DISEASES CAUSED BY PROTOZOA. 121 



some species at least there is development in an inter- 

 mediate host, generally some species of fly. 



There are about 60 species of trypanosomes, many 

 of which are pathogenic for animals, but only 2 are 

 known to* cause disease in man. The Trypanosoma 

 gambiense is the cause of the sleeping sickness, a dis- 

 ease prevalent in equatorial Africa. One of the 

 natural hosts of the parasite is the crocodile, and a 

 species of fly, the Glossura palpales, that feeds on the 

 blood of these animals, transmits the infection to 

 human beings. Trypanosomiasis, or the sleeping sick- 

 ness, is a chronic disorder marked by fever, wasting, 

 and lethargy. The parasites can be found in the blood, 

 but more often in the cerebrospinal fluid. No way of 

 establishing an immunity is known. 



