The Zika virus has been transmitted sexually, top US health authorities have confirmed, fueling fears of the rapid spread of the disease blamed for a surge in the number of brain-damaged babies. With concern growing that an outbreak sweeping Latin America could spread much farther, health authorities in Texas said they had confirmation of the virus being transmitted by sexual contact and not just tropical mosquitoes.

That is a troubling prospect for the United States, Canada and Europe, where Zika had so far only appeared in travelers returning from affected areas. “The patient was infected with the virus after having sexual contact with an ill individual who returned from a country where Zika virus is present” this year, a Dallas County statement read.

The county subsequently tweeted that the virus was contracted from someone who had traveled to Venezuela, and that a second case of Zika imported from Venezuela has also been documented. Dr Tom Frieden, director of the US Centers for Disease Control and Prevention later Tuesday confirmed in an email the case of sexually transmitted infection reported earlier in Texas.

Someone who visited Venezuela and was infected there developed Zika symptoms as did their sexual partner who never left the United States, he said on Twitter. Last month, the CDC said it was aware of one reported case of sexual transmission of Zika and one case of the virus being present in a man’s semen after it disappeared from his blood. Zika, which was first identified in Uganda in 1947, causes relatively mild flu-like symptoms and a rash.

But there is growing alarm over an apparent link between the current outbreak and both a rise in birth defects and a potentially crippling neurological disorder called Guillain-Barre syndrome.

Latin American countries, particularly Brazil, have reported a surge in cases of microcephaly — which causes babies to be born with abnormally small heads — since the Zika outbreak was declared in the region last year. The virus is spread primarily by the Aedes aegypti mosquito, whose habitat is concentrated in the tropics — giving temperate countries an apparent reprieve.

But sexual transmission would complicate matters. “The majority of Zika infections are asymptomatic,” said Peter Horby, professor of emerging infectious diseases and global health at Oxford University. “Is there a risk of sexual transmission from people who had asymptomatic infection? For how long can sperm be infectious? This highlights our ignorance of this virus.”

A day after declaring the spike in serious birth defects in South America an international emergency, the World Health Organization said it had created a global Zika response unit to contain the virus. WHO expert Anthony Costello emphasized the urgency of rapid action, stressing there was no reason to believe the crisis would remain limited to Latin America.

“We know that the mosquitoes that carry Zika virus are present through most of Africa, parts of southern Europe and many parts of Asia, particularly south Asia,” he said. Underlining Costello’s point, Thai officials announced a man had contracted the virus in the country. Cape Verde and Indonesia have also reported domestic Zika cases. The International Federation of Red Cross and Red Crescent Societies joined the WHO in declaring an “emergency,” and appealed for 2.4 million Swiss francs ($2.36 million) to support its response in the Americas.