Zika-carrying mosquitoes are for the first time believed to be spreading the virus within the continental United States, federal disease-control officials said Friday, citing the infections of four people who were apparently bitten in the same Miami neighborhood.
More than 1,650 Zika infections have been reported on the U.S. mainland, but these would be the first not linked to foreign travel or sex with an infected person, said Tom Frieden, director of the Atlanta-based Centers for Disease Control and Prevention.
"As we have anticipated, Zika is now here," he told a news briefing.
Florida Gov. Rick Scott said the four cases likely originated from bites inflicted in the bustling arts district of Wynwood, just north of downtown Miami. The area encompasses less than a square mile.
"This means Florida has become the first state in the nation to have local transmission of the Zika virus," the governor told reporters.
Even though the development had been widely expected, the news is a grim new milestone in the geographical reach of Zika, which, when contracted by pregnant women, can cause serious birth defects. No vaccine exists.
No mosquitoes found in the affected Miami neighborhood have tested positive, but locating them would in any case be almost unheard of, Frieden said – "like finding a needle in a haystack."
Investigators routinely use other methods for trying to identify the means of transmission, including detailed tracking of the movements and activities of people determined to have been infected.
At least two of the four Zika-affected individuals are thought to have been bitten at or near their workplaces in Wynwood, a trendy area filled with restaurants and galleries.
Those who have contracted the virus, three men and a woman, all have "active cases" but have not exhibited sufficient symptoms to be admitted to a hospital, the governor said in a statement.
More cases are almost certain to emerge, Frieden said, adding that there is typically a lag of some days between developing symptoms and receiving a diagnosis. The virus is often asymptomatic or marked only by mild symptoms such as fever and a rash.
It is to unborn children that the biggest threat exists. Many develop microcephaly, a condition associated with incomplete brain development that can leave babies with abnormally small heads and various developmental problems.
The four Miami infections are thought to have been contracted earlier in July, Frieden said.
The CDC dispatched an epidemiologist to Florida and is working closely with health authorities in the state, he said.
Pregnant women living in or visiting the area, or women who are thinking of becoming pregnant, were urged to take diligent precautions against mosquito bites, including wearing bug spray, covering up arms and legs with long sleeves and trousers, and staying indoors or in screened-in areas.
The CDC did not recommend avoiding travel to south Florida, a huge tourist magnet, as it has with more than 50 countries where the virus is endemic. Officials said such a step would not be warranted at this time due to the small number of cases involved.
But blood-donation centers in Miami-Dade and Broward counties have been ordered by the U.S. Food and Drug Administration to halt collections, pending ability to individually screen units of blood for the virus. People who have visited south Florida in the last month are being urged to hold off on donating blood, and adjoining counties have been advised to step up screening efforts.
Previous FDA orders had called for refusing blood donations from people who had traveled to countries where the virus was prevalent.
Health officials have been going door-to-door in the affected area, offering detailed advice on how to avoid being bitten and on getting rid of standing water where the mosquitoes can breed.
The Miami area had already been considered at risk because it is a prime gateway into the United States for travelers arriving from Zika-affected countries in Latin America and the Caribbean. The Aedes aegypti mosquito that carries the virus ranges in parts of the Deep South, Arizona and California, though it is not native to the Americas.
Nearly 100 people in California have been infected with Zika while traveling to a country which has an outbreak or through contact with a person who visited an affected country, according to state health officials. Most of the Zika cases have been in Los Angeles County, with 23 infections, and San Diego County, with 19.
Aedes aegypti mosquitoes are now found in 12 counties in California, including Los Angeles, Orange, Riverside and San Diego counties. In the L.A. region, the mosquitoes are believed to have arrived several years ago in shipments of bamboo plants coming from China to El Monte.
Though Florida and Texas are considered at highest risk for Zika outbreaks, mosquito control workers in many regions of the country have been clearing out backyards and other areas of possible mosquito breeding sites.
Compared to the more common Culex mosquito, Aedes aegypti mosquitoes are difficult to eradicate, able to survive and breed using extremely small amounts of water – as little as a teaspoon -- and lay eggs that can survive months of drought.
Zika has burned its way through dozens of countries in Latin America and the Caribbean.
The virus has also been spreading in the U.S. territories of Puerto Rico, American Samoa and the U.S. Virgin Islands, where more than 4,700 cases have been reported since last year.
Cruelly, the burden of birth defects has fallen chiefly on impoverished populations with fewer resources to deal with often-devastating effects that can last a lifetime.
Because the standard of living is different in the United States than in many areas where the virus has become endemic, U.S. health officials have said that widespread outbreaks are unlikely. Many more people have access to air conditioning and advanced sanitation, and aggressive eradication efforts are already under way in areas where tropical mosquitoes flourish.
Word of the first domestically originating cases drew calls for greater resources to be earmarked for fighting the spread of the virus.
Florida's governor has allocated over $25 million in state funds for Zika response, and the White House and the CDC have provided over $10 million.
But Congress left on a seven-week vacation without giving the Obama administration any of the $1.9 billion it sought to battle Zika.
White House spokesman Eric Schultz called that "regrettable" and said: "Today's news should be a wake-up call to Congress to get back to work."
Times staff writer Soumya Karlamangla in Los Angeles, Naseem S. Miller of the Orlando Sentinel, and the Associated Press contributed to this report.
4:57 p.m.: This article was updated with the conditions of the four Zika patients in Miami.
2:50 p.m.: This article was updated throughout with staff reporting.
1 p.m.: This article has been updated throughout with more information.
9:55 a.m.: This article has been updated with more details and quotes throughout.