MIAMI (AP) — The mosquitoes spreading Zika in Miami are proving more difficult to eradicate than expected, the nation’s top disease-fighter said as authorities sprayed the ground-zero neighborhood, tipped over kiddie pools and handed out cans of insect repellent to the homeless.

Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said Monday that despite aggressive spraying, the mosquitoes are still present in moderately high numbers, suggesting they may be resistant to the insecticide or are managing to hide in crevices and tiny pools of water in the neighborhood.

“In Miami, aggressive mosquito control measures don’t seem to be working as well as we would have liked,” Frieden said.

Mosquito control experts said that’s no surprise to them, describing the Aedes aegypti mosquito as a “little ninja” known for sneaking up on people’s ankles and capable of breeding in a just a bottle cap of standing water.

Fourteen people are believed to have become infected with Zika from bites in Miami’s Wynwood arts district – the first mosquito transmitted cases on record in mainland U.S., which has been girding for months against the epidemic coursing through Latin America and the Caribbean.

On Monday, the CDC instructed pregnant women to avoid the neighborhood, marking what is believed to be the first time in the agency’s 70-year history that it warned people not to travel somewhere in the U.S. The Zika virus can cause severe brain-related defects, including disastrously small heads.

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.