Public health officials in Dallas County have confirmed the first sexually transmitted case of the Zika virus, which is said to be linked to birth defects.

The virus is typically transmitted by mosquitoes, but Dallas County Health and Human Services said Tuesday that an unidentified person had been infected by sexual transmission — the first confirmed case of a person acquiring the virus in the continental United States. The U.S. Centers for Disease Control and Prevention confirmed the transmission Tuesday.

Researchers have warned Zika could be linked with microcephaly, a condition causing children to be born with abnormally small brains and skulls. The World Health Organization called Zika virus a public health emergency on Monday, and the federal government has listed more than 20 Latin American countries, including Brazil, where public health officials say there have been roughly 4,000 reported cases of microcephaly since October and to which they are urging pregnant women not to travel.

"Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others," Dallas County Health and Human Services director Zachary Thompson said in a news release. "Next to abstinence, condoms are the best prevention method against any sexually transmitted infections."

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The most common symptoms of the Zika virus are fever, rash, joint pain and red eyes, according to Dallas County public health officials.

There have been no documented cases of the Zika virus infection by mosquito bite in the continental United States, but such infections have been reported in Puerto Rico. In addition, U.S. travelers returning from affected tropical regions in Latin America have been diagnosed with Zika virus linked to their travel. In Texas, there are seven confirmed Zika cases linked to travel, according to the Department of State Health Services.

U.S. Rep. Gene Green, D-Houston, on Tuesday called for a congressional briefing from public health officials about the virus.

"Houston is a gateway to Central and South America, and because of this, we have a unique role and interest in confronting threats like the Zika virus," he said in a prepared statement.