TRYPANOSOMES, TRYPANOSOMIASIS, AND SLEEPING SICKNESS 37 
TRYPANOSOMA EVANS I. SURRA 
This strain was also given by Professor Laveran, its origin is from one of the 
infected animals in the epidemic which ravaged the island of Mauritius. 
A guinea-pig showing a severe infection was received, from it a series of animals 
have been inoculated. 
Rats — White, Black, and Grey 
These animals are as susceptible to Surra as to Nagana. The incubation 
after subcutaneous inoculation has been three to four-and-a-half days, after intraperi- 
toneal injection two-and-three-quarters to three-and-a-half days. The average 
duration is about five to seven days after the appearance of the parasites in the 
peripheral blood. 
Mice — White and Grey 
The incubation after subcutaneous inoculation is about three-and- three-quarters 
to four days, after intraperitoneal inoculation about three days, duration six to eight 
days after the infection declares itself. In both rats and mice the parasites usually 
continue to regularly augment, so that as death approaches large numbers of para- 
sites are found in the peripheral blood. Careful observation has shown that occa- 
sionally a decrease in the number of the parasites present in the tail blood is to be 
noted, it is, however, only transitory, as the numbers quickly increase, so that at 
death the blood will swarm with the organisms. In rats and mice inoculated with 
attenuated trypanosomes or with the blood of less susceptible animals, the parasites 
being fewer in number, the incubation period is lengthened, and the course of the 
disease prolonged. As in Caderas and the other trypanosomic diseases rats and 
mice inoculated with blood from animals undergoing treatment, especially where the 
parasites are still present, though in very scanty numbers and degenerated, may only 
show a few parasites in the blood. In such cases the parasites are so attenuated that 
they do not increase rapidly, and the animal acquires a chronic form of the disease. 
These animals frequently recover, but they are not immune. 
Rabbits 
The incubation period varies. After intravenous inoculation the parasites appear in 
three-and-a-half to five-and-a-half days. The presence of the parasites in the peripheral 
blood is usually accompanied by a slight rise of temperature. Sometimes the rise is 
marked, being I06 3 to 107 0 F., such a high temperature is usually associated with a 
severe infection. The fever may be marked and of an irregular type, or there may 
