The case involves an individual in Dallas County who was infected with the virus after contracting it in another country where the Zika virus is present. The individual then had sexual contact with the patient, who acquired the virus.

The Texas authorities didn't provide any additional details about the individual except to make clear that the original infection occurred outside the United States.

Until now health officials have focused on mosquitoes as the primary carrier of the virus.

"Now that we know the Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others," said Zachary Thompson, the director of health and human services in Dallas County.

The World Health Organisation (WHO) and other public health authorities are alarmed about the Zika virus because it is suspected of causing a birth defect called microcephaly through mother-to-child transmission. Babies born with microcephaly have unusually small heads and brains.

The symptoms of the virus in adults are relatively minor. They include fever, rash, joint pain and conjunctivitis lasting from several days to a week.

On Monday, the WHO declared a public health emergency over the Zika virus. Brazil, the country most affected, will receive international assistance to counteract the outbreak and spread of the virus.

-News24.com