The director of the Centers for Disease Control and Prevention says officials are finding it challenging to eradicate mosquitoes in a part of Miami where 15 people appear to have contracted the Zika virus.
A new case was announced late Tuesday afternoon.
CDC Director Tom Frieden told ABC's "Good Morning America'' Tuesday that officials issued a rare travel warning advising pregnant women to avoid Miami's Wynwood arts district because mosquito counts are still high in the area.
Frieden said it's possible mosquitoes are resistant to the insecticide being used, but it could take weeks for federal and state officials to figure that out. He said there also could be breeding sites that haven't been destroyed.
He said mosquito control is difficult in the neighborhood because it has industrial, commercial and residential development. The mosquito that carries the virus is generally difficult to eradicate.
In an alert issued Monday, the Centers for Disease Control and Prevention urged expectant mothers who frequent the Wynwood area to get tested for Zika. Officials also said men and women who have recently visited the Wynwood arts district should wait eight weeks before trying to conceive a child.
The CDC also issued the same recommendations for anyone who traveled or lived in the area where those have been affected since June 15th, the earliest known date that one of the people could have gotten the disease.
On Tuesday, Miami Rescue Mission distributed mosquito repellent to the homeless in Wynwood.
Miami-Dade County mosquito control inspectors went door to door in Wynwood on Tuesday, handing out information, checking tires and other objects for standing water, and dipping cups to take water samples from vacant lots, building sites and backyards. In one lush yard, an inspector tipped over a kiddie pool and a cooler full of water.
Daily aerial spraying for adult mosquitoes and larvae has been approved for the next four weeks over a 10-square-mile area around Wynwood, county officials said.
The city of Miami is running extra street-sweeping routes to remove the litter and stagnant water that can serve as breeding grounds.
Because of environmental regulations governing which chemicals can be used as insecticides, mosquito control authorities cannot easily switch to another compound if bugs prove resistant to it.
Nothing has worked to stop this mosquito elsewhere in the world except for the introduction of mosquitoes modified to pass on genes that kill their offspring, said Michael Doyle, executive director of the Florida Keys Mosquito Control District. And the Food and Drug Administration has not given approval to that approach in the U.S.
"We have to totally rethink mosquito control for Aedes aegypti," Doyle said. "It's like a little ninja. It's always hiding."
The U.S. government might have underestimated how difficult it would be to control Zika's spread, said University of Florida public health researcher Ira Longini. But he also said there aren't enough of the disease-transmitting mosquitoes living in and around houses to cause long-term or widespread outbreaks in this country.
"In defense of the CDC and the government, it's a difficult problem to solve," he said.