246 RESEARCH IN PROTOZOOLOGY 



The most available trypanosome for demonstration and study is 

 T. lewisi found in wild rats. It can be readily subinoculated into 

 white rats. The infection lasts as a rule for from thirty to a hun- 

 dred days, after which the animal is immune to reinfection. Talia- 

 ferro and his collaborators (Taliaferro and Taliaferro, 1922; 

 TaHaferro, 1923, 1924; Coventry, 1925) have clearly shown that 

 the lack of pathogenicity of T. lewisi in the rat is due to the 

 elaboration of at least two types of antibodies by the host, one of 

 which destroys the parasites, the other inhibits their reproduction 

 {Cf., Chapter XL). The infection may be transferred from rat to 

 rat by the intraperitoneal injection of trypanosome-containing 

 blood, or, if desired, the transmission may be effected using the 

 natural intermediate host, CeratopJiyllns fasciatus (see Minchin 

 and Thomson, 1915). 



Various strains of trypanosomes pathogenic to rats and other 

 small laboratory animals are carried on in a number of laboratories 

 and are usually available to investigators who care to use them. 

 These strains include species of T. hrucei, T. equiperdum, T. 

 equimmn, T. cruzi, T. rhodesicnse, and T. gambiense. Most of these 

 species are quickly lethal to the rat. In the guinea-pig they cause 

 a discontinuous type of infection where periods of remission 

 alternate with periods of recidivation until, as a rule, the animal 

 succumbs to the infection. In rabbits the course of the infection is 

 similar to that in the guinea-pig except that it may be somewhat 

 more prolonged and not as severe, the animals frequently recover- 

 ing from the infection. 



The species of Crithidia that are most available are the crithidial 

 stage of Trypanosoma nielophagiuin, which is found in the intes- 

 tine of sheep keds, and C. gcrridis from various species of the 

 water strider, Gerris. The crithidial phase of T. melophagium is 

 found in most specimens of the sheep ked, Melophagiis ovinu-s. 

 The parasite may be readily cultivated on Noller's medium 

 (N. N. N. medium plus glucose) from the intestine of the ked or 

 from the blood of infected sheep. In either case, the crithidial 

 phase predominates in the culture. Hoare (1923) has made the 

 most thorough study of the life history of this parasite. 



C. gerridis is found in water-striders throughout the length of 

 the intestine below the anterior half of the stomach (Becker, 

 1923). It has not yet been cultivated, due to the difficulties of 

 separating it from bacteria. 



