44 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
Animal Experiments 
We have infected rats, mice, guinea-pigs, rabbits, and monkeys with human 
trypanosomes taken from cases of Congo Sleeping Sickness, both in its early and its 
latest phases. As was stated in our first Progress Report the course of the infection 
in these animals has given us no reason to suppose that we are dealing with more 
than one species of trypanosomes, or that the parasite is other than Trypanosoma 
gambiense. 
All our inoculations have been made with small or medium doses (i to 5 c.cm.) 
of infective material, either cerebro-spinal fluid or blood — diluted or no — in which, 
in almost every case, living trypanosomes, sometimes in enormous numbers, were 
demonstrated. About 50 per cent, of such inoculations have failed to infect. These 
failures seem to bear no relation either to the source of the material inoculated, to 
the site of inoculation — subcutaneous or intraperitoneal — or to the approximate 
number of parasites injected. 
Once more, no animals inoculated with material in which trypanosomes were 
seen, taken post-mortem from cases of trypanosomiasis, have ever become infected. 
Parasites have been seen in the peripheral blood of infected animals only at more or 
less irregular intervals. Guinea-pigs have, perhaps, shown themselves the most 
satisfactory of ordinary laboratory animals, since, as a rule, when once infected, para- 
sites are constantly present in large numbers. 
Death, in the small number of our infected animals which have died, can in no 
instance be certainly said to have been due to the trypanosome alone. These are all 
points in which the Congo trypanosome resembles Trypanosoma gambiense. In 
addition, the ' incubation period,' that is, the period intervening between the 
inoculation of small or medium doses of infective material and the detection of the 
parasite in the peripheral blood of the experimental animal, is much the same for 
both parasites. 
Congo 
trypanosome 
Trypanosoma 
gambiense 
Rats 
5—20 
7 20 
Mice 
5-16 
6-7 
Rabbits 
I 7 27 
2 1 
Guinea-pigs 
II—25 
1 2 — 2 1 
Monkeys of two different varieties, both belonging to the sub-family Cercopithecus, 
have been infected. Only one has shown slight symptoms of ill-health. On two 
