WASHINGTON – Genital herpes can be transmitted even if a person has no symptoms, suggesting a high risk of transmission from people with unrecognized infections of herpes simplex 2, researchers said on Tuesday.

The study published in the Journal of the American Medical Association followed about 500 people between March 1992 and April 2008. The subjects took daily swabs of their genital secretions for 30 to 60 days.

In the group that reported seeing symptoms of genital HSV, the virus was detected on 20 percent of days, researcher Anna Wald of the University of Washington told reporters as she presented the findings.

Among those who saw no physical signs, such as warts, sores or lesions, HSV was detected on 10 percent of days.

"When we think about people who have symptomatic genital herpes this means that the virus is present in their genital area on one out of five days," Wald said.

"In people who have HSV-2 but who don't have a history of genital herpes, the virus is present on the average of one out of 10 days."

Herpes simplex virus type 2 is one of the most common sexually transmitted diseases, and researchers estimate that 536 million people are infected worldwide.

"Most people who have it do not know they have HSV-2," added Wald.

Ways to prevent the transmission of genital herpes include condom use and medicines that can suppress the shedding of the virus, but these methods about halve the risk and cannot completely prevent infection, experts say.

There is a blood test for HSV-2 but fewer than 30 percent of clinics that treat sexually transmitted diseases have access to it, meaning that many people who are infected may believe they are not, Wald said.