An encouraging reduction in the number of infected mosquitoes indicates the North Texas West Nile virus outbreak may have peaked, state and federal experts said Wednesday.
"It does look like there are some positive indicators in some parts of the state that we may have seen and especially in the North Texas area," Texas Health Commissioner Dr. David Lakey said.
In July and August, about 50 percent of mosquito samples tested positive for the disease, but that number is down to 6 percent in Dallas County after aerial mosquito spraying completed last month, he said.
The numbers amount to a 93 percent reduction in infected mosquitoes in areas where insecticide was applied from the air on two consecutive nights.
The experts said the reduction was lower in areas of North Dallas, the Park Cities and Richardson, where weather interrupted spraying for several days, but they could not say yet how much lower.
Dr. Lyle Petersen, of the U.S. Centers for Disease Control and Prevention, said it could take several weeks to provide conclusive results for those areas.
"It's going to take a while to sort out, because many of these areas were sprayed at different times, sometimes using truck-mounted spraying, sometimes not," he said.
The city of Richardson was divided, with areas west of Central Expressway that were not sprayed on consecutive nights, while areas east of it were.
But Richardson Health Director Bill Alsup said the city’s latest mosquito trap samples show no infected inspects anywhere for the first time all summer.
"That was the whole point of the aerial spraying, to break the cycle of the spread of the disease, and it appears, at this point, it was a success in that regard," he said.
Richardson will continue occasional ground spraying to kill mosquitoes, including on Wednesday night in a neighborhood where a new human case of West Nile virus has been reported.
"Just because we've had a reduction in the mosquito population for the time being doesn't mean all the mosquitoes are gone," Alsup said. "It's incumbent on us as public health officials to do what we can to continue to monitor the mosquito population and control it as best we can based on our observation."
Lakey said the number of West Nile virus deaths statewide rose to 40 this week and the number of human cases rose to 1013.
"As of this week, this is the worst year ever for West Nile in the state of Texas, but we are starting to see some encouraging signs," he said.