The first U.S. case of the Zika virus has been contracted in Dallas County, local health officials said on Tuesday, adding there are no reports of the virus being locally transmitted by mosquitoes in the Texas county.

Dallas County Health and Human Services said the case in Dallas was acquired through sexual transmission, adding that it received confirmation of the infection from the U.S. Centers for Disease Control and Prevention.

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The county did not identify the person infected. The CDC said it did not investigate how the virus was transmitted.

There have been six confirmed travel-related cases of Zika virus disease, all among residents of Harris County, where Houston is located, the Texas Department of State Health Services said.

In medical literature, there has been only one case of Zika transmitted sexually and one case in which the virus was detected in semen.