PARASITISM 



809 



flukes (class Trematoda), both of which live in the blood stream. Para- 

 sites that burrow extensively in body organs include some trematodes, 

 nematodes, and a few fly maggots. 



Trypanosomes. Trypanosomes live in the blood of all kinds of 

 vertebrates and usually are transmitted by blood-sucking arthropods in 

 which a part of the life cycle is passed. Most of them do little harm 

 to their hosts and those that are dangerous are believed to represent 

 instances in which the trypanosomes have invaded new hosts. Such may 

 be the case with African sleeping sickness, a disease of man caused by 

 two species of the genus Trypanosoma (Fig. 39.10). The ancestral 

 species, T. brucei, is common in many African wild mammals where it 

 is harmless. It is virulent in domestic animals such as horses and camels 

 but is unable to attack man. Early in this century in Rhodesia, how- 

 ever, the population of native mammals was greatly reduced and the 

 tsetse flies that carry T. brucei were forced to feed more frequently on 

 humans. In 1909 a case of human sleeping sickness caused by a 

 trypanosome very similar to T. brucei was discovered. Since then there 

 have been numerous instances of human infection by this strain of 

 protozoa called T. rliodesiense although it is probably only a variety 

 of T. brucei. Trypansoma gambiense has had a longer association with 

 man and also is found in monkeys, antelopes, and pigs. It originally 

 was found in central Africa where it produces a serious but not devas- 

 tating disease of man. Late in the nineteenth century, apparently as a 

 result of exploration by whites, the organisms were carried north into 

 Uganda and the lake region where the human population had not 

 previously been exposed to the disease and where tsetse flies were abun- 



y,,,,,,,,,,,,,y,,,,,.y.,y,,,,,y^///^///////////////.Y////////////////^^^^^ 



Fiaure 39 10 A, African sleeping sickness. Active trypanosomes in the blood (B) 

 are sucked up by the tsetse fly (C). The protozoa reproduce nr the d.gestne tract. 

 mTgrate to the^salivary glands where they attach to the walls and finally become infective, 

 (D), passing into a new host during salivary secretion. 



