Study first to document increase risk of herpes infection due to oral sex, particularly in young women

The first clinical study to document risk of acquiring herpes simplex virus type 1 infection based on sexual activity has linked oral sex and vaginal intercourse with a demonstrably higher rate of infection, particularly in young women, researchers from the University of Pittsburgh report in the February issue of Sexually Transmitted Diseases, the journal of the American Sexually Transmitted Diseases Association.

Genital herpes is one of the most common sexually transmitted diseases in the United States. Nationwide, at least 45 million people ages 12 and older – or one out of five adolescents and adults – have had a genital herpes infection. Between the late 1970s and the early 1990s, the number of Americans with genital herpes infection increased by 30 percent, according to the U.S. Centers for Disease Control and Prevention. While there is no cure, antiviral medications can shorten and prevent outbreaks. Daily suppressive therapy can reduce, but not eliminate, risk of transmission to non-infected partners.

Medical students traditionally have been taught that herpes simplex type 1 (HSV-1) infections most often take place above the waist, while herpes simplex type 2 (HSV-2) predominates below the belt. More recent evidence, however, suggests that HSV-1 also is an important pathogen in genital herpes infections, especially in younger women. HSV-1 is more commonly known to cause infections of the mouth and lips, often called fever blisters or cold sores. Viral shedding and disease transmission is possible even in the absence of a visible outbreak.

"Receptive oral sex and vaginal intercourse were found to be significant risk factors for the acquisition of HSV-1," said Thomas Cherpes, M.D., an infectious diseases fellow at the University of Pittsburgh School of Medicine and the study's first author. "Because oral HSV-1 infections are less frequent in childhood and adolescence, future prevention strategies will need to consider increased susceptibility for HSV-1 among young adults, and the important contribution of HSV-1 to the growing genital herpes epidemic."

Researchers enrolled 1,207 non-pregnant women ages 18 to 30 at three Pittsburgh-area health clinics between 1998 and 2000. Participants were asked to return for three follow-up visits at four-month intervals. At each study visit, blood samples were tested for HSV-1- and HSV-2-specific antibodies, and surveys of sexual behavior and demographic information were completed.

Initially, HSV-1 was found in 38 percent of women aged 20 or younger. During the follow-up period, analysis found that women who had vaginal intercourse had a more than six-fold higher risk of acquiring HSV-1 than sexually inactive women, or 6.8 versus 1.2 cases. For those who had only receptive oral sex without vaginal intercourse, however, the risk was even greater – 9.8 versus 1.2 cases.

"The low frequency of infection we detected at enrollment is consistent with other research indicating a reduction in HSV-1 prevalence among younger people," said Sharon Hillier, Ph.D., professor in the departments of obstetrics, gynecology and reproductive sciences and molecular genetics and biochemistry at the University of Pittsburgh School of Medicine and the study's senior author. "As a result, a significant number of young adults are now susceptible to oral or genital HSV-1 infection."

This is important because most current research on genital herpes vaccine development focuses on HSV-2, added Dr. Hillier, who also is a senior investigator at the Magee-Womens Research Institute.

The falling rate of childhood HSV-1 infections is complicating the scenario by leaving larger numbers of young people susceptible to genital infection with HSV-1, according to Dr. Cherpes, particularly since surveys show a high rate of participation in sexual activities that put them at higher risk.

"Lifetime prevalence of receptive oral sex among sexually active women is 75 percent," he said. "In our group, more than 90 percent of study participants reported a history of such activity."

Drs. Cherpes and Hillier note that future vaccine development should include HSV-1 targets to reduce susceptibility.