A doctor draws blood from a man to check for HIV/AIDS at a mobile testing unit in Ndeeba, a suburb in Uganda's capital Kampala May 16, 2014. REUTERS/Edward Echwalu

By Alex Whiting

LONDON (Thomson Reuters Foundation) - A parasitic worm which affects millions of the world's poorest people may hold an important but little-known key to cutting the spread of HIV, researchers said ahead of a conference on the issue in London.

Schistosomiasis affects at least 250 million people. It is caused by parasitic worms, picked up in infested waters, which drill through people's skin and lay eggs in their bodies.

If the worms lay eggs in a woman's genital areas, including the vagina and cervix, they can cause lesions which make women more vulnerable to HIV, experts in the tropical disease said.

Women are three times more likely to be infected with HIV if they have female genital schistosomiasis (FGS), studies carried out in Zimbabwe, Tanzania, South Africa and Mozambique have found.

"It's going completely under the radar," Marianne Comparet, director of the London-based International Society for Neglected Tropical Diseases (ISNTD), said in an interview.

"Treating one could really impact on the other," she told the Thomson Reuters Foundation.

Men with the worms in their genitals show a sharp increase in the amount of HIV virus in their semen, researchers said.

The treatment for schistosomiasis is cheap - the drug has been donated for years to the World Health Organization (WHO), so this could be a relatively easy way to help cut the spread of HIV, experts said.

"In the same way that circumcision came out as something that really changed the way people approached HIV transmission, this could really be the next big thing in controlling HIV transmission," Comparet said.

Circumcision has been found to cut the spread of HIV among heterosexuals and is recommended by the WHO as a means of prevention.

Nearly 37 million people live with HIV, the majority in Africa.

A LITTLE-KNOWN DISEASE

It is not known how many people have FGS, but estimates range from 20 million to 80 million, the vast majority in Africa.

According to the WHO, most cases of FGS are undiagnosed and few medical staff are aware of its existence. It gets no mention in medical textbooks or nursing curricula in any of the countries where schistosomiasis is endemic, WHO says.

The U.N. agency recommends the regular treatment of young girls through mass drug administration in schools and communities to prevent FGS from developing.

Treatment kills adult worms but it cannot reverse damage they have already done to people's organs and tissues.

"It starts early on, and then when you are a young woman, without any treatment it becomes really serious, and when women become sexually active they are very vulnerable to HIV," Jutta Reinhard-Rupp at Merck Serono said in an interview.

Merck Serono produces praziquantel, the only treatment available for schistosomiasis.

FGS can also cause other complications including infertility and ectopic pregnancies.

The link between FGS and HIV is very difficult to prove in a clinical study because it not possible to have a control group that is left untreated, Reinhard-Rupp said.

Another possible link is between schistosomiasis in men's genitals and the spread of the HIV virus.

Men with both diseases had an HIV viral load in their semen 10 times bigger than that of men without schistosomiasis, according to initial findings from a small-scale study carried out in Zimbabwe last year.

After treatment for schistosomiasis, the viral load returned to normal levels.

The findings will be made public at the ISNTD Coinfections conference in London on Friday.

Many countries in southern Africa are seriously affected by both schistosomiasis and HIV, Peter Leutscher, who is a professor at Aarhus University Hospital and helped carry out the research, said in a telephone interview.

"This overlap of HIV and schistosomiasis is really striking," he told the Thomson Reuters Foundation.

"It's a neglected risk factor" in the fight against HIV, he said.

Leutscher wants genital schistosomiasis to be included alongside other risks involved in the spread of HIV like the number of sexual partners, condom use, circumcision, and other sexually transmitted diseases.

(Reporting by Alex Whiting, editing by Tim Pearce.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit http://news.trust.org)