The World Health Organization (WHO) voiced concern on Wednesday over a report the Zika virus had been sexually transmitted in the United States and called for further investigation into the mosquito-borne virus.
The first known case of Zika virus transmission in the United States was reported in Dallas, Texas on Tuesday by local health officials, who said it likely was contracted through sex and not a mosquito bite.
READ MORE: Dallas reports first case of sexual transmission of Zika virus
"We certainly understand the concern. This needs to be further investigated to understand the conditions and how often or likely sexual transmission is, and whether or not other body fluids are implicated," WHO spokesman Gregory Hartl told Reuters.
"This is the only the second mooted case of sexual transmission," he said, 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 virus, linked to babies born with abnormally small heads and birth defects in Brazil, is spreading rapidly in the Americas and the WHO declared an international public health emergency on Monday about the condition known as microcephaly.
The United Nations agency, which is leading international coordination on the outbreak, said on Tuesday the virus could spread to Africa and Asia, which have the world's highest birth rates, as well as to southern Europe.
In a statement on Wednesday to European member states, WHO said the risk of the virus spreading into Europe increases with the onset of spring and summer.
"Now is the time for countries to prepare themselves to reduce the risk to their populations," WHO's Europe chief Zsuzsanna Jakab said. "Every European country in which Aedes mosquitoes are present can be at risk for the spread of Zika virus disease.
"A number of travelers infected with Zika have entered Europe, but the disease has not been transmitted further, as the mosquito is still inactive. With the onset of spring and summer, the risk that Zika virus will spread increases."
The WHO has not recommended travel or trade bans with affected countries, but says that it is drawing up advice to pregnant women.
The WHO global response team will discuss the sexual transmission report among other issues at its daily meeting later on Wednesday, Hartl said.
"There are many things we don't know about Zika," he said. "Lots of surveillance is needed ... We have our team set up and are sure there will be lots of progress quickly."
For now, the key in infected areas is to try to control mosquitoes and for people to wear adequate clothing, use insect repellent and sleep under bednets, Hartl said.
Pfizer Inc, Johnson and Johnson and Merck & Co Inc said they were evaluating their technologies or existing vaccines for their potential to combat Zika.
Japan's Takeda Pharmaceutical Co Ltd said it had created a team to investigate how it might help make a vaccine, a day after Sanofi SA said it would launch a Zika vaccine program.
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