Orlando, Florida: Florida authorities on Friday reported what is believed to be the first evidence of local Zika transmission in the continental United States, concluding that mosquitoes likely infected four people with the virus that can cause rare but serious birth defects.

Governor Rick Scott said the state believed active transmission of the virus was occurring within an area of the city about the size of 2.6 square kilometres. Testing showed that one woman and three men had been infected, Gov. Scott said.

While health officials have yet to identify mosquitoes carrying the virus, the state has ruled out other means of transmission, including travel to another country with a Zika outbreak, and sexual contact.

"We have worked hard to stay ahead of the spread of Zika and prepare for the worst," Gov. Scott said in a statement. "We will continue to put every resource available to fighting the spread of Zika in our state."

Zika's greatest risk is believed to be posed by infection in pregnant women, given its ability to cause microcephaly in babies, a condition defined by small head size that can lead to developmental problems. The current outbreak was first detected in Brazil last year and has since spread rapidly through the Americas.

Florida Surgeon General Celeste Philip said that health officials are not advising pregnant women in the transmission area to move.

"We do not believe there will be ongoing transmission," Dr Philip said at a press conference in Orlando, citing daily efforts to control the mosquito population in the area.

The local health department is searching for other potential infections, with more than 2,300 people tested so far in the state, is ramping up mosquito control programs and is distributing Zika protection kits to pregnant women at their doctors' offices, Florida officials said.

Residents in Miami neighbourhoods thought to harbour Zika said that local spread of the virus had been inevitable, given the large numbers of tourists from other countries with outbreaks.

Damian Jose Delgado, a 35-year-old father of two, said news of Zika's arrival would make him think twice about expanding his family.

"I think I might be done having kids," Mr Delgado said.

Early warnings

US health officials have cautioned for months that the summer mosquito season was likely to bring local outbreaks, with Gulf Coast states such as Florida, Texas and Louisiana, on the frontlines. Some have said Zika's spread could be more limited than in places like Brazil, given widespread use of screens on windows, air conditioning and mosquito control programs.

The Zika outbreak in Brazil has been of particular concern given this summer's Olympic Games in Rio.

The US Centres for Disease Control and Prevention (CDC) said on Friday that it would not recommend limiting travel to Florida. The agency has recommended that pregnant women not travel to other countries where Zika is active.

"We anticipate that there may be additional cases of 'homegrown' Zika in the coming weeks," said Dr Lyle Petersen, a CDC official helping lead the agency's Zika response effort. "Our top priority is to protect pregnant women from the potentially devastating harm caused by Zika."

Up to 80 per cent of people infected with Zika may experience no signs of illness, while those who do generally have relatively mild symptoms, including fever and rash. There are no specific treatments or vaccines for the virus.

The World Health Organisation declared a global health emergency in February, reflecting alarm over the discovery that Zika was linked to microcephaly and other severe neurological abnormalities. Brazil has confirmed over 1,600 cases of microcephaly linked to Zika infection in pregnant women.

Public health officials say Zika is also a likely cause of Guillain-Barre syndrome, a rare neurological disorder in adults that can cause temporary paralysis.

Zika's arrival in the United States comes with Congress in recess after failing to reach agreement over how much funding could be used to fight an outbreak. The Obama administration has requested $US1.9 billion to finance research, mosquito control and other prevention efforts. Gov. Scott said he has authorised $US26.2 million to help fight the virus in the state.

Until now, the more than 1,600 Zika cases in the United States have stemmed from travel to another country with active transmission, as well as a small number of cases of apparent sexual transmission by a person infected outside of the country.

Puerto Rico is grappling with a Zika outbreak, with more than 4,600 cases of local transmission. US health officials have predicted there will be hundreds of thousands of cases on the island territory before the current outbreak ends due to the prevalence of Zika-carrying mosquitoes and a lack of infrastructure to protect against insect bites. 

Reuters