ORLANDO (Florida) • Four cases of Zika infection in Florida are very likely to have been caused by mosquitoes there, the state's Department of Health has said - the first documented instances of local transmission in the continental United States.

"Zika is now here," Dr Thomas Frieden, director of the Centres for Disease Control and Prevention (CDC), said in a press statement on Friday.

The CDC and Florida officials said that for now, the area of concern is limited to a square mile (2.6 sq km) in the Wynwood neighbourhood of Miami, a gentrifying area with restaurants and art galleries just north of downtown.

Health authorities are not advising people to stay away from the neighbourhood, Dr Frieden said, but this could change if the number of cases increases substantially. The four people appear to have been infected early this month. Since then, mosquito control efforts have been stepped up in the area, and additional cases have not been identified.

The Florida cases signal a new stage in an epidemic that has left a trail of birth defects in Brazil and strained healthcare resources throughout Latin America.

The epidemic is raging in Puerto Rico, CDC officials reported last week. Two per cent of blood donors there have been recently infected, and hundreds of pregnant women have tested positive.

Researchers had long predicted that the Zika virus would gain a toehold in the continental United States, likely in Florida and along the Gulf Coast.

While the outbreak is not expected to escalate sharply, its course is uncertain. There are now more than 1,600 confirmed Zika cases in the continental US.

Until the announcement on Friday, all of them had been a result of travel abroad: The virus was contracted either by a mosquito bite elsewhere or by intercourse with someone who had been to a Zika- affected area.

None of the four patients in Miami had travelled to Zika-affected areas in Latin America or the Caribbean.

NEW YORK TIMES