TRYPANOSOMES, TRYPANOSOMIASIS, AND SLEEPING SICKNESS 21 
Chronicity 
The artificially produced disease in animals may pursue an acute or chronic 
course. With the ordinary strain all the animals show a more or less chronic and 
lengthened infection as compared with Nagana and other pathogenic trypanosome 
infections. It appears more virulent for kittens, pups, and monkeys (Rhesus) than 
for cats, dogs, rabbits, guinea-pigs, rats, and mice ; in some of these animals the 
parasites, after appearing almost constantly for a time, disappear and are then found 
only at rare intervals. Such animals usually show an increase of parasites a short time 
before death. The large animals, goats, sheep, cows, donkeys, and horses, though 
susceptible to the infection, with the exception of goats, appear to be able to withstand 
the disease. It is interesting to note that a horse still shows infection twenty-seven 
months after inoculation. 
Virulence 
In the majority of animals infected with the various strains of T. gambiense the 
disease pursues a normal course, and the animal dies with a fair number of parasites 
in its blood. Occasionally the numbers of parasites increase enormously, and such an 
animal quickly dies. The subinoculations, even with very diluted blood containing 
no more parasites than control blood, may show that it is extremely virulent, the 
incubation and duration of the disease being shortened. Such virulency has been 
noted in baboon 747, rabbit 823, and in rabbits, rats, and mice. Though parasites 
are numerous in guinea-pigs, in no case has an increased virulency of the parasite been 
observed. As noted elsewhere if a strain be repeatedly and quickly run through 
animals of the same species it will acquire a certain virulency for such a species, but 
this continues only so long as the strain is not run through other species of animals. 
Immunity 
Certain species of animals appear to possess a more or less stable resistance to 
infection with trypanosomes. Baboons are peculiarly hard to infect, the immunity is 
not an absolute one as they have been on occasion infected with T. gambiense and 
7. aimorphon. In every case the incubation has been lengthened, the parasites have 
been scanty but the animals have succumbed to the disease. Certain species of 
monkeys, as the Jew and Sooty, are somewhat more difficult to infect than the Rhesus 
and Calothrix. 
All attempts to infect birds and fish with T. gambiense and T. dimorphon have 
failed. Goats' and sheep, though susceptible to infection with various pathogenic 
trypanosomes, do not easily succumb. Some species of animals, ordinarily susceptible 
1. As Bruce suggested it is with such animals that the histo-pathology of the disease ought to be investigated. In our 
report a comparison of the lesions found in animals dying after lengthy period from a chronic type of the Gambian Horse disease 
has b;en made uith those found in sleeping sickness. A comparative study of a large number of goats, etc., infected would be 
of great aid in determining if the various trypanosomes pathogenic for animals produce lesions akin to those found in natives 
'lying from sleeping sickness. 
