MIAMI — Mosquitoes have apparently begun spreading the Zika virus on the US mainland for the first time, health officials said Friday in a long-feared turn in the epidemic that is sweeping Latin America and the Caribbean.
Four recently infected people in the Miami area — one woman and three men — are believed to have contracted the virus locally through mosquito bites, Governor Rick Scott said at a news conference.
No mosquitoes in Florida have actually been found to be carrying Zika, despite the testing of 19,000 by the state lab. But other methods of Zika transmission, such as travel to a stricken country or sex with an infected person, have been ruled out.
‘‘Zika is now here,’’ said Dr. Thomas Frieden, director of the US Centers for Disease Control and Prevention.
Still, US health officials said they do not expect widespread outbreaks in this country of the sort seen in Brazil, in part because of better sanitation, better mosquito control, and wider use of window screens and air conditioners.
The virus has triggered alarm across the warmer latitudes of the Western Hemisphere. While most people who get Zika don’t even know they are sick, infection during pregnancy can cause babies to born with disastrously small heads and other severe brain-related defects.
More than 1,650 people in the mainland United States have been infected with Zika in recent months. The four people in Florida are believed to be first ones to contract the virus within the 50 states from mosquitoes.
‘‘This is not just a Florida issue. It’s a national issue — we just happen to be at the forefront,’’ Scott said.
Florida agricultural officials immediately announced more aggressive mosquito-control efforts, and Florida politicians rushed to reassure tourists that it’s still safe to visit the state.
Some medical experts said pregnant women should not travel to the Miami area, especially if it involves spending time outdoors. However, the CDC is not issuing such advice.
US health officials said the country might see small clusters of infections. But ‘‘we don’t expect widespread transmission in the continental United States,’’ the CDC’s Frieden said.
The four Florida infections are thought to have occurred in a small area just north of downtown Miami, in the Wynwood arts district, the governor said.
The area, known for bold murals spray-painted across warehouses, art galleries, restaurants, and boutiques, is rapidly gentrifying and has a number of construction sites where standing water can collect and serve as a breeding ground for the tropical mosquito that carries Zika.
People in Florida’s Miami-Dade and Broward counties are being tested to learn whether there are more cases, the governor said.
‘This is not justa Florida issue. It’s a national issue — we just happen to be at the forefront.’
‘‘If I were a pregnant woman right now, I would go on the assumption that there’s mosquito transmission all over the Miami area,’’ warned Dr. Peter Hotez, a tropical medicine expert at the Baylor College of Medicine.
He said that there are probably more cases that have not been diagnosed, and that people should not be surprised if mosquitoes are soon found to be spreading Zika in Louisiana and Texas as well.
Earlier this week, federal authorities told blood centers in the Miami-Fort Lauderdale areas this to stop collecting blood until they screen it for the virus.
Frieden said the evidence suggests that the mosquito-borne transmission occurred several weeks ago over several city blocks.
So far, there have been than 4,700 cases of mosquito-borne Zika in Puerto Rico and other US territories.