22 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
to infection, show occasional resistance. One rat, 285 (see p. 25), was difficult to 
infect with T. dimorphon, and then only showed the parasites for a few days. It 
recovered and was able to withstand inoculations of enormous quantities of virulent 
blood. In the preliminary note on T. gambiense it was shown that sometimes, of two 
rats inoculated at the same time the one acquired the disease the other did not, the 
resistant animal even having received the greater amount of the virulent blood. 
With these resistant animals and those in which the disease pursues a markedly 
chronic course experiments have been made to see it an exacerbation of the disease 
could be produced. Such animals have been daily, weekly, or monthly reinoculated 
with small quantities of virulent blood. In a few cases a rise of temperature 
corresponding to the incubation period has been observed. In a couple of cases the 
parasites have somewhat increased. Various experiments in breeding infected and 
healthy animals together have been done. The progeny of such unions have not 
shown any immunity or resistance. 
Endeavours to protect animals, or to at least mitigate the disease by injection 
of attenuated parasites or large quantities of blood from which the parasites have been 
freed or killed off, have proved of little value. The same applies to injection of 
animals with sera from animals which have recovered from the disease in which the 
chronic type prevails, or from animals naturally immune. Here again very meagre 
results have been obtained. Efforts to produce an immunity by the administration of 
drugs is of little value. Arsenic, which has an effect on the parasite, is of no use when 
attempts are made by first treating the animal with the substance to render it later 
proof against inoculation of virulent blood. Ehrlich and Shiga in their work on 
'Trypanroth ' were able to protect 1 animals by preliminary treatment with the dye. If 
large enough amounts are administered a certain amount of protection is afforded. 
Toxicity 
All efforts to isolate a toxine have proved negative. The various procedures 
which Plimmer and Bradford had used were employed. They had found that 
animals could be inoculated with large quantities of blood from dogs infected with 
Nagana, the blood being filtered through porcelain bougies, without producing any 
effect. As much as 150 c.c. of filtered serum have been used without producing any 
toxic effect. 
On one or two occasions the injection of blood containing trypanosomes has 
seemed to us to possess toxic properties. A rabbit infected with T. gambiense showed 
this. The blood as long as there were few parasites present could be inoculated into 
rats with impunity. Later on, as the parasites became numerous, it was noticed 
I. Such a line of investigation with this and other substances ought to be tried on large animals. The history of punitive 
expeditions, etc., in the past has shown how handicapped man is when a trypanosomic disease breaks out among-st che transport 
animals. If this dye were administered before passing through a fly-belt, and the animals carefully examined ana tested before 
and afterwards, valuable data wouid be available. 
