314 PARASITES OF THE DOMESTIC ANIMALS 



by a protozoan which, though not identical with it, is allied to the 

 malarial parasite, and, like it, enters and destroys the red blood cells. 

 In this case the infecting organism has been found to be conveyed from 

 animal to animal by a certain species of tick {Margaropus annulatus, 

 p. 144), and it is now known that the presence of the tick is essential to 

 such transmission. 



Trypanosomes were first studied in mammahan blood by Lewis in 

 1877, who observed them in the blood of a rat. Three years later 

 Trypanosoma evansi was studied as the cause of surra, a disease of horses 

 of Asiatic countries, the transmitting agent of which is thought to be a 

 blood sucking fly (Tabanus, p. 332). 



Bruce, in 1894, demonstrated that a trypanosome {Trypanosoma 

 hrucei) was the specific organism causing the fatal nagana or tsetse 

 fly disease of horses and other domestic animals of Africa. He showed 

 conclusively that blood-sucking invertebrates, mainly the tsetse flies 

 (Glossina, p. 44), are responsible for its transmission from the blood of 

 wild immune to the blood of susceptible domesticated animals. 



The relationship of the tsetse fly to human trypanosomiasis was shown 

 in much the same way as that followed in the researches of Bruce. 

 African sleeping sickness of man was originally confined to the West 

 Coast; it has spread eastward and is now a serious menace to the develop- 

 ment of Central Africa. In 1902 the infecting organism of this fatal 

 disease was discovered to be a trypanosome {Trypanosoma gambiense) 

 carried from host to host mainly by a tsetse fly. Students of protozool- 

 ogy have since shown that mosquitoes, lice, and leeches may carry 

 trypanosomes, and that piercing flies, therefore, may not alone be 

 responsible for the spread of the diseases which are caused by these 

 Protozoa. 



The instances above cited will serve to direct attention to the im- 

 portance of the Protozoa from the viewpoint of their pathogenicity both 

 in its economic relation and as regards disease in man. Up to the present 

 time the Protozoa as disease-producing organisms have not received 

 the attention in the United States that has been given them by inves- 

 tigators in Africa and Europe. This is probably due to the fact that, 

 though this country is not free from pathogenic trypanosomes, it has 

 thus far escaped the ravages of the trypanosomiases of Africa, Asia, 

 and South America, to which countries sleeping sickness, kala-azar 

 (leishmaniasis), nagana, surra, and mal de caderas have to the present 

 time confined their plague. A sHght acquaintance with the subject, 

 however, is sufficient to dispel a feeling of security based upon the 

 erroneous impression that these diseases are restricted to tropical 

 countries or that their spread depends upon the presence of a certain 

 kind of fly. It has already been noted that the African trypanosomiases 

 may not depend wholly upon the tsetse flies for their existence and 



