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A 500-Square-Foot Area in Miami Is Ground Zero for the Zika Virus

Around July 4, a patient entered an emergency room in Miami-Dade County with a fever, a rash and joint pain — three of the four classic symptoms of the Zika virus. By this point, there had already been about 1,600 other Zika cases in the continental United States, but it soon became clear that this one was different.

All the other patients had either traveled to Latin America or the Caribbean, where Zika had been raging for months — or they had sex or close contact with someone who had been there. Not this patient.

It was the case public health officials had been expecting and dreading: A person in the continental United States had been infected from the bite of a local mosquito.

It would turn out to be the first of a wave of cases health officials are now scrambling to identify and contain. They are investigating 17 suspected cases of locally transmitted Zika — including 13 linked to a 500-square-foot area that touches two neighboring businesses in the Wynwood section of Miami.

While officials are confident that Zika will never run rampant in the United States, the chase is on in South Florida as more local cases are identified and Aedes aegypti mosquitoes, the type that carry Zika, stay one step ahead of the spray.

Public health officials are also grappling with, well, the public: Some think that the authorities should warn pregnant women away from much more than one square mile, and still others seem unaware that Zika, while mild or inconsequential for most people, can cause devastating brain damage to the babies of infected pregnant women.

“Obviously when people detect local transmission, there’s a lot of different opinions,” said Dr. Lyle R. Petersen, who is managing the Zika response of the Centers for Disease Control and Prevention. “People panic and there’s potential for irrational thinking in either direction, not doing enough and doing too much.”

Last week the agency took the unprecedented step of urging people to stay away from a place in the continental United States, warning pregnant women to avoid the square-mile area of Wynwood that contains the 500-square-foot patch and the two unidentified businesses.

Dr. Petersen said because the continental United States has better mosquito control, more air-conditioning and less standing water than other countries dealing with the Zika virus, there are most likely to be only “handfuls of local transmission and very rare outbreaks,” which will be containable with a targeted response.

“It’s not the whole city — it’s a very small part of the city,” Dr. Petersen said of the possible risk in Miami. “So the recommendation is just don’t go there, particularly if you’re pregnant. In the rest of the city, you’re more likely to get killed in a car crash than you are to get Zika virus.”

The story of the homegrown Zika cases demonstrates both the value and the limits of planning when the enemy is an unpredictable and stealthy virus delivered by a hardy mosquito adept at hide-and-seek. In a 60-page blueprint this year, the C.D.C. outlined detailed steps to take, and officials have been assiduously tracking the patients, testing people close to them and amping up mosquito control.

Still, battling Zika in Wynwood is challenging because its mixture of businesses, apartments and warehouses makes it a veritable urban mosquito mecca. Slices of the gentrifying neighborhood are bursting with art galleries, boutiques and condominiums, but they give way to a still-tattered section of run-down buildings where residents struggle in poverty.

“This is low-income,” Tony Fonseca, 45, a construction worker, said as he stood outside the La Fama Supermarket at Northwest Second Avenue and 31st Street. “People live on welfare, they use drugs. You walk around here at night, you can get assaulted — they’ll steal your Ray-Bans.”

That part of Wynwood, Mr. Fonseca said, has “lots of standing water,” but he said people in this predominantly Latino neighborhood tend to blame foreign visitors to the arts district for bringing the Zika virus.

“Maybe someone brought it from Latin America,” said Mr. Fonseca, born in Miami of Nicaraguan parents. “But no one around here is worried about it.”

A firefighter taking a break at a coffee shop on Northwest Second Avenue said two of his female colleagues, both pregnant, were temporarily transferred to a station several miles south, near Coconut Grove. But as Florida health department workers go door to door asking for urine samples to test, seeking to learn the extent of the Zika risk in Wynwood, not everyone sees the need.

Diana Ozuna, 27, declined to be tested, even after her 53-year-old mother, who lives nearby, submitted a urine sample of her own. Ms. Ozuna said she lowered her window screens and used repellent, especially on her 20-month-old daughter. Still, because she is not pregnant and has no immediate plans to be, she does not perceive Zika to be a great menace.

“It’s bad,” she said, “but it’s not something that you die from.”

Zika is an enemy most people cannot see. While its effects can be catastrophic to developing fetuses, in adults the effects are usually mild or negligible, and health officials assume that for every person with symptoms, four more have undetected Zika infection.

So when a homegrown case arises, officials need to test people close to the infected patient to see if the case is isolated or if there is active local transmission taking place in that community.

