46 PARASITES OF THE DOMESTIC ANIMALS 



trypanosome in their blood. The parasites were never numerous, so 

 that it was only after a long search that they could be discovered by the 

 microscope alone. The wild animals did not seem to be affected by the 

 trypanosomes in any way; they showed no signs or sjanptoms of the 

 disease, and it, therefore, appeared probable that the tiypanosomes lived 

 in their blood as harmless guests, just as the trypanosome of the rat lives 

 in the blood of that animal." 



As Trypanosomu brucei is now known to be the organism causing the 

 fatal nagana of horses and mules of Africa, so T. gamhiense is known to 

 be the cause of sleeping sickness of man. The relationship of the tsetse 

 fly to human trypanosomiasis was shown in a way very similar to that 

 by which Bruce reached his conclusions. While the tsetse species 

 Glossina morsitans and G. longipalpis are especially concerned in the 

 transmission of nagana, and G. palpalis likewise related to sleeping sick- 

 ness, it has been shown by students in the field of protozoology that not 

 only biting flies, but mosquitoes, lice, and leeches may carry trypano- 

 somes from one vertebrate host to another. 



Experiment has shown that the trypanosomes adhering to the pro- 

 boscis of the biting fly after it has fed upon the blood of an infected 

 animal rapidly lose their vitality, becoming sufficiently attenuated 

 within fortj'-eight hours to be noninfective. The fl}', therefore, can 

 only inoculate mechanicalh^ that is by the puncture of its soiled pro- 

 boscis," within a few hours after it has become a carrier of the infecting 

 organism. It is now known, however, that trypanosomes taken into 

 the stomach of the fly with its meal of blood pass through a metamor- 

 phosis involving sexual forms, and that at the end of about twentj'-eight 

 days the fly may again become infective. At this time the parasites 

 have reached the salivary glands and here they remain during the re- 

 mainder of the life of the fly. How long such a fly may retain its power 

 to infect is yet a question, though it has been found by the Sleeping 

 Sickness Commission to be at least three months. The duration of the 

 life of the tsetse fly has only been observed upon specimens in captivit}^, 

 but it is probable that it is about four to six months. 



Control. — Measures looking to the control of the breeding of the 

 flies are limited practically to exclusion owing to the fact that the larval 

 period is passed within the body of the female, hence offers no opportu- 

 nity for attack through sources of larval food supply. The fact that 

 tsetse flies seek the vicinity of water courses surrounded by wooded 

 areas may be taken advantage of in excluding them from locations of 

 settlement. With a view to this it has been recommended that clearings 

 be made over an area of six hundred to eight hundred yards at some 

 distance from streams of water, the water supph' l^eing obtained from 

 wells. The difficulties presented, however, in the control of the fly are 

 numerous and in many features seem unsurmountable. The ultimate 



