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The most common sexually transmitted disease can double a woman’s risk of ovarian cancer, according to new research released Thursday.
Women who had chronic chlamydia infections had twice the risk of ovarian cancer compared to women with no evidence of ever having been infected, researchers are expected to report at a meeting of the American Association for Cancer Research in April.
"Our data is lending support for there being a role of pelvic inflammatory disease in ovarian cancer and the prime cause of pelvic inflammatory disease, particularly in the U.S., is chlamydia infection,” the National Cancer Institute’s Britton Trabert told a briefing ahead of the meeting.
“We are seeing a doubling in ovarian cancer risk with a prior history of pelvic inflammatory disease.”
Ovarian cancer is not common, but it will be diagnosed in more than 21,000 American women this year and it will kill more than 14,000. It is the fifth leading cause of cancer deaths among women, killing 55 percent of patients within five years.
Chlamydia is very common. More than 1.5 million Americans have chlamydia, the Centers for Disease Control and Prevention reports. Chlamydia is easily treated with antibiotics, but it often causes no symptoms, so people can be infected for months or even years and not know it.
Over time, chlamydia causes widespread inflammation that that cause infertility. This new study suggests it may also cause cancer.
The team was unable to test for gonorrhea, another extremely common sexually transmitted infection that also causes pelvic inflammatory disease.
A doubling of risk doesn’t raise the number of cases significantly, but there are not many known causes now, aside from genetics.
The single best-known risk factor is a family history of ovarian cancer. But most cases are in women with no known family history of ovarian cancer. Women with endometriosis — an overgrowth of the uterine lining —have a higher risk of ovarian cancer.
Screening for ovarian cancer is tricky because the symptoms are vague. They can include bloating, which can be caused by something as simple as a heavy meal.
That's why knowing that an infection raises risks can be useful. Trabert said more studies will be needed to show if quickly treating chlamydia lowers risk of cancer and, of course, to confirm whether infection raises the risk in the first place.
Trabert’s team looked at two different studies — one in Poland, which involved 278 ovarian cancer cases and 556 women without cancer, and a U.S. study comparing 160 ovarian cancer patients and 159 people without.
They tested blood samples of the patients for an antibody that shows when someone has been infected with chlamydia.
In both studies, women with evidence of previous chlamydia infection were twice as likely to be in the ovarian cancer group. Women with antibodies against other infections, such as herpes or hepatitis, were not more likely to have ovarian cancer. And women infected with HPV, which is known to cause other types of cancer, did not have a higher risk of ovarian cancer.