Lutzomyia longipalpis (Lutz & Neiva), the vector of American visceral leishmaniasis (AVL), is much more abundant in animal sheds than in houses on Marajó Island, Pará State, Brazil. This difference in abundance is known not to reflect host preference. We show here that it also cannot be explained in terms of variable trapping efficiency, or insecticide application, and we exclude animal sheds as important daytime resting sites. In experimental sheds, the number of L. longipalpis increased markedly with the openness of the walls, though artificially large aggregations of flies could be generated in closed houses by using caged flies and hosts as attractants. We conclude that L. longipalpis tend to congregate at sites outdoors, including animal sheds, because these are the places where leks can most easily form on abundant, stationary (sleeping) and accessible hosts. These results help to explain why the seroprevalence of Leishmania chagasi infection is generally much higher among dogs than humans. They also indicate that human exposure to sandfly bites varies with the quality of house construction.
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