A woman in Texas likely infected her female partner with HIV through sexual contact, the Centers for Disease Control and Prevention reported Thursday.

The case offers the strongest evidence to date that HIV transmission between women, although rare, is possible.

“There were cases where it was suspected, but not all the pieces were there to say it so clearly as this one,” says Patrick Sullivan, an epidemiologist at Emory University who wasn’t involved in the study.

Unique Case

The circumstances in this case were unique, a spokeswoman for the CDC says. The serodiscordant couple frequently had sexual contact without a barrier and exchanged blood through rough sex with toys.

The HIV virus can be found in vaginal fluid and menstrual blood. But it’s been tough for researchers to determine the risk of infection between women. In many cases other transmission routes, such as intravenous drug use and heterosexual intercourse, can’t be ruled out.

These other risk factors weren’t present in the current case, a CDC team wrote in the current issue of the Morbidity and Mortality Weekly Report.

The women, both in their 40s, were in a monogamous relationship with each other for six months prior to HIV transmission. One of the women had been HIV positive since 2008. Her partner had no history of drug use. And she hadn’t gotten tattoos or blood transfusions for five years prior to the infection.

When the partner tested positive for HIV in 2012, the team at the CDC analyzed the DNA of the viruses from each women. The gene sequences almost matched up perfectly.

“That gives really strong evidence that the women were sharing the virus — that it moved from infected partner to uninfected partner,” Sullivan says.

“This type of transmission is rare,” says Amy Lansky, a deputy director at the CDC’s Division of HIV/AIDS Prevention. “But still, it’s important for discordant lesbian couples — when one is HIV positive and the other is negative — to get medical counseling and HIV treatment.”

Source