The Zika virus has been transmitted sexually, top US health authorities have confirmed, fuelling 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 travellers 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 travelled 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 on 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.

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