(CNN) -- A Texas woman apparently contracted HIV through sexual contact with another woman, the Centers for Disease Control reported Thursday, a rare female-to-female transmission of the virus.
Testing confirmed the 46-year-old woman with newly diagnosed HIV "had a virus virtually identical to that of her female partner, who was diagnosed previously with HIV and who had stopped receiving antiretroviral treatment in 2010," according to the CDC's Weekly Morbidity and Mortality Weekly Report.
HIV transmission through female-to-female sexual contact "has been reported rarely and is difficult to ascertain," officials said.
Past confirmation of such transmission "has been difficult because other risk factors almost always are present or cannot be ruled out," the report said. "In this case, other risk factors for HIV transmission were not reported by the newly infected woman, and the viruses infecting the two women were virtually identical."
The CDC said it was contacted by the Houston Department of Health about the case in August 2012.
The woman newly diagnosed with HIV did not report any other risk factors, such as injection drug use, tattooing, transfusions or transplants, officials said. She supplemented her income by selling plasma, and tested negative for HIV in March 2012.
Ten days after donating plasma, however, in April 2012, she went to an emergency room complaining of a sore throat, fever, vomiting and decreased appetite, among other symptoms. She again tested negative for HIV.
However, she tested positive for HIV when attempting to sell plasma 18 days later, and further testing confirmed the diagnosis.
The woman is believed to have been infected by her 43-year-old sexual partner, the CDC said. She told officials the woman was her only sexual partner during the six months before she tested positive.
The couple said they routinely had unprotected sexual contact and shared sex toys between them. At times, the contact was "rough to the point of inducing bleeding in either woman," according to the CDC. The women said some of the unprotected sexual contact occurred during menstruation.
Commonly, HIV infections in women who have sex with women are traced to risk behaviors such as intravenous drug use or heterosexual sex, the CDC said.