Endotoxin-induced non-specific resistance in rats examined by Trypanosoma equiperdum challenge

In rats weighing 100 g, intravenously administered Trypanosoma equiperdum organisms started logarithmic growth immediately, whereas in rats weighing 150-300 g growth started after a lag phase of 0.7-3.1 hr. The lag phase lasted for 15-17 hr when the rats were pretreated serially with endotoxins and this time course was not modified by changes either in the host's body weight or in the germ count of the inocula. Maximum resistance to infection was achieved with gradual doses of the endotoxin given at 48 hr intervals on 6 occasions. The total dose of endotoxin was fourfold of its LD50. Additional doses failed further to increase the resistance. Serial passages of the strain in rats pretreated with endotoxin disclosed that the trypanosomes growing after the lag phase had developed resistance to the factor responsible for the lag phase. Thus the method does not allow a quantitative estimation of resistance induced by serial endotoxin treatment, yet it represents a rapid and simple procedure in every case in which qualitative assessment is sufficient.