TRANSMISSION OF DISEASES BY INSECTS 



249 



from the fly country and fed to healthy horses failed to convey the 

 disease. 



Then the influence of the fly was tested. Tsetse flies caught in the 

 lowland, carried to the mountain and placed at once on healthy animals 

 gave rise to the disease ; but the flies never retained the power of infecting 

 a healthy animal for more than forty-eight hours after feeding "upon a 

 sick animal. Thus wild flies, kept without food for three days and then 

 fed on a healthy dog, never gave rise to the disease. The fly alone trans- 

 mitted the disease; and this by means of trypanosomes adhering to the 

 proboscis either inside or out. Bruce found these organisms in the diges- 

 tive tract also, but with no change in their form. 



He discovered further that buffaloes, antelopes and many other wild 

 animals carried the parasite in their blood, and was 

 able by injecting this blood to transmit the disease 

 to healthy domesticated animals. The parasites 

 were never numerous in the blood of their wild hosts, 

 however, and the latter seemed to be unaffected by 

 their presence. The "big game" of Africa serves, 

 generally speaking, as a reservoir for supplies of 

 trypanosomes. 



The species of parasite that Bruce studied is 

 named Trypanosoma brucei (Fig. 273). The flies 

 concerned are Glossina morsitans, G. pallidipes and 

 G. fusca, particularly the first two, the distribution 

 of which coincides with that of nagana. 



No certain remedies for the disease are yet 

 known. Human serum injected into infected ani- 

 mals causes the trypanosomes to disappear, at least 

 temporarily; but this fact is of more scientific in- 

 terest than practical importance. The precaution of traveling by night 

 is often adopted. Creolin and some other substances' rubbed on animals 

 serve to repel the flies, and the smoke of encampments drives them away. 

 The protection of horses by means of screens is of course effective. 



Human Trypanosomiasis. Sleeping sickness is most prevalent in 

 the Congo basin, whence it has spread rapidly in equatorial Africa, where 

 it kills about fifty thousand natives every year. The reported cases of 

 recovery are so extremely rare that the mortality is placed at one hundred 

 per cent. 



In the first stage of the disease, marked by the appearance of trypano- 

 somes in the blood, negroes show no symptoms as a rule, though whites 



FIG. 273. Trypano- 

 soma brucei. Greatly 

 magnified. 



