The first known case of Zika virus transmission in the United States has been reported in Texas by local health officials, who say it was likely contracted through sex and not a mosquito bite, a day after the World Health Organisation 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 the WHO say it could hit Africa and Asia as well.
Zika was thought to be spread through bites of the mosquito of the Aedes genus, making sexual contact as a mode of transmission a potentially alarming development.
The US Centers for Disease Control and Prevention confirmed it was the first US Zika case in someone who had not travelled abroad, CDC Director Dr Tom Frieden said on Twitter.
However, the CDC has not investigated how the virus was transmitted.
After this case, the CDC advised men to use condoms after travelling to affected areas, while pregnant women should avoid contact with semen from men exposed to the virus.
The Dallas County Department of Health said on Twitter that the person was infected through sexual contact with someone who had travelled to Venezuela and had not themselves travelled to the South American country.
The Texas Department of State Health Services said the details were still being evaluated, but the possibility of sexual transmission from an infected person to a non-infected person is likely in this particular case.
Authorities said there were no reports of the virus being transmitted by mosquitoes in the Texas county.
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 transmission. The medical literature also has one case in which the virus was detected in semen.
The virus has been reported in more than 30 countries and linked to microcephaly, in which babies have abnormally small heads and improperly developed brains.
The American Red Cross has asked blood donors who have travelled to Zika virus outbreak areas to wait at least 28 days before donating.
The WHO said the virus could infect four million people in the Americas and has launched a global response unit to fight the mosquito-borne virus, with a focus on low and middle income countries.
Brazil is the country hardest hit and President Dilma Rousseff says all resources will be used to combat the mosquito that transmits the virus, as no vaccine or treatment was yet available.
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.
French drug maker Sanofi SA has launched a project to develop a vaccine against the virus.
Other organisations are also working on a vaccine with the University of South Australia saying it would work with Australian biotech Sementis Ltd and US drug developer NewLink Genetics Corp saying it has also started work.
The first Irish cases of Zika virus have been detected in two people with a history of travelling to a country affected by the mosquito-borne infection, the Health Service Executive of Ireland said.
Chilean health officials said they have confirmed three cases in Chile of people infected with the Zika virus, all of whom were infected while travelling elsewhere in Latin America.
An Australian state health service said two Australians were diagnosed with the virus after returning from the Caribbean, confirming the first cases of the virus in the country this year.
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