The first known case of Zika virus transmission within the United States has been reported in Texas, and was likely contracted through sex rather than a mosquito bite.
The announcement was made on Tuesday by local health officials in Dallas, one day after the World Health Organization (WHO) declared Zika an international public health emergency.
The infected individual's partner had recently returned from Venezuela, though they themselves had not traveled abroad.
The virus, linked to severe birth defects in thousands of babies in Brazil, is spreading rapidly in the Americas, and WHO officials on Tuesday expressed concern that it could hit Africa and Asia as well. Zika had been thought to be spread by the bite of mosquitoes of the Aedes genus, so sexual contact as a mode of transmission would be a potentially alarming development.
Related: Brazil Steps Up Its War Against Zika
The US Centers for Disease Control and Prevention (CDC) confirmed it was the first US Zika case in someone who had not traveled abroad during the current outbreak.
After this case, the CDC advised men to consider using condoms after traveling to areas with the Zika virus. Pregnant women should avoid contact with semen from men exposed to the virus.
Previously, international health officials had noted one US 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.
While the WHO declared Zika an international public health emergency on Monday, it said it had found no public health justification for restrictions on travel or trade to prevent the spread of the virus.
The WHO has said the virus could infect 4 million people in the Americas. On Tuesday, the organization launched a global response unit to fight the mosquito-borne virus.
"Most important, we need to set up surveillance sites in low- and middle-income countries so that we can detect any change in the reporting patterns of microcephaly at an early stage," Anthony Costello — WHO's director for maternal, child and adolescent health — said in Geneva.
More than 20 countries in South and Central America have reported cases, as have several states further afield, including Australia and Ireland. The virus has been linked to thousands of babies being born with microcephaly — abnormally small heads and improperly developed brains.
There is currently no vaccine or specific treatment available to stop it.
Brazil is the country hardest hit by Zika. In an address to a joint session of Brazil's Congress on Tuesday, President Dilma Rousseff said her government would spare no resources in mobilizing to combat the mosquito that transmits the virus. With no vaccine or treatment for Zika, efforts to curb its spread have focused on eradicating mosquito breeding sites.
Brazil, which has more than 4,000 suspected cases of microcephaly that may be linked to Zika, is scheduled to host the Olympics in Rio de Janeiro in August.
Several governments in the region — including Brazil — have advised women to avoid getting pregnant until 2018.
The US Red Cross has also issued a statement asking potential blood donors to who have returned from affected areas such as Mexico, the Caribbean, or Central or South America, to wait 28 days before giving their blood.
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