TRYPANOSOMA BRUCEI 543 



multiplies rapidly in the blood till near the end enormous numbers are 

 present, and death may take place quite suddenly, suggesting a blockage 

 of some important part of the circulatory system. It must not be forgotten 

 that the virulence of any particular strain increases with passage through 

 animals. Many laboratory strains have acquired a high virulence for 

 rats, mice, and guinea-pigs after long maintenance in these animals. 

 Some strains will kill mice in two or three days and rats in less than 

 a week. 



Hornby (1919rt), iii Rhodesia, has noted that if a convoy of horses 

 and cattle are taken through fly areas, the general rule is for the horses or 

 other equidse to become infected with T. brucei, while the cattle acquire 

 T. congolense and T. vivax. With few exceptions this rule seems to apply 

 fairly regularly throughout the tsetse fly areas of Africa. It is possible 

 therefore that nagana is a disease caused by several distinct trypanosomes. 



As regards the susceptibility of birds, Mesnil and Martin (1906) showed 

 that T. brucei could survive in geese for as long as three months, as proved 

 by inoculation of blood into guinea-pigs. Durham (1908) had similar results 

 in the case of an inoculated falcon {Cerchneis tinnunculus). Wendelstadt 

 and Felmer (1909) showed that T. brucei could survive in the circulation 

 of snakes and tortoises for about a week. Trypanosomes inoculated into 

 the body cavity of two beetles survived seven and two days respectively, 

 as proved by the inoculation of rats. Similarly in a snail the trypano- 

 somes survived for two days. 



Morphology. — T. brucei, being a polymorphic trypanosome, varies 

 considerably in size (Plate V., b, p. 456). The measurements of a series 

 given by Bruce (1911) are shown in the curve compared with T. evansi 

 (Fig. 196). There occur the same three forms as are found in T. gambiense 

 — namely, the short broad or stumpy form without a flagellum, the long 

 slender one with a fairly long flagellum, and the intermediate form 

 (Fig. 225). The short forms, however, tend to be broader than the corre- 

 sponding ones of T. gambiense, and there is more variation in the position 

 of the nucleus. In a certain number of broad forms, especially in infection 

 in small laboratory animals, the nucleus becomes displaced towards the 

 posterior end (posterior nuclear forms). Sometimes it may actually lie 

 posterior to the kinetoplast. These posterior nuclear forms occur, not 

 only in the undoubted T. brucei, but also in T. pecaudi and other West 

 African strains, a fact which lends support to the view that they are 

 identical. As will be seen below, they also occur in the human strain of 

 T. brucei [T. rhodesiense), and afford a means of distinguishing this try- 

 panosome from T. ga^nbiense. It must be remembered, however, that it is 

 only in the large infections seen in rats, mice, and guinea-pigs that the 

 characteristic posterior nuclear forms are met with to any extent. The 



