AUSTIN, Texas (Reuters) – The first known case of Zika virus transmission in the United States was reported in Texas on Tuesday by local health officials, who said it was contracted through sexual contact and not the bite of a mosquito, a day after the World Health Organization declared an international public health emergency.

Dallas County Health and Human Services said it received confirmation of the case in Dallas from the U.S. Centers for Disease Control and Prevention.

A Dallas County health official said in a tweet that the case was transmitted through sexual contact with someone who had traveled to Venezuela. The person infected did not travel to the South American country, county health officials said.

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 WHO has said the virus, linked to severe birth defects in Brazil, has been spreading rapidly in the Americas and could infect 4 million people. It said it had launched a global response unit to fight the mosquito-borne virus, which is spreading rapidly in Latin America. Africa and Asia are also seen as being vulnerable.

The virus has been linked to microcephaly, in which babies have abnormally small heads and improperly developed brains.

“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,” Dr. Anthony Costello said in Geneva. Costello is WHO’s director for maternal, child and adolescent health.

Twenty to 30 sites could be established worldwide, mainly in poor countries without robust healthcare systems.

Brazil, the country hardest hit by Zika, has reported 3,700 suspected cases of microcephaly that may be linked to Zika.

In an address to a joint session of Brazil’s Congress, President Dilma Rousseff said her government will 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.

“There will be no lack of funding,” Rousseff said.

Brazil, which has 3,700 suspected cases of microcephaly that may be linked to Zika, is scheduled to host the Olympics in Rio de Janeiro in August.

Rousseff also said 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.

The virus was first identified in 1947 in rhesus monkeys in Uganda while scientists were studying yellow fever, according to the WHO. It was identified in humans in 1952. Zika is spread by mosquitoes of the Aedes genus.

Sanofi on Tuesday announced that it has lauched a project to develop a vaccine against the virus, the most decisive commitment yet by a major vaccine maker. The company said its Sanofi Pasteur vaccines division would use its expertise in developing vaccines for similar viruses such as yellow fever, Japanese encephalitis and dengue.

The WHO called for urgent development of better tests to detect the virus in pregnant women and newborn babies.

“The reason it’s a global concern,” Costello said of Zika, “is that we are worried that this could also spread back to other areas of the world where the population may not be immune.”

Costello said the Aedes mosquitoes that carry the Zika virus “are present … through Africa, parts of southern Europe and many parts of Asia, particularly South Asia.” Africa and Asia have the world’s highest birth rates.

‘GUILTY UNTIL PROVEN INNOCENT’

WHO Director-General Margaret Chan said on Monday it was “strongly suspected but not yet scientifically proven” that Zika causes microcephaly.

“We believe the association is ‘guilty until proven innocent,'” Costello said, referring to whether Zika causes microcephaly.

A new method to render male mosquitoes infertile by nuclear radiation could help reduce populations of the insect carrying the virus, the Vienna-based U.N. International Atomic Energy Agency U.N. atomic agency said.

Small biotech companies and academic institutions also have plans to develop a Zika vaccine, and GlaxoSmithKline PLC has said it is concluding feasibility studies to see if its vaccine technology was suitable. And on Tuesday other companies joined the effort.

The University of South Australia said it was working on a Zika vaccine with Australian biotech Sementis Ltd.

U.S. drug developer NewLink Genetics Corp said it has started a project to develop Zika treatment options.

Experts have said a Zika vaccine for widespread use is months if not years away.

A Zika case has been identified on mainland Chile for the first time in a man who had traveled to Colombia, where the virus is circulating, local media reported.

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.

(Additional reporting by Dominique Vidalon in Paris, Stephanie Nebehay in Geneva, Shadia Nasralla in Vienna, Ben Hirschler in London, Anthony Boadle in Brasilia, Jane Wardell in Sydney, Amy Sawitta Lefevre in Bangkok, Pedro Fonseca in Rio, Rosalba O’Brien in Santiago, Ankur Banerjee and Amrutha Penumudi in Bengaluru; Writing by Will Dunham; Editing by Toni Reinhold and Jonathan Oatis)