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 territories impassable for horses or cattle (fig. 46B). 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 trypanosome 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. 46D), 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 Dutton discovered a trypanosome in the blood of a West African patient; and a few other cases were noted. This trypanosome of human blood was called by Dutton _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 seriously attacked by sleeping sickness, as is that of Uganda, as many as 28 per cent. of the people have trypanosomes in their blood. There is no ground at present known for distinguishing Dutton’s _T. Gambiense_ of human blood from that which Bruce has found to be so terribly abundant in Uganda, and to be the cause of sleeping sickness. Indeed all the trypanosomes of the blood of the larger mammalia are singularly alike in appearance; and the figure which is here given (fig. 50) of the trypanosome of sleeping sickness (_T. Gambiense_) might quite well serve to represent the _T. Evansii_ of surra disease, the _T. Brucei_ of nagana disease, or the _T. equinum_ of the South American _mal de caderas_. [Illustration: FIG. 50. _Trypanosome Gambiense_, from the blood of men suffering from the early symptoms of sleeping sickness. A, after Bruce and Navarro; B, after Castellani. They show a large oval nucleus (drawn as a black mass), and a small black “micronucleus,” or “blepharoplast” in front.]