A case of the Zika virus being sexually transmitted in the United States has been reported in Texas.
Local health officials said it is likely the virus was contracted through sex and not a mosquito bite, a day after the World Health Organization (WHO) declared an international public health emergency.
The virus, linked to severe birth defects in thousands of babies in Brazil, is spreading rapidly in the Americas, and WHO officials yesterday expressed concern that it could hit Africa and Asia as well.
Zika had been thought to be spread by the bite of the Aedes mosquitoes, so sexual contact as a mode of transmission would be a potentially alarming development.
Dallas County Health and Human Services said it received confirmation of the case in Dallas from the US Centers for Disease Control and Prevention.
The county department said on Twitter that the person was infected through sexual contact with someone who had travelled to Venezuela.
The person infected did not travel to the South American country, county health officials said.
The Texas Department of State Health Services was slightly more cautious in its assessment, saying in a statement; "Case details are being evaluated, but the possibility of sexual transmission from an infected person to a non-infected person is likely in this case."
County authorities said there were no reports of the virus being locally transmitted by mosquitoes in the Texas county.
A CDC spokesman confirmed the results of a test for Zika infection but said local officials investigated the mode of transmission.
Previously, international health officials had noted one case of possible person-to-person sexual transmission. But the Pan American Health Organization said more evidence was needed to confirm sexual contact as a means of Zika transmission. The medical literature also has one case in which the virus was detected in semen.
The virus has been linked to microcephaly, in which babies have abnormally small heads and improperly developed brains.
The WHO has said the virus could infect 4 million people in the Americas. It said on Tuesday it launched a global response unit to fight the mosquito-borne virus.
Twenty to 30 sites could be established worldwide, mainly in poor countries without robust healthcare systems.
Brazil is the country hardest hit by Zika and has 3700 suspected cases of microcephaly that may be linked to Zika, is scheduled to host the Olympics in Rio de Janeiro in August.
Brazil and the United States will enter a partnership to develop a Zika vaccine as soon as possible to stem the spread of the virus.
The Pan American Health Organization said Zika was now spreading in 26 countries and territories in the Americas.
Meanwhile the American Red Cross said blood donors who had traveled to Zika virus outbreak areas should wait at least twenty-eight days before donating their blood.
The waiting period applies to US blood donors who have visited Mexico, the Caribbean, or Central or South America during the past four weeks.