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Bait and Predict: Bracing for Dengue in Brazil

By Patrick McGuire

Every year since 1996, a serious epidemic of dengue fever has struck Belo Horizonte, a city of 2.2 million and the third largest in Brazil. Without a dengue vaccine, control of the disease there—as in most of Brazil—is all about mosquito control.

But from year to year, authorities are never sure which dengue serotype will be borne by the annual attack of mosquitoes, when it might come and in what intensity. Thus they are forced into a deadly game of catch-up, applying their control strategies mostly after the fact.

Two MMI scientists, however, are heading an early warning project in Belo Horizonte that may give citizens a jump on the type of disease coming, the severity and, to some extent, a time frame for when it may strike.

With a $2 million Gates Foundation grant, associate professor Douglas Norris and professor Gregory Glass, both in Molecular Microbiology and Immunology, are using a relatively new “olfactory attractant” mosquito trap in Brazil. Baited with a proprietary scent known to attract the Aedes aegypti mosquito that carries the dengue pathogen, the 60 strategically placed traps are expected to snare thousands of mosquitoes.

The captured insects will be sorted and identified by a corps of Brazilian collaborators, then “whisked back to the lab.” There they will be analyzed to see if they are carrying dengue and, if so, which serotype. “It’s a matter of warning people that conditions may be building for an epidemic,” says Norris, the project’s principal investigator.

Knowing ahead of time which serotype infects the annual mosquito swarm may give investigators an idea of the severity of the coming epidemic season. “If the mosquitoes we find are carrying a serotype that circulated last year,” says Norris, “we know that it may not build up to epidemic proportions, because the population may have built up immunity.”

But if the new crop of mosquitoes is carrying a different serotype, it could not only threaten the same people who were infected in the previous year, it could turn their illness into the most severe form of dengue hemorrhagic fever, which can cause bleeding, shock and death.

“Essentially,” says Norris, “the idea is to get ahead of the typical epidemic season, to identify infected mosquitoes early enough to provide local authorities with the time necessary to implement existing mosquito control strategies to reduce the ‘infected’ vector/mosquito population.”

The traps may collect 20 to 30 different species of insects a day, says Norris, most likely attracted by the scent of the bait and the carbon dioxide the trap is also baited with. Many bloodfeeding arthropods are attracted to carbon dioxide [exhaled from hosts], so we might expect many other mosquitoes and biting flies in the traps.

”The idea for using the trap and its attractant came from Jorge Arias, PhD, of the Fairfax County, Virginia Health Department. Arias has worked with mosquito control related to the West Nile virus and is active in PAHO. Norris, Glass and Arias are working with Paulo Pimenta, PhD, from the Centro de Pesquisas Rene Rachou, in Belo Horizonte.