TRYPANOSOMES, TRYPANOSOMIASIS, AND SLEEPING SICKNESS 25 
III. TRYPANOSOMA DIMORPHON. GAMB1AN HORSE DISEASE 
This strain was brought back by Drs. Dutton and Todd from the Senegambia. 
Our results in general coincide with those of Dutton and Todd and Laveran and 
Mesnil. 
Rats — White and Black 
The incubation varies ; in the case of the more attenuated strains the incubation 
period may be as long as twenty days ; with the ordinary strain, which has been 
passed through many rats, the incubation period is from three to twelve days, the 
average being from four to seven days. Duration of infection, seven to forty-two 
days ; average, eighteen days. Periodicity in some of the rats is fairly well marked. 
A white rat in the course of thirty-three days showed almost daily an irregular 
number of trypanosomes in the blood — after amounting to thirty to forty to a field 
they would diminish to one to thirty to forty fields, and then gradually rise. Each 
time when the parasites commenced to disappear a marked leucocytosis was present. 
On account of the smallness of the animal no blood count could be made. 
One rat showed a marked resistance to the parasite: — Experiment 285, 
rat (white), one hundred and eighty-five grammes weight. Inoculated intra- 
peritoneally January 5, 1904, with 0-75 c.c. of blood containing three trypano- 
somes to a field. Up to February 5, this animal never showed parasites in its 
blood. It was then reinoculated intraperitoneally with the whole blood from a rat 
(Experiment 242), there being two parasites to a field. Five ciays later four parasites 
were seen to the cover-slip preparation. These continued present for three days ; its 
blood then became negative. On February 22, nine days later, it was once more 
inoculated intraperitoneally with 3-5 c.c. of pure blood from a dog (Experiment 314), 
there being twelve trypanosomes to a field in the mixture. It never became infected, 
nor was its blood infective to small rats when inoculated in quantities of o - 5 c.c. to 
0*75 c.c. If some of its serum was added to blood containing the parasites a marked 
and permanent agglutination occurred. Up to May 10 it was repeatedly inoculated 
with large quantities of virulent blood, it then succumbed to an epidemic of broncho- 
pneumonia. Unfortunately, very little serum could be obtained for agglutination 
and other work. Artificially a certain degree of resistance to this parasite can often 
be established by injecting a mixture of blood and citrate solution attenuated by 
preserving, in the incubator at 35° C. for four to six hours. Out of seven white 
rats so treated five of them were able to withstand at the end of eighteen days inocu- 
lations of i - o to 3*o c.c. of virulent blood. Their blood, however, caused very slight 
d 
