BETHESDA, MD. — Herpes simplex virus 1 has emerged as an important genital pathogen and is more likely than herpes simplex virus 2 to be the cause of primary genital herpes infections in young women, Sharon L. Hillier, Ph.D., said at a conference on vulvovaginal diseases.
Young, sexually active women are more susceptible to HSV-1 because most do not have protective antibodies to HSV-1 due to the dramatic drop in childhood HSV-1 infections, said Dr. Hillier, director, reproductive infectious disease research, Magee-Womens Hospital, Pittsburgh.
Among the major implications of this trend is the potential utility of the genital herpes vaccine that is being developed, since it targets only HSV-2, and HSV-2 antibodies do not confer protection against HSV-1, she pointed out.
Since estimates of genital herpes from national seroprevalence studies include only HSV-2 infections, they “probably greatly underestimate” the amount of U.S. cases, Dr. Hillier said at the conference, held by the American Society for Colposcopy and Cervical Pathology.
Studies documenting the emergence of HSV-1 as a cause of primary genital herpes infections date to 1990, when HSV-1 was found to have replaced HSV-2 as the most common cause of genital herpes in Scotland. Studies published in 2000 reported that HSV-1 was the cause of 85% of all primary genital HSV infections in Sweden and 70%–90% of all first episodes of genital herpes in women younger than 21 in Norway. In the United States, a 2003 study found that the proportion of newly diagnosed genital herpes infections due to HSV-1 in a university student health service increased from 31% in 1993 to 78% in 2001.
In a recent study, Dr. Hillier and her associates found only 29% of a sample of college students at a University of Pittsburgh student health clinic had antibodies to HSV-1, making the majority susceptible to infection. The study enrolled 1,207 women aged 18–30 years at three different health clinics and found that the HSV-1 seroprevalence was 46.6% overall, but 60% at the primary care clinic and 51% at an STD clinic (Sex. Transm. Dis. 2005;32:84–9).
Their results indicated that age and number of sex partners was associated with HSV-1 seroprevalence: 38% of the women aged 18–20 years were positive for HSV-1 antibodies, increasing to 49% among those aged 21–25 years, and 64% among those aged 26–30 years. HSV-1 seroprevalence was 26% among those who previously had no sex partners, 41% among those who had one to four sex partners in their lifetime, and 53% among those who had five or more lifetime sex partners.
Follow-up of these women determined that cunnilingus and vaginal intercourse were risk factors for the acquisition of HSV-1 infections: Of the 516 HSV-1 seronegative women who returned for 1,833 visits, 29 acquired antibodies to HSV-1. That means that 6% of the women per year were acquiring herpes infections due to HSV-1 infection, said Dr. Hillier, also, professor of obstetrics, gynecology, and reproductive sciences, and of molecular genetics and biochemistry at the University of Pittsburgh.
In terms of sex practices, the acquisition rate for HSV-1 was highest among those who had receptive oral sex only, at 9.8 per 100 woman-years, compared with 1.2 cases per 100 woman-years among those who were not sexually active.
Among women having vaginal intercourse, the rate for HSV-1 was 6.8 cases per 100 woman-years, or 6.8% per year, she said. The HSV-2 rate was 5.7 cases per 100 woman-years among those reporting vaginal intercourse, but zero among those reporting receptive oral sex only.
Therefore, “women who report only oral sex are just as likely to acquire HSV-1 as women reporting vaginal sex,” she said. These data are similar to findings from Scotland and Finland, indicating oral sex raises the risk of HSV-1 “and has really changed the way we have begun to think about a lot of the women saying they have herpes,” she added.