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updated 4/12/2011 5:20:55 PM ET 2011-04-12T21:20:55

Even people who don't show symptoms of genital herpes can harbor active forms of the virus that can be spread to sexual partners, according to a new study.

People who are infected with the herpes simplex virus type 2, but don't have symptoms, still have active forms of the virus detected on their genital tract 10 percent of the time, the study said.

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And people who do have symptoms of herpes infection carry active forms of the virus on their genital tract 20 percent of the time, said study researcher Dr. Christine Johnston, acting assistant professor of medicine at the University of Washington.

It's long been known that the herpes simplex virus type 2 is contagious even if a person doesn’t have symptoms of the virus, Johnston said. But the new finding shows what percent of the time the virus is actually active and infectious, she added.

"These findings will help us counsel patients who are diagnosed with [herpes simplex virus type 2] infection about their risks of transmission to others, which is often a key concern for people diagnosed with genital herpes," Johnston told MyHealthNewsDaily.

Herpes simplex virus type 2 is sexually transmitted and causes most cases of genital herpes, which manifests as blisters that form on the genitals and rectum, according to the Centers for Disease Control and Prevention.

Herpes simplex virus type 1 can also cause genital herpes, but typically this form of the virus only causes cold sores in or around the mouth.

The study will be published April 13 in the Journal of the American Medical Association.

Viral shedding
Johnston and her colleagues compared the rates of viral shedding, the rate at which the virus replicates and is therefore considered contagious, in 498 people with genital herpes between 1992 and 1998.

Researchers had each participant collect a swab of genital secretions once a day for at least 30 days, the study said.

People with symptoms of genital herpes had more episodes of viral shedding over a year (17.9 episodes), than those without symptoms (12.5 episodes), the study said.

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However, these results show that even people without symptoms of herpes are still contagious and may not realize they are passing the virus on to their sexual partners, Johnston said.

What's at stake
Transmission of herpes occurs through direct person-to-person contact with an area of the body that is shedding the virus, she said.

"We do not know if there is a threshold quantity of virus above which people are contagious," she said, but "we presume that if people are shedding virus, they are at risk of transmitting virus."

More than 16 percent of people ages 14 to 49 in the United States are infected with the herpes simplex virus type 2, according to the CDC.

There is no cure for herpes, but the study highlights the need for vigilant prevention of the virus, Johnston said.

Prevention strategies include using condoms, suppressive antiviral therapy and disclosure of having herpes to sexual partners. Each of these strategies is 50 percent effective in reducing the risk of genital herpes transmission, Johnston said.

Pass it on: People infected with genital herpes but don't show any symptoms can still be contagious and spread the virus to sexual partners.

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