386 TRYPANOSOMA THEILERI [CH. XXIII 



an infection. Out of four experiments made in this way, two 

 were successful. It must be stated here that the experimental 

 animals were kept together with control animals in a stable, to 

 exclude spontaneous infection, and that none of the control 

 animals shewed a spontaneous infection. The incubation periods 

 coincided typically with the period which is observed after 

 artificial infection with small quantities of virus " (Theiler). 



Two specimens of the Hippohosca used in these experiments 

 were identified by Speiser as Hippohosca rufipes v. Offers, and 

 H. maculata Leach, respectively. As the latter is excessively 

 rare in South Africa, it is probable that H. rufipes is the usual 

 carrier of T. theileri. 



The parasite may also be experimentally transmitted to 

 cattle by the injection of small quantities of infected blood. 

 Under these circumstances the incubation period depends, to a 

 large extent, on the number of trypanosomes introduced ; it 

 averages between four and six days, but in one case a period of 

 i8 days was noticed. 



The trypanosomes are only present in the peripheral 

 circulation for a comparatively short time and then disappear, 

 leaving the animal immune against any further infection. 

 The longest period in which the presence of the parasite was 

 observed in the blood was 13 days, the average nine days, and 

 the shortest period, one day. While present the numbers of 

 the parasites may rise to 30 trypanosomes in each microscopic 

 field, but the average is about five per field. The disappearance 

 of the parasites is only apparent, however, for the blood of a 

 cow has been shewn to be still infective 11 months after the 

 animal was inoculated. 



REFERENCES. 

 Laveran, A. (1902). Compt. Rend. Acad. Sci. vol. cxxxiv. p. 512. 

 Theiler, A. (1903). ■ Journ. Com- Path, and Therap. vol. xvt. p. 193. 



