In an uncommon occurrence, a Texas woman appears to have acquired HIV through sex with another woman, the CDC is reporting.
Although such cases have been reported previously, they are rare and difficult to pin down to sexual contact, the agency said in the March 14 issue of Morbidity and Mortality Weekly Report.
"Past confirmation of HIV transmission during female-to-female sexual contact has been difficult because other risk factors almost always are present or cannot be ruled out," the agency report said.
But in this case, other risk factors -- such as injection drug use or heterosexual intercourse -- were not present. Both women also had HIV that was genetically almost identical when three genes were sequenced., the CDC report said.
Such an event is "an extremely rare occurrence," commented Jeffrey Klausner, MD, of the University of California Los Angeles.
Given that more than 70 million people have ever been infected with HIV worldwide, he told MedPage Today, "the absence of more well-documented cases of sexual transmission between women means that such transmission is very, very difficult."
But the level of verification in this case is such that it's "sewed up," commented Michael Saag, MD, of the University of Alabama Birmingham. "I think this is real," he told MedPage Today.
The Texas case came to light, the agency report said, when a 46-year-old woman attempted to sell her plasma in April 2012 and was found on an enzyme immunoassay (EIA) test to be HIV-positive.
The same test had been negative when she donated plasma the month before.
She was also negative on an EIA test 10 days after the March donation, when she went to an emergency department with sore throat, fever, vomiting, decreased appetite, pain on swallowing, dry cough, frequent diarrhea, and muscle cramps.
She had been in a monogamous relationship for 6 months with a 43-year-old woman, who had been diagnosed with HIV in 2008 but who had stopped treatment 2 years later.
After the second woman was diagnosed as HIV-positive, the couple reported routinely having oral and vaginal contact without using barrier precaution. They also said they used insertive sex toys that were shared between them but not with anyone else.
And they reported their sexual contact was occasionally "rough to the point of inducing bleeding," the agency report said.
On the other hand, the newly infected woman reported no heterosexual sex in the previous 10 years, injection drug use, tattoos, acupuncture, transfusions, transplants, or any other HIV risk behavior.
Saag noted that there have been cases in which swallowed fluids -- semen or breast milk -- have been shown to transmit HIV. And if there was inflammation or abrasions in the genitalia, infected blood might also have carried the virus from one woman to the other.
Phylogenetic analysis of the HIV pol, gag, and env genes in specimens from both women showed nearly complete pairwise nucleotide identity -- 98.7% in gag and 98.0% in both env and pol.
The genetic similarity, combined with the lack of other risk factors, makes the case one of "likely female-to-female transmission," the agency report argued.
Klausner told MedPage Today that the case underlines the need for treating every person infected with HIV.
"Persons not on treatment can spread HIV," he said.
"The public health and medical sector must work harder to ensure that every case of HIV is diagnosed and every diagnosed case is on effective treatment," he added.
The sequence of events, Saag noted, suggest that the newly infected woman went to the emergency department in the window period after infection and before seroconversion, so that the HIV was not picked up by the rapid EIA test.
A second test, looking for HIV RNA or antigens, might well have been positive at the time, Saag said.