NEW YORK (Reuters Health) - Findings from two new studies add to growing evidence that herpes simplex virus type 1 (HSV-1) is now the principal cause of genital herpes.

For years, clinicians have been taught that HSV-1 is responsible for orolabial herpes, while HSV-2 is the main cause of genital herpes. However, the current reports, which are published in the October issue of Sexually Transmitted Diseases, suggest that HSV-1 may be the main etiology for both types of herpes.

In one study, Dr. Michael Dan, from Wolfson Hospital in Holon, Israel, and colleagues determined the HSV type present in 285 positive genital and 659 positive nongenital cultures obtained between 1993 and 2002 in the Tel Aviv area of Israel.

Nearly 100% of the nongenital specimens and 66% of the genital specimens contained HSV-1, the authors found. Although the virus showed no gender bias, about 66% of both sexes harbored HSV-1, its prevalence was inversely related to patient age. The annual HSV-1 isolation rate did not change dramatically during the study period.

In a similar study, Dr. Scott J. Spear and colleagues, from the University of Wisconsin at Madison, retrospectively evaluated the cause of genital herpes in 499 positive isolates obtained from university students between 1993 and 2001.

The percentage of genital herpes cases caused by HSV-1 rose from 31% in 1993 to 78% in 2001, the authors found. In contrast to the other study's findings, HSV-1 was more common in females than in males.

"These studies...argue that HSV-1 is increasing and may be the major cause of new genital infections in certain populations," Dr. Gregory J. Mertz, from the University of New Mexico in Albuquerque, and colleagues note in a related editorial.

One explanation for this trend could be a decline in HSV-1 infections during childhood so that more "young adults are HSV seronegative when they become sexually active," the editorialists speculate. Other possibilities include the emergence of more virulent HSV-1 strains and an increase in oral-genital sex.

Regardless of the reason, this trend could have important implications for vaccine strategies designed to control genital herpes, they add.

Sex Transm Dis 2003;30:794-802.

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