The Morbidity and Mortality Weekly Report, a publication of the Centers for Disease Control (CDC), has a paper that makes the case for HIV transmission between women.
In the past it was thought that HIV transmission probably didn’t occur between women who have sex with women (WSWs) exclusively. There were a few studies done that showed that women didn’t seem to be able to pass on HIV successfully. There was the occasional report of a rare woman who seemed to have gotten HIV without getting it in one of the “traditional” ways: heterosexual sex with a positive male; sharing used needles; etc., but there was often some other possible mechanism that might have caused the infection.
For example, the woman might have only had sex with other women, but she was also using intravenous drugs. Or she may have been occasionally having sex with men. Therefore, the source of the HIV infection might have been from contaminated needles, or that occasional male partner. So a definite connection showing woman-to-woman transmission was unproven.
In this case from Texas, the woman seems to clearly have gotten HIV from her partner.
The woman (initially HIV negative) who became infected said that she hadn’t had heterosexual sex for years. She had no history of IV drug use, tattoos, acupuncture, or other possible modes of HIV transmission. She entered into a monogamous relationship with another woman (who was HIV positive) about 6 months before she developed her HIV infection. During that relationship, they used sex toys, and that use caused some bleeding. They also had unprotected sex.
Upon subsequent testing, the previously-negative woman was found to have become HIV positive.
Additional testing on the patient’s and her partner’s blood showed that the HIV that she carried was virtually identical to the strain that infected her partner. Her partner had been on ART briefly after her diagnosis in 2009, but had stopped her medications. At the time of testing, the partner’s viral load was 69,000 copies/mL.
The CDC recommends that women who have sex with women, especially those who are serodiscordant (one HIV positive and one HIV negative), receive counseling and information on safer-sex practices as well as being referred to health care providers. They also mention, in a related article, that using safer-sex practices might have decreased the risk of HIV transmission here. In this case, safer-sex practices would have included: using dental dams, placing condoms over shared sex toys (and replacing the condom every time the toy moves to the other partner), wearing gloves, etc.
So, there is a risk for transmission of HIV between women having sex with each other. Though, at least to date, the risk seems to be extremely low. But since this is now recognized as a mode of transmission, additional studies and case reports may produce better statistics on what the risk is for acquisition of HIV in WSW serodiscordant couples.
You can read more on this from the CDC.