Sleeping sickness as with other vector-borne diseases is determined by the habits of the vector. In the gambiense type, the tsetse fly breeds in the tunnel of the forest, along the course of rivers (Fig. 15.17). Although powerful flyers, they do not range far from this shaded protection, but travel extensively through this tunnel of forest in search of blood meals. Any mammals, including humans, that come to the river to drink or cross are attacked and fed upon.
Humans are the main reservoir of T. b. gambiense infection (although the domestic pig may be involved) and people whose jobs brings them into contact with the infected fly are more likely to succumb to infection. Since women are involved in the collection of water for domestic use, the preparation of food and the washing of clothes, they are more commonly infected with Gambian sleeping sickness.
The disease can occur in both endemic and epidemic forms. There are well-known foci from which people become infected at a constant rate (Fig. 15.16), but movements of infected flies or more commonly people, into new areas can initiate epidemics. Generally, infected flies are comparatively few in number, so that a large number of bites are required before a person becomes infected. Where the community that is fed upon is small and stable (less than ten persons/ km2), only a few cases will occur. When
the community is much larger (above ten persons/km2), as when an infected person travels to a more densely populated area, the infection can be transmitted to other people, who in turn form a reservoir to infect more flies and an increasing number of cases occur. Epidemic sleeping sickness is more likely in T. b. gambiense infection, as it is a more chronic disease and cases provide a reservoir (to infect flies), before symptoms cause them to seek medical attention. While endemic foci are difficult to eradicate, control measures should prevent epidemics from occurring.
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