4i4 POPULAR SCIENCE MONTHLY 



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 trans- 

 mitted from man to man by the tsetse fly, Glossina palpalis; Trypano- 

 soma brucei, the ' tsetse fly disease' of horses, by Glossina morsitans; 

 Trypanosoma lewisi is carried from rat to rat by the louse, Haima- 

 topinus; T. ziemanni from owl to owl by the mosquito, Culex pipiens. 

 Nor are the intermediate hosts limited to the insects. Trypanoplasma 

 oorreli, 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, Haimatopinus, 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 in- 

 termediate 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 para- 

 site 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 estab- 

 lished, 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 



