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in number with the virulence of the trypanosome. From the available evidence 
one can only conclude that sometimes 7’. rhodesiense is a more virulent strain of 
trypanosome than 7’. gambiense but is similar to it in other respects. During 
the past year Duke, Kleine, and Lavier, of the League of Nations International 
Commission on Human Trypanosomiasis, have concluded that not only may 
posterior nuclear forms be found in 7’. gambiense, but that the difference between 
the two human parasites is not yet established. 
Many observers have suggested that 7’. rhodesiense is also identical with 7. 
brucei, the common parasite of nagana of horses and cattle and of large game in 
Africa. Indeed, Bruce, Kinghorn and Yorke and several other investigators 
regarded the two as identical. Apparently the great objection that has been 
made to this opinion is that 7’. brucei in wild animals and G. morsitans have a very 
much wider distribution than 7’. rhodesiense in man. Duke has described a 
human epidemic of sleeping sickness which he believed was due to a lack of wild 
game in the vicinity, the tsetse fly being driven to attack man for food. Infection 
was said to be always traced to contact with the previous human case and was 
said to be direct by the fly, no cyclical development of the trypanosome in the gut 
of the fly and transmission to the salivary glands occurring. 
Dye ” has also reported upon the study of a similar local epidemic of trypano- 
somiasis in a village in Tanganyika Territory. His study convinced him that the 
source of the infection was man and that game animals played no part in the 
spread of the disease, the 7'’rypanosoma rhodesiense being conveyed directly from 
man to man by the bite of the fly. Van Hoof * also in an epidemiological inquiry 
involving the examination of more than 45,000 natives, came to the conclusion 
that there was nothing to suggest that there was any other reservoir of the virus 
than man. 
It is important to recall that Taute and Huber were unable to infect 131 men 
with trypanosomes by inoculating them with the blood from four horses and two 
mules, containing 7’. brucez. Nevertheless, it seems not unlikely that the try- 
panosome of human sleeping sickness is probably the same species as that which 
infects certain animals, notably wild game, but that it is a species which has 
become gradually accustomed to a new environment and finally specially adapted 
to life in the blood of man. Man is probably by the natural mode of infection 
immune to the trypanosomes of animals. It is perhaps only in those instances in 
which an individual especially susceptible becomes infected through a large 
number of virulent trypanosomes that the trypanosome becomes adapted to 
life in human blood and then may more frequently infect other human beings. 
The idea is not impossible that a somewhat similar relationship may sometimes 
occur in connection with the malarial parasites of human beings and of monkeys. 
Duke * points out that monkeys are much more susceptible to the pathogenic 
1 Maclean: (Ann. Trop. Med. and Parasit., Dec. 1929, XXIII, 519) has recently observed and 
described posterior nuclear forms in a strain of Trypanosoma brucei. 
2 Dye: Trans. Royal Soc. Trop. Med. and Hyg. (1927), X XI, 187. 
’ Van Hoof: Interim Report, League of Nations Intern’! Commission on Human Trypanosomiasis, 
Geneva (1927), p. 103. 
4 Duke: Parasitology (1928), XX, 427. 
