A patient acquired Zika virus in the U.S. through sex with a person who had traveled to a place where the virus is circulating, Dallas County, Texas, health officials and the Centers for Disease Control and Prevention reported Tuesday.

This is not the first time that the virus has been sexually transmitted, and it most likely isn't the first time it's been sexually transmitted in the U.S.

In 2008, two scientists returned to Colorado after months of field work in Senegal, where they'd been bitten by Aedes aegypti, the species of mosquito that transmits Zika virus.

One of them ended up passing the virus to his wife, most likely during intercourse. The couple noticed that the husband's semen had been bloody for a few days before the wife felt sick. She later tested positive for Zika, even though she had not left the U.S. in years. The pair co-authored a paper on their case, which has been called the first documented case of sexual transmission of an insect-borne disease.

During a Zika virus outbreak in French Polynesia in 2013, the virus was isolated from the bloody semen of a man in Tahiti. This was a few weeks after he had symptoms, and while his blood no longer contained traces of the virus, his urine did, and his semen contained live virus capable of replicating. The authors speculate that the virus may have replicated in the man's genital tract.

Similarly, Japanese researchers studying boars infected with a virus in the same family as Zika isolated virus from the urine and semen of boars that was capable of infecting a female through artificial insemination.

Is sexual transmission definitely possible? "Well, it sounds like it," says Dr. Robert Tesh, a professor at the University of Texas Medical Branch who studies emerging infectious diseases. But if it is, it's probably quite rare.

"I know it's sexy, talking about sexual transmission, but it's still the mosquito that's the important vector," says Tesh, who co-authored the case report from Colorado.

The silver lining is that both the Colorado case and the Texas case happened in the winter, when it's too cold out for the species of mosquito that transmits the virus to be out and about. So Zika couldn't have spread to other people by mosquito.

Though the virus has been connected with birth defects in Brazil, in adults the symptoms, if any appear, are often mild and short-lived: rash, joint pain, conjunctivitis and slight fever. The CDC is trying to figure out if an uptick in cases of Guillain-Barre syndrome, a neurological disorder, that was reported by the Brazil Ministry of Health is connected to Zika.

Research on a 2013 outbreak in French Polynesia, the largest known, has yielded the most information on which bodily fluids Zika hangs out in, and when.

One study found signs of the virus in the saliva of patients shortly after the onset of symptoms. A small study in New Caledonia detected it in patients' urine more than 10 days after their first symptoms, and more than a week after it became undetectable in blood.

A third study found the virus in the breast milk of infected mothers, and concluded that two babies who tested positive for Zika virus within days of birth possibly acquired it from their mothers' bodily fluids during pregnancy or birth.

Tesh says it's unclear how the virus remains in bodily fluids, but hypothesizes that the virus could hide in white blood cells.

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