Kleine states (1912) that Glossina palpalis can no longer be regarded as the sole transmitter of sleeping sickness. Taute (1911) had shown that under experimental conditions Glossina morsitans was capable of transferring the disease and Kleine calls attention to the fact that in German East Africa, in the district of the Rovuma River, at least a dozen cases of the disease have occurred recently, though only Glossina morsitans exists in the district. It appears, however, that these cases are due to a different parasite, Trypanosoma rhodesiense. This species, found especially in north-east Rhodesia and in Nyassaland, is transferred by Glossina morsitans.
Other workers maintain that the disease may be transmitted by various blood-sucking flies, or even bugs and lice which attack man. Fülleborn and Mayer (1907) have shown by conclusive experiments that Aedes (Stegomyia) calopus may transmit it from one animal to another if the two bites immediately succeed each other.
It is not possible that insects other than the tsetse-flies (and only certain species of these), play an important rôle in the transmission of the disease, else it would be much more wide-spread. Sambon (1908) pointed out that the hypothesis that is spread by Aedes calopus is opposed by the fact that the disease never spread in the Antilles, though frequently imported there by West African slaves. The same observation would apply also to conditions in our own Southern States in the early part of the past century.
Since Glossina palpalis acts as an essential host of the parasite and the chief, if not the only, transmitter, the fight against sleeping sickness, like that against malaria and yellow fever, becomes primarily a problem in economic entomology. The minutest detail of the life-history, biology, and habits of the fly, and of its parasites and other natural enemies becomes of importance in attempts to eradicate the disease. Here we can consider only the general features of the subject.
Glossina palpalis lives in limited areas, where the forest and undergrowth is dense, along the lake shore or river banks. According to Hodges, the natural range from shore is under thirty yards, though the distance to which the flies may follow man greatly exceed this.
It is a day feeder, a fact which may be taken advantage of in avoiding exposure to its attacks. The young are brought forth alive and full-grown, one every nine or ten days. Without feeding, they enter the ground and under favorable conditions, complete their development in a month or more.
Methods of control of the disease must look to the prevention of infection of the flies, and to their avoidance and destruction. Along the first line, much was hoped from temporary segregation of the sick in regions where the fly was not found. On the assumption that the flies acted as carriers only during the first two or three days, it was supposed that even the "fly belts" would become safe within a few days after the sick were removed. The problem was found to be a much more difficult one when it was learned that after a given brief period the fly again became infective and remained so for an indeterminate period. Nevertheless, isolation of the sick is one of the most important measures in preventing the spread of the disease into new districts. Much, too, is being accomplished by moving native villages from the fly belts. (c.f. [fig. 137].)
All measures to avoid the flies should be adopted. This means locating and avoiding the fly belts as far as possible, careful screening of houses, and protection of the body against bites.