Feb. 12 (iVillage Total Health) -- Women who contract a common sexually transmitted disease (STD) are 50 percent more likely to be infected with HIV than women who do not have the STD, according to a new study.

Researchers said the results of the study point to the need for HIV prevention programs to focus on stopping the spread of STDs in general, including trichomoniasis. Also known as "trich" or "trick," the disease infects 7.4 million women and men every year in the United States, according to the U.S. Centers for Disease Control and Prevention (CDC). Caused by a parasite (trichomonas vaginalis), trichomoniasis is common in young women and affects about 5 to 10 percent of all women, according to the National Women's Health Resource Center. The infection may cause symptoms such as vaginal discharge, but when it occurs alone it usually does not have serious consequences.

Researchers from the University of Washington in Seattle, the University of Nairobi and the Coast Provincial General Hospital in Kenya tracked 1,335 female sex workers in Kenya. Participants were HIV-negative at the start of the study and received monthly checkups, examinations and lab tests. The study lasted 11 years and each woman was tracked for a median of 566 days.

Women who were diagnosed with pelvic inflammatory disease or vaginal discharge -- common symptoms of trichomoniasis -- were given oral metronidazole, the medication used most often to kill the parasite. There were 806 cases of trichomoniasis and 265 women contracted HIV. Other studies have shown a generally increased risk of contracting HIV among patients who have other STDs. In this study, researchers found a 1.5-fold increase in risk of HIV among participants with trichomoniasis.

"What this means is that a woman with trichomoniasis is at about 50 percent greater risk for acquiring HIV than a woman without trichomoniasis," lead researcher Dr. R. Scott McClelland, said in a press release. Researchers believe trichomoniasis may make women more susceptible to HIV because the parasite causes bleeding in the mucus membranes of the vagina. This could allow HIV to enter the bloodstream. Another theory is that the parasite may also break down an enzyme important in blocking HIV attachment to healthy cells.

Results of the study will be published in the March 1 issue of The Journal of Infectious Diseases and was published online January 25.

Copyright 2007 iVillage Total Health.