WASHINGTON -- A 46-year-old U.S. woman has likely acquired HIV from her female partner in a rare case of female-to-female sexual transmission of the AIDS virus, the U.S. Centers for Disease Control and Prevention (CDC) reported Thursday.

Laboratory testing confirmed that the woman from Texas 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," the CDC said in its Morbidity and Mortality Weekly Report.

The woman with newly acquired infection did not report any other recognized risk factors for HIV infection, such as injection drug use, tattooing, acupuncture and piercing, the agency said.

She supplemented her income by selling her plasma and first tested negative for HIV after donating plasma in March 2012. The Houston Department of Health reported the case to the CDC in August 2012.

The CDC said the likely source of the patient's new HIV infection was her 43-year-old female sex partner who tested positive for HIV in September 2008.

The couple reported routinely having unprotected sexual contact during a six-month monogamous relationship and the recently infected woman reported that her partner was her only sexual contact at that time, the agency said.

According to the CDC, transmission of HIV between women who have sex with women (WSW) has been reported rarely and is difficult to ascertain because other risk factors almost are present or cannot be ruled out.

"Although rare, HIV transmission between WSW can occur," it said.

"The potential for HIV transmission by female-to-female sexual contact includes unprotected exposure to vaginal or other body fluids and to blood from menstruation, or to exposure to blood from trauma during rough sex."

The CDC described in the report one case in the Philippines who reported sexual contact exclusively with women and said she did not use injection drugs. However, no source of transmission was confirmed about this case.

Another instance of female-to-female HIV transmission was reported for a 20-year-old woman with no other risk behaviors who said she had a two-year relationship and unprotected intercourse with a female partner known to be HIV-infected. The woman and her partner had identical HIV-1 drug resistance mutations, but no phylogenetic linkage testing was conducted, the CDC added.