like parasites.