Why the Menace of Mosquitoes Will Only Get Worse

The outbreak began so slowly that no one in Dallas perceived it at first. In June 2012, a trickle of people began showing up in emergency rooms broiling with fever, complaining that their necks were stiff and that bright lights hurt their eyes. The numbers were initially small; but by the middle of July, there were more than 50 victims each week, slumping in doctors’ offices or carried into hospitals comatose or paralyzed from inflammation in their brains. In early August, after nine people died, Dallas County declared a state of emergency: It was caught in an epidemic of what turned out to be West Nile virus, the worst ever experienced by a city in the United States. By the end of the year, 1,162 people had tested positive for the mosquito-borne virus; 216 had become sick enough to be hospitalized; and 19 were dead.

West Nile was not new to the United States. It had been a minor summer threat since August 1999, when it made 17 people sick in New York City. That was the virus’s first entry into the country, and it expanded through it thereafter. It landed in Dallas in 2002, sickening 202 people and killing 13. When it moved on toward the West Coast, epidemiologists in the city thought West Nile would no longer be a threat. And events seemed to prove them right: Each year, there were just a handful of cases. In 2011, the year before the epidemic, there was only one.

“We all thought these things come as a flash in the pan: one big outbreak and then you don’t see them again,” Dr. Robert Haley says. Haley is the director of epidemiology at the University of Texas Southwestern Medical Center in Dallas and a former disease detective at the C.D.C. After the last cases were recorded in the final days of 2012, he and a team of researchers studied the episode. Right away, they could see the geography of the illness: Victims were clustered in affluent ZIP codes where many owners had walked away from over mortgaged mansions. Haley and his team knew that there would be abandoned swimming pools and potted plants there — perfect places for mosquitoes to breed unbothered. But the financial crisis was four years old in 2012. The homes had been neglected for years without triggering an epidemic; no matter how many mosquitoes bred in the summer, the deep cold that blankets central Texas a dozen days every winter would knock the bugs down again.

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