(12 Sep 2001)

Couples may not realise it but by having oral sex, when one of the partners has a cold sore, they risk transmitting the herpes simplex virus (HSV) to the genital area. Recent research shows that a significantproportion of genital herpes is caused in this way. See Liana's story in our shared experience section.There are two distinct types of herpes simplex virus (HSV). The majority of genital herpes is caused by herpes simplex virus type 2 (HSV-2), which is generally transmitted through vaginal or anal sex. Herpes simplex virus type 1 (HSV-1) is most commonly associated with cold sores (oral herpes) and is usually passed on through kissing. There is, however, some crossover between the two types of HSV and the areas of the body that they infect. HSV-2 infection can sometimes lead to cold sores; likewise, HSV-1 is capable of causing genital herpes.

A number of studies have investigated what proportion of first episodes of genital herpes are caused by HSV-1, and the results ranged from 20% to over 40% of cases.

The good news

Groups such as the International Herpes Management Forum (IHMF) believe that genital herpes caused by HSV-1 usually recurs less frequently than genital herpes caused by HSV-2. Genital HSV-1 is also thought to be less contagious than HSV-2 partially due to the fewer outbreaks.

Both HSV-1 and HSV-2 can be passed on in the absence of symptoms through a mechanism known as asymptomatic viral shedding. There is evidence to suggest that this is less likely to happen with HSV-1 than with HSV-2. Additionally, people who already have oral HSV-1 have antibodies that help protect them from catching genital HSV-1.

‘If one person has genital HSV-1, and the other doesn’t but has oral HSV-1, the risk of transmission is very low,’ says Professor Hunter Handsfield, Professor of Medicine at the University of Washington and Director of the Public Health, Seattle and King Country STD Control Program.

However, asymptomatic shedding of HSV-1 from the oral region does occur and HSV-1 can be transmitted genitally through oral sex even when an open lesion is absent.

How to avoid contracting genital HSV-1

Abstaining from oral sex if your partner has an active cold sore is probably all you need to do to avoid catching genital HSV-1, according to experts. The only exception where extra precautions are necessary is during late pregnancy.

‘If a pregnant woman has no history of oral herpes, and her partner does, I would recommend they avoid oral sex during the third trimester,’ says Professor Handsfield.

Alternatively, there are new type-specific blood tests available that pregnant women can use to can find out whether or not they are already carrying HSV-1. Should a woman find that she is already infected, unless a genital outbreak occurs at the time of delivery, the risk to the baby from HSV-1 is extremely low.

Further advice for expectant parents on protecting their baby from genital herpes is available in our information leaflet: Herpes Simplex and Pregnancy.