76 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
we believe that a definite increase of virulence, as well as an attenuation, can be 
achieved, the former by the passing of the trypanosomes through a large series of 
animals of the same species, and the latter by constantly changing the species of 
animal employed. 
Rats. — Young rats, thirty experiments, shortest incubation two days, longest 
thirty-five, average from four-and-a-half to seven-and-a-half days. Usual duration 
trom twenty to forty-one days. Some have survived one hundred and thirty-two days. 
Adult rats, thirty-three experiments, incubation from four to forty-seven days, average 
eight-and-a-half days. Duration from forty-five to three hundred and eighty-eight 
days. Some rats inoculated in Senegambia in November, 1902, still survive, but 
their blood is negative microscopically, and non-infective. The parasites are present 
in the blood of both young and adult rats in fair quantities, though in many of the 
animals they are not constantly present, being at one time numerous in the blood, 
and a few days later absent altogether, or only demonstrated after a most careful 
search. 
Mice. — Twenty-five experiments. Incubation period from one to thirty-seven 
days ; average from four to seven days. Usual duration from eleven to fourteen 
days. In some cases the parasites have been very numerous, and continually present 
in the blood, but very often the animal shows only a few parasites, or there may be 
a marked irregularity as to the appearance and disappearance of the organism. 
Guinea-pigs. — Ten experiments. Three guinea-pigs have become infected after 
twelve, fifty-three, and seventy-three days, respectively, and have lived three, four, 
and eight months. One, Experiment 114, 1 inoculated May 13 and positive twelve 
days later, never showed parasites again in its peripheral blood until September 1 1. 
On November 1 thirty to a field 1 were observed. The trypanosomes continued to 
increase in numbers, and on December 3 reached eighty to one hundred to a field. 
On December 20 the numbers decreased and death ensued on January 4. A guinea- 
pig, Experiment 133, 3 inoculated on July 20 from a rat, was positive on September 1 1, 
parasites became very numerous on November 1 (twenty to field), and continued so 
to the end on November 20. Rupture of the spleen was the cause of death. Four 
guinea-pigs, inoculated one hundred and twenty-four days ago with large quantities 
of blood containing numerous trypanosomes, have remained negative up to the 
present time, despite re-inoculations. Their controls — i.e., rats and rabbits — became 
infected in the usual time. Three guinea-pigs, inoculated on March 7 from rat 
279A with large doses of blood containing countless trypanosomes, showed parasites 
in their blood in twelve hours in two cases, and in three days in the other. The 
1. J. E. Dutton and J. L. Todd, First Report of the Trypanosomiasis Expedition to Senegambia (1902), Liverpool School of 
Tropical Medicine. Memoir XI. 
2. We have, throughout, used three-quarter inch square cover-slips, and examined with one-sixth inch objective, and 
No. II eye-piece. 
3. Dutton and Todd, he. cit. 
