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SEATTLE, Jan. 29 -- For women, infection with a common sexually transmitted disease sharply increases the risk of contracting HIV, according to researchers here.

In a prospective observational study in Mombasa, Kenya, women infected with Trichomonas vaginalis had a 50% increase in the risk of HIV infection as well, found R. Scott McClelland, M.D., of the University of Washington, and colleagues.

Because 170 million people worldwide each year are newly infected with T. vaginalis, the parasite "could account for a high attributable risk for HIV-1 acquisition," Dr. McClelland and colleagues reported in the March 1 issue of the Journal of Infectious Diseases.

"What this means is that a woman with trichomoniasis is at about 50% greater risk for acquiring HIV than a woman without trichomoniasis," after adjusting for confounding factors, Dr. McClelland said.

Dr. McClelland and colleagues said the association is biologically plausible for several reasons:

• T. vaginalis leads to an inflammatory response, bringing CD4-bearing lymphocytes and macrophages to the vaginal and cervical mucosa.

• The parasite causes mucosal hemorrhages that could compromise the mechanical barrier to HIV infection.

• T. vaginalis degrades secretory leukocyte protease inhibitor, which can block HIV attachment to cells.

• And trichomoniasis could increase the risk of HIV infection by increasing susceptibility to bacterial vaginosis or persistence of abnormal vaginal flora.

With colleagues at the University of Nairobi, and the Coast Provincial General Hospital in Kenya, Dr. McClelland studied a cohort of 1,335 female sex workers in Mombasa, all HIV-negative at the beginning of the study.

They were monitored for a median of 566 days, using monthly check-ups, combined with appropriate treatment for sexually transmitted diseases, at a Mombasa clinic.

Over the course of the study, the researchers reported, there were 806 incident T. vaginalis infections (or about 23 per 100 person-years), and 265 women became HIV-positive (or 7.7 per 100 person-years).

After adjusting for a range of confounding factors, including other sexually transmitted infections, age, condom use, and education, the researcher found that trichomoniasis was associated with 52% increased risk of getting HIV. The odds ratio was 1.52, with a 95% confidence interval from 1.04 to 2.24, which was statistically significant at P=0.03.

A meta-analysis in 2001 also found a 50% increase in HIV risk for women with trichomoniasis, the researchers noted. The current study provides "the strongest evidence to date" of the link, they said.

"Little attention has been given to the potential role of vaginal health as an HIV-1 prevention strategy," the researchers concluded. "Interventions to prevent and treat trichomoniasis and to improve vaginal health in general could provide important female-controlled methods for reducing the risk of HIV-1 transmission to women."

The authors pointed out a potential limitation. "Because T. vaginalis and HIV-1 share a common route of transmission, it is important to control for behaviors that simultaneously increase the risk for both infections. Accurate assessment of sexual risk can be difficult. We have previously shown that self-reported behavioral data from this population were associated with STIs which may provide objective markers of sexual risk. Nonetheless, there remains a potential for residual confounding."

The research was supported by the National Institutes of Health and Fogarty International Center. The researchers reported no conflicts.

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