death in from three to twelve months. Death usually is due to the disorganization of the nervous system, but in chronic cases it may follow from weakness and emaciation, or, in acute cases, it may result from the blocking of the capillaries by the parasites.
The full life-history of the organism of sleeping sickness is not yet known. In all blood-dwelling protozoa, however, the infection is carried from individual to individual by some intermediate host, usually a blood-sucking invertebrate. Trypanosoma gambiense is thus transmitted from man to man by the tsetse fly, Glossina palpalis; Trypanosoma brucei, the 'tsetse fly disease' of horses, by Glossina morsitans; Trypanosoma lewisi is carried from rat to rat by the louse, Hæmatopinus; T. ziemanni from owl to owl by the mosquito, Culex pipiens. Nor are the intermediate hosts limited to the insects. Trypanoplasma borreli, for example, is transmitted from carp to carp by the fish leech, Piscicola geometra.
The history of the trypanosomes in the digestive tracts of these various carriers has been made out in several cases, although by no means in all. In the gut of the leech, of the louse, Hæmatopinus, and of the mosquito, three different species of Trypanosoma have been worked out by different and competent observers, and in all cases it has been found that this environment is the scene of the most important phases in the life history of the parasite, viz., the conjugation stages, which lead to renewal of vitality. The flagellate parasites thus agree in the main features of their life cycles with the malaria organisms in man and in the insect host Anopheles. Indeed, so widespread is this phenomenon that we are justified in assuming, where actual evidence is not forthcoming, that similar important processes take place in all intermediate hosts, and that we must look for conjugation phases of the parasite of Texas fever (cattle) in the tick (Boophilus bovis), and of sleeping sickness in the fly (Glossina palpalis). On the other hand, we are justified in assuming a protozoon parasite, even though the parasite is not known, in cases where its existence has been proved in an intermediate host. This is the case, for example, in yellow fever, where a definite incubation period of the parasite of about twelve days in the mosquito Stegomyia fasciata is known, and where it is fully established that, apart from this mosquito, no other means of transmission of the disease exists.
Associated with the Trypanosoma diseases, although not yet established, are those curious maladies of India and similar countries, known as dum-dum fever, kala-azar, splenomegaly, etc. The infection may be general or localized, and curious structures, known as the Donovan-Leischman-Wright bodies, have been observed in the spleen of dum-dum fever patients, in the lesions in 'tropical ulcer,' Delhi boil, 'oriental sore' and the like, and these bodies have been interpreted as stages in
