WASHINGTON – People infected with parasitic worms may be much more susceptible to the AIDS virus, according to a study published Tuesday that may help explain why HIV has hit sub-Saharan Africa particularly hard.

The study involving monkeys demonstrated how a type of parasitic worm that causes schistosomiasis, which affects 200 million people globally, may make HIV infection more likely.

Much lower amounts of the AIDS virus – 17 times lower – were needed to cause infection in monkeys who had the parasitic worms than in the parasite-free monkeys, the researchers said.

“The presence of the worm is like adding fuel to the fire – it creates more fertile ground for the virus to take hold,” Dr. Ruth Ruprecht of Harvard Medical School, one of the researchers, said in a telephone interview.

Evan Secor of the U.S. Centers for Disease Control and Prevention, another of the researchers, said the findings likely apply to people as well. This may confirm suspicions that parasitic worm infections like those common in parts of sub-Saharan Africa with unsanitary water supplies make people more vulnerable to HIV, Secor said.

“Sub-Saharan Africa has only like 10 percent of the world's population but almost two-thirds of the world's HIV/AIDS,” Secor said in a telephone interview.

“So there's an apparent disproportionate amount of HIV/AIDS there, and it's very severe. So the hypothesis is that one of the things that may contribute to the more intense nature of HIV/AIDS in sub-Saharan Africa is the presence of these parasitic worms,” Secor added.

Schistosomiasis, seen primarily in developing countries, is caused by tiny flatworms that live in snail-infested freshwater like rivers and lakes.

When people wade, swim or bathe in contaminated water, worms bore through the skin and travel in the blood, causing anemia, diarrhea, internal bleeding, organ damage and death.

Secor said the parasitic worm infection may undercut the immune system's ability to fight off HIV infection and may make it easier for HIV to get into white blood cells.

HIV MORE LIKELY TO SPREAD

The researchers conducted experiments with rhesus monkeys, some of which had an acute infection with the Schistosoma mansoni parasitic worm and some of which were parasite-free, normal and healthy.

They then exposed the monkeys to a hybrid AIDS virus – a genetically engineered version that combined elements of the monkey AIDS virus and the human immunodeficiency virus that causes AIDS in people.

Ruprecht said having parasitic worms not only made a monkey more susceptible to AIDS virus infection, but once infected they had far higher concentrations of the virus in their bloodstream, meaning they became more likely to infect others.

“If the virus is extremely high in the blood, then the chances are that the virus is also going to be high in the genital fluids. And therefore such a host would be more likely to be spreading the infection to others,” Ruprecht said.

Ruprecht said the findings emphasized the need for public health measures to control parasitic worm infections in regions where HIV infection is common. A drug called praziquantel is available to treat schistosomiasis.

The study, published in the Public Library of Science journal PLoS Neglected Tropical Diseases, can be read at www.plosntds.org/doi/pntd.0000265.

(Editing by Eric Beech)