A Neglected Aspect of the Epidemiology of Sleeping Sickness: The Propensity of the Tsetse Fly Vector to Enter Houses

Abstract

BackgroundWhen taking a bloodmeal from humans, tsetse flies can transmit the trypanosomes responsible for sleeping sickness, or human African trypanosomiasis. While it is commonly assumed that humans must enter the normal woodland habitat of the tsetse in order to have much chance of contacting the flies, recent studies suggested that important contact can occur due to tsetse entering buildings. Hence, we need to know more about tsetse in buildings, and to understand why, when and how they enter such places.Methodology/Principal FindingsBuildings studied were single storied and comprised a large house with a thatched roof and smaller houses with roofs of metal or asbestos. Each building was unoccupied except for the few minutes of its inspection every two hours, so focusing on the responses of tsetse to the house itself, rather than to humans inside. The composition, and physiological condition of catches of tsetse flies, Glossina morsitans morsitans and G. pallidipes, in the houses and the diurnal and seasonal pattern of catches, were intermediate between these aspects of the catches from artificial refuges and a host-like trap. Several times more tsetse were caught in the large house, as against the smaller structures. Doors and windows seemed about equally effective as entry points. Many of the tsetse in houses were old enough to be potential vectors of sleeping sickness, and some of the flies alighted on the humans that inspected the houses.Conclusion/SignificanceHouses are attractive in themselves. Some of the tsetse attracted seem to be in a host-seeking phase of behavior and others appear to be looking for shelter from high temperatures outside. The risk of contracting sleeping sickness in houses varies according to house design.

Factors Affecting the Propensity of Tsetse Flies to Enter Houses and Attack Humans Inside: Increased Risk of Sleeping Sickness in Warmer Climates

“…1, Detransformed catch) showed that catches peaked in the early part of the hot season, i.e. , in September and October, consistent with the expectation from other work that catches would increase with temperature [4]. However, catches dropped sharply in November, despite high temperatures then, but according with the fact that tsetse densities vary throughout the year, with the greatest decline occurring in the late dry season [12].…”

“…, female G. m. morsitans and both sexes of G. pallidipes , that normally alight very little on humans. This proportion is even higher than the already high proportions of 17–47% (pooled value = 43%, N = 257) found in samples taken throughout the year from humans in buildings in two previous studies at Rekomitjie [3], [4].…”

“…Details of the station, the floor plan of the house and the diurnal variations of temperature in the house, are given in [4]. For present purposes it need be noted only that the house had a net-windowed veranda along the whole of its West side, i.e.…”

“…While it is common to assume tacitly that almost all contact between humans and tsetse occurs when humans enter the woodland habitat of the flies, two recent papers [3], [4] showed that much contact occurs in Zimbabwe when tsetse flies, G. morsitans morsitans and G. pallidipes , approach or enter buildings in large clearings. Moreover, these papers indicated also that a high proportion of the tsetse attacking men inside buildings were females, i.e.…”

“…First attempts to answer the above questions [4] suggested that at all times of year some of the tsetse responding to various types of house did so in a phase of behavior analogous to the response to host-like traps; other flies entered the houses to find a cool refuge from high temperatures during hot weather. This preliminary work was performed with houses that were occupied for only a few minutes every two hours, and so was useful in showing that the houses were themselves attractive, irrespective of a prolonged human presence.…”