Researchers and health officials in Maryland are reporting a case of a man spreading Zika to a female partner, despite the fact he didn’t have any symptoms of the virus. Most cases of sexually transmitted Zika reported at this point have been spread by people who had signs of the virus. While experts have said it’s possible for someone without symptoms to spread the virus, such cases were not well documented until this point.

In a new case study published Friday in the U.S. Centers for Disease Control and Prevention (CDC)’s Morbidity and Mortality Weekly Report, the authors discuss a case where a woman who had not traveled anywhere with ongoing Zika transmission came down with symptoms of the virus, including a fever and a rash, and her samples tested positive for the virus in a laboratory in Maryland.

While she had not traveled, the woman did have sexual contact with a man who had recently traveled to the Dominican Republic where he had exposure to mosquitoes. However, the man did not have any symptoms of the virus.

Health authorities have reported only one other case where a man without symptoms of the virus may have spread it to his female partner. However, in that case, both the man and the woman had traveled to places with Zika and could have been bitten by mosquitoes.

The report authors conclude that not having symptoms of Zika does not mean a person can’t spread the virus to their sex partners. “Ongoing surveillance is needed to determine the risk for sexual transmission of Zika virus infection from asymptomatic persons,” the researchers write. They also add that it might be worthwhile to consider people who have condomless sex with people who have traveled to areas with ongoing Zika virus as possibly exposed to the virus.

So far there have been slightly more than 20 cases of Zika virus from sexual transmission in the United States. There are 2,487 travel associated cases, and Florida has reported 43 cases of locally transmitted Zika. As of Friday, the U.S. Food and Drug Administration (FDA) is advising that all donated blood be tested for Zika in the U.S. and territories.
 
 
This article originally appeared on Time.com.