A sexually transmitted disease known as Trichomonas vaginalis is twice as common as previously thought, and is particularly prevalent in women over 40, US doctors said Tuesday. Using new genetic assay technologies for testing, the research led by Johns Hopkins University found that the overall infection rate among US women was 8.7 percent, compared to previous findings of about four percent. Analyzing data from 7,593 American women between the ages of 18 and 89, women 50 and older showed the highest trichomonas infection rate at 13 percent. Women in their 40s showed an 11 percent infection rate, said the study which spanned 28 states and \"is believed to be the largest and most in-depth analysis of the STD ever performed in the United States,\" the study authors said. Since the parasite often causes no symptoms in women but can lead to severe health problems, senior study investigator Charlotte Gaydos of Johns Hopkins in Maryland said she is urging all women over 40 to get tested. \"What we are really witnessing with trichomonas, especially in older women, is that no one ever looked, no one ever tested and diagnosed, and no one is really getting treated, so the infection persists year after year,\" she said. \"Trichomonas infections are quite treatable with antibiotics,\" she added. \"And these high numbers really warrant older women getting screened by their family physicians and gynecologists during routine check ups to make sure they are not infected and are not inadvertently spreading it to others.\" Infection rates were highest in African American women -- 20 percent -- compared to white women at 5.7 percent. The parasitic disease often causes no symptoms, but can result in painful urination or genital itching and discharge. Trichomonas infection can also make it easier for HIV to be transmitted, and can cause pelvic inflammatory disease and pregnancy and birth complications. The findings were presented at the annual meeting of the International Society for STD Research, in Quebec City, Canada.