After the first case appeared in July, the Florida Department of Health tested 54 people who had some connection to the patient or lived within 150 meters of the patient’s apartment building, the maximum distance experts say mosquitoes that carry Zika can typically travel. None of those 54 tested positive, the department said.

Days later, around July 8, another homegrown Zika case showed up in Broward County, adjacent to Miami-Dade: a person who had visited a family doctor, complaining of fever, rash, headache and joint pain.

In Broward, the health department tested 70 contacts and neighbors of Patient No. 2. All tested negative. And Patient No. 2 had no connection to Patient No. 1.

“They had never been even close to each other,” Dr. Petersen said. And “there was no third person. It wasn’t like you could say, ‘Oh, Harry in the apartment next door has the same symptoms.’”

That there were two isolated cases in different neighborhoods was, in some ways, a relief. It meant that there was no danger zone, no place rife with infected mosquitoes. Most likely, a person who had traveled to Latin America had landed in Miami with Zika, been bitten by an Aedes aegypti mosquito in Florida, which, now infected itself, had bitten a person in Miami-Dade. The same thing had probably happened via a different mosquito in Broward, they believe.

County workers unleashed mosquito control tactics where those patients lived, and the health department determined that those neighborhoods were not active Zika zones

Then two other cases appeared — one in Broward and one in Miami-Dade, people whose symptoms started on July 9 or July 10. They were not connected to Patient No. 1 or Patient No. 2. But they were connected to each other.

“They worked in businesses that were very close to each other,” Dr. Petersen said. Because both businesses were in Wynwood, “obviously it looked like there was a potential link between the two. They were in close proximity, two people who were sort of in the same area working.”

On July 29, officials announced the four cases of local transmission. But it was still unclear whether Wynwood had an outbreak or just two local cases. Determining the answer would depend on two things: Were there other Wynwood-linked cases besides the two workers, and was the mosquito-killing campaign launched when those two cases were identified killing enough mosquitoes?

The Florida health department began asking for urine samples from employees and people who lived or worked near the businesses, which are small and do not draw much public traffic, but have outdoor spaces that might have been attractive to mosquitoes.

Over the weekend of July 30, lab results came in. Of 26 people closely connected to the Wynwood workers, four were considered to have Zika. Six of 52 people tested in the neighborhood did too. There were now 12 infected people with Wynwood connections, all between the ages of 20 and 45.

“All the people that were positive spent time either at those businesses or in the immediate 150 meters around those businesses,” Dr. Petersen said.

The core transmission zone was even smaller. “The area that has demonstrated the spread of Zika is only a 500-square-foot area,” Dr. Thomas R. Frieden, the director of the C.D.C., said at a news conference in Miami.

None of the people infected in Wynwood was related or had sexually transmitted the virus to each other. Some were part of a group of “friends that hang out together,” Dr. Petersen said, and two were housemates. Officials declined to say if either of the women was pregnant.

The earliest infection in the group probably occurred in late June, meaning that Zika had been in Wynwood for at least a week longer than officials had previously known. “This had been going on for practically a month,” Dr. Petersen said, so “you have to basically assume that there’s ongoing transmission.”

Moreover, officials responsible for mosquito control reported in the last weekend of July that “despite daily use of spraying,” they were “still seeing new larval mosquitoes and moderately high Aedes aegypti counts,” Dr. Frieden said, adding that the mosquitoes might be resistant to the insecticides being used, or simply hiding in pockets unreachable by backpack- and truck-spraying. “Aggressive mosquito control measures don’t seem to be working as well as we would like.”

On Friday, after aerial spraying with a different insecticide began in Wynwood, the mosquito numbers were coming down, Dr. Frieden said. The C.D.C. will likely keep its Wynwood warning in place for weeks, lifting it only when about 45 days have passed without a new case being diagnosed, or when mosquito counts decrease substantially, Dr. Petersen said. As of yesterday, the health department said it had tested 437 people in the square-mile active zone and identified only one additional case — someone among the 26 close contacts of the two Wynwood workers.

Wynwood is still the only area authorities consider an active local transmission zone. So far, a 10-block area in the northwest section of the square mile has been cleared, since no one there was found to be infected, Gov. Rick Scott of Florida said.

But authorities expect more cases to be identified, some in Wynwood, some elsewhere. On Monday, they announced a 17th case: a person in Palm Beach County who had traveled to Miami-Dade.

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