The virus, spreading quickly across the Americas, is usually transmitted by mosquitoes, but Dallas County reported on February 1 that the first known case contracted in the United States was a person infected after having sex with someone who had returned from Venezuela.
The WHO declared a global health emergency on Monday, citing a "strongly suspected" causal relationship between Zika infection in pregnancy and microcephaly, a condition marked by abnormally small head size that can cause permanent brain damage in newborns.
Health ministers from across South America gathered in Uruguay's capital, Montevideo, to discuss the public health emergency and how the region could coordinate its fight against the outbreak.
Sexual transmission could add a new dimension to the threat Zika poses, but WHO spokesman Gregory Hartl stressed that "almost a 100% of the cases" are transmitted by the bite of a mosquito.
"This reported case in the U.S. of course raises concerns," Hartl said at the U.N. agency's headquarters in Geneva. "This needs to be further investigated to understand the conditions and how often or likely sexual transmission is."
But he said that for the WHO "the most important thing to do is to control people's exposure to mosquitoes."
The WHO estimates as many as 4 million people could become infected in the Americas.
Hartl called the Texas case only the second worldwide linked to sexual transmission, referring to media reports about a case of an American man who returned from Senegal in 2008 and is suspected of having infected his wife. The medical literature also has a case in which the virus was detected in semen.
"If you swap enough bodily fluid, most viruses can probably be sexually transmitted to some extent," said Ben Neuman, a virologist at Britain’s University of Reading.