27 o 
animal, hard work, lack of food and concomitant disease sometimes 
seem to lead to a great increase in the number of trypanosomes 
present and (apparently) in their pathogenic effect. 
( 'b ) Laboratory work has also shown that the virulence of trypano¬ 
somes propagated by direct inoculation from animal to animal may 
vary very greatly indeed. 
1 homas and Breinl (7, pages 14 and 21) obtained a very virulent strain of 
Trypanosoma gambiense. The animal reactions of the common strain of Tryfm »• 
soma brucei are no longer the same as those originally described by Bruce (10, page 
140). l'he strain of Trypanosoma dimorphon employed by Thomas and Breinl 
finally became much more virulent for laboratory animals than it was at first. 
1 he strains of trypanosomes obtained from rats in whom Trypanosoma iruai 
had recurred after treatment by atoxyl were repeatedly found to be more virulent 
than the control strains (page 276). 
Whether natural variations in virulence are of the same type as in these 
laboratory examples is unknown. An observation bearing on this point is that 
there was no difference between the reactions either at the first or succeeding 
passages of animals infected with Trypanosoma gambiense in the Gambia, where 
sleeping sickness is sporadic, and those infected at Leopoldville or in Uganda, 
where that disease is epidemic (n, 12). 
XI. CONCLUSIONS 
A. C attle trypanosomiasis is very widely distributed in the Congo 
Free State. 
B. Trypanosoma dimorphon is probably the usual infecting 
parasite. 
C. All cattle transport should be interrupted or severely controlled. 
D. The posts for cattle raising must be carefully chosen and 
maintained. 
E. Domestic animals probably acquire a relative immunity to some 
strains of trypanosomes, and may even recover spontaneously (see 
Section IX). 
