"That gives you the plausibility of spread, but the science is clear to date that Zika virus is primarily transmitted to people through the bite of an infected mosquito", Dr. Anne Schuchat of the Centers for Disease Control said during a recent news conference.

Costa Rican health authorities have only detected two people infected with the mosquito-borne virus - both suspected of having contracted Zika elsewhere in Latin America. They could not determine how long it had persisted, because he had had more than one episode of fever that year that might have been related to Zika virus.

The illness is usually mild with the symptoms lasting a week.

A 2011 study looked at two cases of the virus in USA scientists who lived in Senegal while they were performing a mosquito-sampling project.

Based on that, the CDC says it will soon provide guidance on sexual transmission, with a "focus on the male sexual partners of women who are or who may be pregnant". "CDC is aware of a few scientific papers that suggests Zika virus may be transmitted through sex".

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.

A CDC spokesman confirmed the results of a test for Zika infection but said local officials investigated the mode of transmission.

The Zika virus, which is transmitted via the Aedes aegypti mosquito, has been linked to a birth defect called microcephaly.

In west-central Florida, in Pinellas County, officials plan to educate people about removing standing water where mosquitoes breed, and are looking into expanding spraying to specifically target mosquitoes that transmit Zika.

Australians probably tolerate the odd mosquito bite and Zika-carrying vectors are not necessarily circulating in the kind of numbers which might trigger repellent use.

Babies with microcephaly are born with unusually small heads, and can go on to have incomplete brain development.

The World Health Organisation this week classified the spread of the virus as a global public health emergency.

Health officials say a patient in Texas has acquired the Zika virus through sexual transmission.

There is no treatment or vaccine for Zika, which is in the same family of viruses as dengue. "Through these experiences, the department remains ready to protect residents and visitors from the Zika virus".

The WHO has estimated there will be 3 to 4 million cases of Zika in the Americas over the next 12 months.