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Dear student BMJ, is it possible to transmit HIV through oral sex? Kitty Smith replies

HIV can be transmitted in any situation where body fluids containing HIV are able to enter the tissues of the uninfected person.

Generally speaking, oral sex involves contact between the mouth and genitals or mouth and anus. This can incorporate licking, sucking, biting, or ejaculating in the mouth.

Saliva has been shown to contain HIV, although its viability is usually low and its concentration extremely low, compared to many other body fluids, notably semen. Semen, cervical and vaginal fluid contains high concentrations of the virus in HIV infected individuals.

Although HIV cannot breach intact keratinised skin, there is some evidence that it may be transported into mucous membranes actively by Langehans cells or similar antigen presenting cells. Additionally, there may be small breaches in the oral mucosa, sores on the lips or genitals. The presence of these greatly increase the risk of transmitting HIV.

Case studies suggest that the sexual practice which carries most risk of HIV transmission is receptive anal sex, with receptive vaginal sex as a close second. Oral sex has a much lower risk, but there are well documented cases where transmission seems to have occurred and many HIV doctors have patients whom they believe have been infected in this way. Risk is probably increased to the receptive partner in oral penile sex, especially if the insertive partner ejaculates in the mouth and the ejaculate is swallowed.

Oral sex carries other health risks. Oral anal sex is more likely to transmit hepatitis A or B than HIV and may also transmit bowel pathogens such as giardia and E coli. Gonorrhoea and syphilis can also be transmitted in this way.

Many people who work in HIV prevention and advise on safer sex take the pragmatic view that oral sex is safer than unprotected penetrative vaginal or anal sex. There is a move to encourage safer sex while acknowledging that totally risk-free sex is probably an unobtainable goal.

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