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Fighting West Nile

November-December 2001

Behind Harvard Stadium, in a dark facilities-maintenance shed, Paul Reiter is stirring a witch's brew he uses to attract female mosquitoes. He prepares the infusion according to a simple recipe. Add hay, with perhaps a touch of fecal matter, to a 55-gallon, water-filled barrel fitted with a spigot on the bottom. Let the hay steep in the water for seven days, stirring occasionally. Keep the infusion well covered until ready to use. Reiter, a scientist with the dengue branch of the Centers for Disease Control and Prevention(CDC) in San Juan, Puerto Rico, has been dispatched to Harvard for a special reason. West Nile virus, which appeared in New York City two years ago, is "spreading quite rapidly throughout the whole country," he says. By last year, the virus had extended its range from New Hampshire to North Carolina; this past summer, it was found from Ontario to the Florida Keys and as far west as Indiana. The CDC wants Reiter to evaluate the efficacy of a variety of human interventions in controlling the main vector, Culex pipiens, the common house mosquito. Because of residential opposition to spraying with insecticide, Cambridge is a good place to do research. Reiter can evaluate the kinds of interventions that the city does allow, and in the event of an outbreak, Cambridge will provide Reiter with a baseline for comparison to other communities that do spray.

"There are a number of well-known measures that are used," says Reiter, "and they've been extensively tested in the laboratory for efficacy against mosquitoes and for safety with respect to the public, but using them in the field presents a lot of problems." For example, how can researchers monitor the mosquito population, and assess the impact of these various control strategies? That, says Reiter, "requires knowing not only some of the basics of insecticide-insect interaction, but also something about the private life of the mosquito": when is it active, where is it concentrated, how does weather affect its activity? How is this related to its behavior of feeding on birds versus feeding on people?

Culex pipiens, though it lives in houses, feeds primarily on birds, which act as amplifying hosts in which the West Nile virus multiplies rapidly. Only rarely does Culex bite humans. That's a problem, because, as Reiter puts it, "the dangerous mosquito is the one that doesn't bite people most of the time, but focuses its biting on a good reservoir host, like a bird, and only later bites a person."

Reiter uses special traps of his own invention to attract and catch mosquitoes. He drains a week-old brew of his hay infusion into black plastic basins and sets them out at various locations in the Boston area. During the night, when Culex mosquitoes are most active, gravid females will find the basins and, flying low above them, circle in a figure eight pattern for some time before deciding whether to lay eggs, which are deposited in canoe-shaped rafts. Reiter observes this nocturnal activity in his own backyard with the aid of an infrared camera. Suspended above each of the other brew-basins is a modified tackle box: a hole in the side, fitted with a battery-powered exhaust fan, creates a slight vacuum, causing gentle suction through a circular opening in the bottom of the box. A rubber collar hangs down from the bottom hole to within a few inches of the water's surface. Inevitably, the mosquitoes are drawn in and trapped for study by Reiter and others. Back in his laboratory two doors down from Andrew Spielman's office at the School of Public Health, Reiter identifies and dissects the mosquitoes; other researchers crush and test liquefied mosquitoes for virus. Reiter's work, says Spielman, "makes us the center of the U.S. effort to develop anti-West Nile intervention methodologies."