Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

Tsetse-flies of the species Glossina morsitans, which fed upon diseased animals, were found capable of giving rise to the disease in healthy animals up to forty-eight hours after feeding. Wild tsetse-flies taken from an infected region to a region where they did not normally occur were able to transmit the disease to healthy animals. It was found that many of the wild animals in the tsetse-fly regions harbored Trypanosoma brucei in their blood, though they showed no evidence of disease. As in the case of natives of malarial districts, these animals acted as reservoirs of the parasite. Non-immune animals subjected to the attacks of the insect carrier, quickly succumbed to the disease.

A question of prime importance is as to whether the insect serves as an essential host of the pathogenic protozoan or whether it is a mere mechanical carrier. Bruce inclined to the latter view. He was unable to find living trypanosomes in the intestines or excrements of the fly or to produce the disease on the many occasions when he injected the excrement into healthy animals. Moreover, he had found that the experimental flies were infective only during the first forty-eight hours and that if wild flies were taken from the infected region, "kept without food for three days and then fed on a healthy dog, they never gave rise to the disease."

Koch had early described what he regarded as sexual forms from the intestine of the fly but it remained for Kleine (1909) to experimentally demonstrate that a part of the life cycle of the parasite was undergone in the fly. Working with Glossina palpalis, he found that for a period of ten days or longer after feeding on an animal suffering from nagana it was non-infective, but that then it became infective and was able to transmit the disease for weeks thereafter. He discovered and described developmental stages of the parasite within the intestine of the insect. In other words, the tsetse-fly (in nature, Glossina morsitans), serves as an essential host, within which an important part of the life cycle of the parasite is undergone. These conclusions were quickly verified by Bruce and numerous other workers and are no longer open to question. Klein and Taute are even inclined to think that mechanical transmission plays practically no rôle in nature, unless the fly is interrupted while feeding and passes immediately to a new animal.

Tsetse-flies and Sleeping Sickness of Man—About the beginning of the present century a hitherto little known disease of man began to attract great attention on account of its ravages in Uganda and the region of Victoria Nyanza in South Africa. It was slow, insiduous and absolutely fatal, characterized in its later stages by dullness, apathy, and finally absolute lethargy all day long, symptoms which gave it the name of "sleeping sickness."

It was soon found that the disease was not a new one but that it had been known for over a hundred years on the west coast of Africa. Its introduction into Central and East Africa and its rapid spread have been attributed primarily to the development of the country, the formation of new trade routes and the free mingling of native tribes formerly isolated. It is estimated that in the first ten years of the present century there were approximately two hundred thousand deaths from the disease in the Uganda protectorate. In the British province Bugosa, on the Victoria Nyanza there were thirty thousand deaths in the period from 1902-1905.

While the disease is peculiarly African there are a number of instances of its accidental introduction into temperate regions. Slaves suffering from it were occasionally brought to America in the early part of the last century and cases have sometimes been imported into England. In none of the cases did the disease gain a foothold or spread at all to other individuals.

In 1902 Dutton described a trypanosome, T. gambiense, which he and Forde had found the year before in the blood of a patient suffering from a peculiar type of fever in Gambia. In 1902-1903 Castellani found the same parasite in the cerebro-spinal fluid of sleeping-sickness patients and definitely reported it as the causative organism of the disease. His work soon found abundant confirmation, and it was discovered that the sleeping sickness was but the ultimate phase of the fever discovered by Dutton and Forde.

When Castellani made known his discovery of the trypanosome of sleeping sickness, Brumpt, in France, and Sambon, in England, independently advanced the theory that the disease was transmitted by the tsetse-fly, Glossina palpalis. This theory was based upon the geographical distribution and epidemiology of the disease. Since then it has been abundantly verified by experimental evidence.

Fortunately for the elucidation of problems relating to the methods of transfer of sleeping sickness, Trypanosoma gambiense is pathogenic for many species of animals. In monkeys it produces symptoms very similar to those caused in man. Bruce early showed that Glossina palpalis "fed on healthy monkeys eight, twelve, twenty-four and forty-eight hours after having fed on a native suffering from trypanosomiasis, invariably transmitted the disease. After three days the flies failed to transmit it." In his summary in Osler's Modern Medicine, he continues "But this is not the only proof that these flies can carry the infective agent. On the lake shore there was a large native population among whom we had found about one-third to be harboring trypanosomes in their blood. The tsetse-flies caught on this lake shore, brought to the laboratory in cages, and placed straightway on healthy monkeys, gave them the disease in every instance, and furnished a startling proof of the danger of loitering along the lake shore among those infected flies."

As in the case of nagana, Bruce and most of the earlier investigators supposed the transmission of the sleeping sickness trypanosome by Glossina palpalis to be purely mechanical. The work of Kleine (1909) clearly showed that for Trypanosoma gambiense as well as for Trypanosoma brucei the fly served as an essential host. Indeed, Kleine and many subsequent investigators are inclined to think that there is practically no mechanical transmission of trypanosomes from animal to animal by Glossina in nature, and that the many successful experiments of the earlier investigators were due to the fact that they used wild flies which already harbored the transformed parasite rather than directly inoculated it from the blood of the diseased experimental animals. While the criticism is applicable to some of the work, this extreme view is not fully justified by the evidence at hand.