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updated 11/8/2012 12:50:05 PM ET 2012-11-08T17:50:05

A sexually transmitted parasite that's extraordinarily widespread gets even more dangerous when infected with a virus, researchers say.

Scientists investigated the sexually transmitted disease trichomoniasis, which is caused by the parasite Trichomonas vaginalis, a type of microbe known as a protozoan. Rather than invade human cells, the parasite latches onto their surfaces and feeds on them.

Trichomoniasis is more common than all bacterial sexually transmitted diseases combined, annually affecting nearly 250 million men and women worldwide. People infected with the parasite become especially vulnerable to other sexually transmitted diseases, including HIV, which causes AIDS, and HPV, which is linked with cervical and prostate cancers. In addition, complications from trichomoniasis include miscarriage, preterm delivery, low birth weight and infertility.

"Trichomoniasis is associated with devastating consequences for women due to inflammation and related risks of reproductive disease," said researcher Raina Fichorova, a reproductive immunobiologist at Harvard Medical School and Brigham and Women's Hospital in Boston.

Parasite-virus duo

Intriguingly, more than 80 percent of these parasites are themselves infected, with a virus simply dubbed Trichomonasvirus. [ Quiz: Test Your STD Smarts ]

"Unlike flu viruses, for example, this virus can't spread by jumping out of the cell into another one," said researcher Max Nibert at Harvard Medical School. "It just spreads between cells when they [the hosts] divide or mate."

The virus seems to have no detrimental effect on the parasite, and the widespread nature of this virus led researchers to suspect it might actually benefit the parasite in some way. To learn more, the researchers collected Trichomonas from infected women and tested how both virus-infected and virus-free versions of the parasite affected human cells grown in lab dishes.

Currently, trichomoniasis is most often treated with the antibiotic metronidazole. However, when researchers killed virus-infected Trichomonas with this drug, they found the dying or injured parasites released viruses that inflamed the human cells.

The virus does not infect the human cells, but it nevertheless aggravates the harmful effects of the parasite. These findings may explain why metronidazole does not prevent the harmful effects trichomoniasis can have on women's reproduction, and may actually make things worse — it forces the parasite to release harmful viruses.

"Antibiotic treatment can actually make things worse by amplifying inflammation," Fichorova said.

Making the body more vulnerable

The inflammation also might help explain why the parasite makes people more vulnerable to other sexually transmitted diseases (STDs). "The inflammation may destroy or weaken the barriers that keep other infections out," Fichorova said.

Scientists aren't sure whether the virus actually helps the parasite in some way. "The virus may change the parasite's makeup — for instance, help generate proteins on the parasite's surface that may subdue immune response and make the parasite more acceptable to its host," Fichorova said.

Future research should explore what part of the virus' structure or life cycle may be most vulnerable to drugs, Fichorova added. "To treat trichomoniasis safely, we have to not only attack this parasite, but also its virus at the same time."

The scientists detailed their findings online Nov. 7 in the journal PLOS ONE.

© 2012 LiveScience.com. All rights reserved.

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