TRANSMISSION OF DISEASES BY INSECTS 265 



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

 whites are subject to fever. The symptoms may appear as early as 

 four weeks after infection or as late as seven years. 



In the second stage trypanosomes appear in the cerebro-spinal fluid 

 and in large numbers in the lymphatic glands, those of the neck, axillae 

 and groins becoming enlarged. There is tremor of the tongue and 

 hands, drowsiness, emaciation and mental degeneration. The drowsi- 

 ness passes into periods of lethargy which become gradually stronger 

 until the patient becomes comatose and dies. Some victims do not 

 sleep excessively, but are lethargic, and "profoundly indifferent to all 

 going on around them." 



There is some disagreement among authors as to the precise effects 

 of trypanosomes on human tissues and organs, but the evidence indi- 

 cates at least that trypanosomes produce a toxin which sets up irrita- 

 tions of the lymphatic glands in general and those of the brain in 

 particular. Many of the symptoms of trypanosomiasis are traceable 

 primarily to inflammation of the lymphatics of the nervous system. 



The specific cause of sleeping sickness is T. gambiense, discovered in 

 1901 by Forde and named by Button. Two eminent English investi- 

 gators of sleeping sickness, Button and Tullock, sacrificed their lives to 

 the disease they were studying. 



As the result of the labors of many investigators, human trypano- 

 somiasis is now well understood. Bruce and Nabarro demonstrated by 

 means of inoculation experiments with monkeys that T. gambiense is 

 transmitted chiefly, if not solely, by a tsetse fly, Glossina palpalis. 

 They and Greig showed that the distribution of the disease in Uganda 

 coincided with that of the fly. In some regions where the fly is present 

 the disease is unknown; which means simply that cases of the disease 

 have not yet been introduced. 



Notwithstanding the great activity in the study of this disease no 

 good remedy for it has been found. Wise travelers in tropical Africa 

 take every precaution against being bitten by tsetse flies. Much effort 

 is being exerted to check the spread of the disease among the natives in 

 some of the infected regions; chiefly by removing patients from the fly 

 region, by screening dwellings or by building them away from the damp 

 and marshy areas where the flies breed. 



FILARIASIS 



The first disease found to be transmitted by an insect was filariasis, 

 the subject of important investigations by Manson, Bancroft and others. 





