PROTOZOA AND DISEASE 209 



also illustrates the relationship of insects to disease-producing 

 organisms. The following are representative cases affecting man: 

 yellow fever, the germ of which, like that of malaria, is carried 

 by a mosquito; typhus fever and trench fever, transmitted 

 through the body louse; Rocky Mountain spotted fever, trans- 

 mitted by a wood tick; the Japanese flood fever, transmitted by a 

 small mite ; elephantiasis, in which the parasite is one of the thread 

 worms, transmitted by mosquitos; relapsing fever in man, and 

 Texas fever in cattle, transmitted by ticks; bubonic plague, by 

 fleas. The list might be further extended. It is for this 

 reason that the insects have assumed great importance to 

 medicine, particularly in tropical countries, since the stages of the 

 malaria parasite, the first life cycle of this nature to be discovered, 

 were ascertained in 1898. 



African Sleeping Sickness and Trypaiiosomes. — Another exam- 

 ple of an insect-borne disease is the sleeping sickness, occurring in 

 equatorial Africa and caused by one of the flagellated protozoa 

 called trypanosojnes (Fig. 109). By the bite of the blood-sucking 

 tsetse fly, the stages of the parasites found in human blood are 

 transferred to the intestine of the insect, which serves as an 

 intermediate host. Here a scries of stages occur, until some 

 three to four weeks later the parasites appear in the salivary 

 glands of the tsetse fly, from which they may be again transferred 

 to the blood of man or some other mammal. In the final stages of 

 the cycle, they invade the cerebro-spinal fluid, causing the sleep 

 that characterizes this disease and finally ends in death. It is a 

 curious fact that while they cause a fatal disease in human beings, 

 they apparently produce no very serious effects when they invade 

 the blood of some of the larger African mammals. Such a condi- 

 tion can perhaps be explained on the theory that the mammals 

 in question have acquired an immunity through being subjected 

 to the infection for many generations, whereas man has perhaps 

 but recently come into contact with these parasites. 



Protozoa, as well as bacteria, have thus become recognized as 

 the germs of certain diseases, although the majority are harmless or 

 even beneficial to man. A science of Protozoology, comparable 

 with Bacteriology, has arisen in the past twenty-five years. Med- 

 ical schools in which tropical diseases demand special considera- 

 tion have established professorships in this subject. It seems 

 unlikely that the protozoa will ever assume the importance of the 



