LUSAKA, Zambia -- Since the rise of AIDS in Africa 15 years ago, researchers have been baffled by the wide disparity in infection rates from city to city.
A study released Tuesday gives the clearest reason yet: cultural practices such as circumcision and early sexual activity.
The U.N. research, released at a major African AIDS conference, found the disease spread more quickly in places where girls became sexually active at an earlier age and where male circumcision was uncommon, leaving men with a greater risk of contracting the disease during unprotected sex.
The findings give the first clear pointers on how to deal with the problem, said lead researcher Dr. Michel Carael of the U.N. AIDS office in Geneva.
"The prevalence of the virus in very young girls is a major dynamic in this epidemic," he said. "We now know the importance of working to reduce early experiences of risky sex among girls and young women."
The researchers found that the younger a girl was in having her first sexual experience, the more likely she would be to contract the disease -- partly because of the belief among many sexually active men that young girls are "safe," and even that sex with a virgin can cure AIDS.
The study focused on four cities: Ndola, in central Zambia, where 31 percent of women were infected with the HIV virus that causes AIDS; Kisumu, in western Kenya, where 27 percent were infected; Yaounde, the capital of Cameroon, with a 7 percent infection rate; and Cotonou, the capital of Benin, with a 3 percent rate. The rates among men were lower but followed a similar pattern.
The study, based on a random sampling of 1,000 men and 1,000 women in each town, found that in Kisumu, 27 percent of girls had had sex before age 15, compared with 5 percent in Cotonou.
"Young girls think the older men can provide for them," said Anne Buve, of the Institute of Tropical Medicine in Antwerp, Belgium. "The men think the young are innocent, free of the virus and cheaper to keep than a mature woman. Tragically, this is a false but self-perpetuating notion we have to start putting right through education, training and awareness."
Maina Kahinda, a former head of Kenya's AIDS control program, said Kenyan education authorities refused for years to acknowledge that school-age children were having sex.
"From the study, we know many children have already had their sexual debut and we must go to the education sector and the children themselves and make the serious dangers of sex that is forced, coerced or bought with `sugar-daddy' gifts better known," he said.
Buve said the risk of HIV transmission was higher where men were not circumcised, making them prone to "genital ulceration" from sex diseases, a major transmitter of HIV.
The study showed that 10 percent to 30 percent of men were circumcised in Kisumu and Ndola. Up to 97 percent of men were circumcised in the two cities with lower infection rates.
Carael said the research did not touch on factors that led people to engage in risky sex in the cities studied, "but if you want to change sexual behavior you have to tackle poverty, migration and the broad picture."
In the four cities, the scale of poverty-driven prostitution was comparable, but more sex workers generally reported using condoms in the low-incidence cities.
Two-thirds of the 31 million people worldwide infected with the virus live in Africa. AIDS, infecting five Africans every two minutes, is seen as the main obstacle to growth and development in the largely impoverished continent.