Local health officials in Dallas on Tuesday reported a case of the Zika virus being sexually transmitted, heightening fears of the spread of the mosquito-born virus.

It comes a day after the World Health Organization declared Zika an international public health emergency.

"The patient was infected with the virus after having sexual contact with an ill individual who returned from a country where Zika virus was present," a statement from Dallas County Health and Human Services said. It later said on Twitter the country was Venezuela.

The county said it had received confirmation of the case from the US Centers for Disease Control and Prevention (CDC). A CDC spokesman confirmed the results for a Zika infection but said local officials investigated the mode of transmission.

Authorities said there were no reports of the virus being locally transmitted by mosquitoes.

Sexual transmission not proven

Only one possible person-to-person case of sexual transmission has been reported internationally. But health officials have said more evidence is needed to confirm whether Zika can be transmitted through sexual contact.

The WHO has said the virusis spreading rapidly in the Americas and could infect 4 million people. A global response unit has been launched to fight the virus.

Caution urged for women

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

Researchers believe that if a pregnant woman is bitten by an infected mosquito, particularly in the first trimester, she faces a higher risk of having a child with birth defects.

The virus has now spread to 26 countries and territories including Brazil, which is the country hardest hit, with 3,700 suspected cases of microcephaly that may be linked to Zika. Brazilian authorities have vowed to proceed with the 2016 Olympics despite the health scare.

Ireland reports first cases

The first Irish cases of the virus were detected in two people with a history of traveling to an affected country, the country's Health Service Executive said on Tuesday.

The two individuals are unrelated and neither is at risk due to pregnancy. They've been described as currently well and fully recovered.

Meanwhile, Nicaragua confirmed its first two cases in pregnant women on Tuesday and Chile reported its first case of the virus.

The race is on to find a vaccine to prevent the virus taking hold. There is no treatment or vaccine for Zika, which is in the same family of viruses as dengue fever. However, Germany has developed the first test for Zika.

mm/jr (AFP, AP, Reuters)