Bug collecting

Each week, Dallas County health workers set out scores of small traps in neighborhoods across the city and its suburbs, looking for infected mosquitoes that could offer an early warning about a possible disease risk.

The small traps are baited with a homemade broth called “stinkwater” that replicates the odors of the fetid swamps preferred by females for laying eggs. A small fan sucks the insects into the trap’s mesh net, which in a week’s time will catch scores or even hundreds of live insects.

At the lab, the mosquitoes are killed and then segregated by species. Then vials of dead mosquitoes are ground up and tested genetically for traces of West Nile virus. If mosquitoes from any neighborhood test positive, county officials return in their white pickup trucks and unleash a fog of pesticides to kill any insects nearby.

The spray trucks were initially controversial; some Dallas residents feared the chemicals more than the disease. But after 19 people died of West Nile in a single season, county officials felt compelled to act. Budgets were adjusted to accommodate dramatically higher levels of insect monitoring, and spray trucks returned to county streets for the first time since the 1960s.

“There is no way you can predict it. You just have to respond to it,” said Zachary Thompson, the director of the Dallas County health department. “We’re in America, where most citizens don’t want to stay in the house on a nice day.”

Local officials believe they are better equipped to detect and prevent a potential outbreak than they were three years ago, but they also acknowledge that the battlefield is changing. Long-term climatic data shows that Dallas, like the rest of the country, is warming, with shorter and generally milder winters. Fewer hard freezes means virus-infected mosquitoes have a greater chance of surviving the winter. And West Nile itself, which passes from birds to mosquitoes and then to humans, can begin to replicate and spread early in the season.