Aerial spraying of insecticide began Thursday in the one-mile-square area of Miami where mosquitoes have infected people with the Zika virus, and officials reported some glimmers of progress.
“We are very encouraged by the initial results, which showed a large proportion of the mosquitoes killed,” Dr. Thomas R. Frieden, the director of the Centers for Disease Control and Prevention, said at a news conference here. But Dr. Frieden added, “This is going to take an intense effort.”
On Monday, faced with 14 locally transmitted infections, the C.D.C. took the unprecedented step of advising people to stay away from a location in the continental United States, urging pregnant women not to travel to the one-square-mile area where 12 of the cases are linked.
That area is part of the Wynwood section of Miami, north of downtown. Since then, mosquito-control efforts have been accelerated and teams from the Florida Department of Health have been testing people in the neighborhood, a mix of industrial and residential buildings.
Gov. Rick Scott of Florida in Miami on Thursday. Credit Joe Raedle/Getty Images
On Thursday, the health department said it had completed testing in a 10-block section of the neighborhood and found no additional cases.
“We feel comfortable now that in that one-mile radius, we can take 10 blocks in that northwest corner and say we don’t believe there is any active transmission of Zika,” Gov. Rick Scott said at the news conference.
Nevertheless, Dr. Frieden said, “we would not be surprised to see additional infections diagnosed” in a core area that measures 500 square feet.
“That’s the way Zika works,” he said. “But what we want to see is the mosquito counts coming down.”
This week, the department has reported only one other case of local transmission of Zika, bringing the total to 15, but that was in another part of Miami-Dade County with no connection to Wynwood. Officials said they were testing people in the community to see if the virus had spread but so far had found no evidence.
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The aerial spraying of a pesticide called Naled, conducted early Thursday morning, was undertaken after other methods of attacking the Aedes aegypti mosquito, which spreads the Zika virus, had only marginal success.
“The Aedes aegypti mosquito has several bad things associated with it,” Dr. Lyle Petersen of the C.D.C., who is managing the agency’s Zika response, said in an interview. “It tends to breed in small pools of water, which are ubiquitous in any urban environment, and tends to be hard to reach. It tends to breed in cryptic environments that are hard to find. And it’s just very hard to get rid of.”
The traditional methods used in Miami — truck-mounted spraying and backpack spraying with two types of pyrethroid insecticide — have not killed enough mosquitoes, Dr. Petersen said. He said Naled, an insecticide that has been widely used in Florida but not in Miami, might work against city mosquitoes that could have become resistant to pyrethroids, the insecticides that had been used. He said the plan was to spray once a week with Naled to kill adult mosquitoes and once a week with insecticide to kill larvae.
Also, Naled is good at providing a “rapid knockdown of mosquitoes,” Dr. Frieden said. “It can get where no truck can get, where no backpack sprayer can get.”
Dr. Frieden stressed that Zika was not like West Nile virus, which spreads easily because it is carried by a different kind of mosquito. It is possible to stop Zika in its tracks more efficiently because the Aedes aegypti mosquito flies only a short distance, he said.