THE SLEEPING SICKNESS 175 



A little later, namely, in the year 1895, came Bruce's 

 discovery of a trypanosome associated with a tsetze fly 

 in the production of the terrible nagana disease of the 

 "fly-belts" of South Africa, which renders whole terri- 

 tories impassable for horses or cattle (fig. 466). The 

 remarkable and important observation was made by Bruce 

 that this trypanosome (known as T. Brucei) inhabits 

 the blood of big game without injuring them, just as 

 the rat's trypanosome inhabits the rat's blood without 

 producing disease ; and that it is only when the try- 

 panosome is carried from these natural wild "hosts" to 

 domesticated animals introduced by man, such as horses 

 asses, cattle, and dogs, that disease results. The wild 

 animals are " immune " to Bruce's trypanosome ; the 

 introduced animals are poisoned by the products of its 

 growth and fissile multiplication in their blood. 



Since Bruce's researches on nagana, a trypanosome, 

 T. equinum (fig. 460), has been discovered in the horse- 

 ranches of South America, where it causes deadly disease, 

 the mal de caderas, among the collected horses ; and a 

 curiously large-sized trypanosome has been found by 

 Theiler in the blood of cattle in the Transvaal. Down 

 to a recent date no trypanosome had been found in 

 the blood of man ; and indeed it is almost certain that 

 none of the kinds hitherto mentioned can survive in his 

 blood. But in 1902 Button discovered a trypanosome 

 in the blood of a West African patient ; and a few 

 | other cases were noted. This trypanosome of human 

 I blood was called by Button T. Gambiense. It was not 

 found to be connected with any serious symptoms, a 

 little fever being the only disturbance noted. It now, 

 however, appears that this trypanosome in the blood 

 is the preliminary stage of the infection which ends in 

 sleeping sickness ; and, as we have seen, in a population 



