284 BACTERIOLOGY FOR NURSES 



in a hanging drop preparation of freshly drawn blood. If tryp- 

 anosomes are present attention is soon attracted to their neigh- 

 borhood by a disturbance amongst the red blood corpuscles, and 

 soon the rapidly undulating organisms come into view. Blood 

 films may also be made and colored with Romanowsky stain. 



With the exception of dourine, a disease occurring amongst 

 horses, the trypanosomes are transmitted from one animal to an- 

 other through the agency of blood-sucking insects or leeches. The 

 organism undoubtedly passes through a definite cycle of develop- 

 ment within its invertebrate host, the details of which, however, 

 are not yet known. 



T. Lewisi. The first of the species to be more or less fully 

 described was T. lewisi, a comparatively non-virulent form seen 

 in the blood of the rat as early as 1845, but more fully studied and 

 described by Lewis in 1879. Since then it has been noted by ob- 

 servers in various parts of the world. 



The organism is peculiar to the rat and of no importance so 

 far as human infection is concerned, but in that animal it produces 

 a malady which runs a definite course although it is rarely fatal. 



When a rat is experimentally infected by injecting infective 

 material into the peritoneal cavity the organisms soon appear 

 in the blood, and So rapid is their multiplication there that within 

 a few days they seem to be almost as numerous as the red blood 

 cells. The infection continues for about two months, during 

 which time the animal may show no symptoms of disease. At the 

 end of that period the parasites gradually disappear and the rat 

 is immune against further infection. The serum of such a rat 

 has highly protective properties and a marked agglutinative capac- 

 ity, causing the trypanosomes to gather together in a rosette for- 

 mation in which the flagella are directed outward. 



The rat flea is responsible for the transmission of the parasite. 

 It becomes infective about a week after biting a diseased animal 

 and remains so for the rest of its life, passing the trypanosomes 

 in its dejecta. The organism may enter through the puncture 

 made by the flea in the act of sucking or may be swallowed by 

 the rat when licking its fur. 



