Immunity against trypanosomes

It is an establised fact that rats which have recovered from an infection with Tr. Lewisi are immune to subsequent inoculation with that species of flagellate. The same holds true for cattle, sheep, goats, etc., that have recovered from the infection caused by the pathogenic trypanosomes, such as nagana, surra and dourine. This condition of active immunity is seemingly possible only in those species of animals that are relatively insusceptible, for with really susceptible species the infection is always fatal.

Heretofore all experiments on artificial immunity against trypanosomes have been made on animals that have recovered from the effects of the parasite which has been living and multiplying in the blood-vessels of that animal. Now that cultures of some of these organisms, as for example Tr. Lewisi of the rat and Tr. Brucei of nagana, are possible it was desirable to ascertain whether or not they could be used to immunize against the virulent organisms. It may be said, in passing, that cultures of both of these trypanosomes, even after they have passed through a hundred generations or subcultures in the course of two years, do not become attenuated by such prolonged consecutive passage but readily infect susceptible animals.

We have shown, however, that cultures of Tr. brucei can be attenuated by exposure for about two days at 3 4 ° C. By repeated injections of cultures thus treated, attempts have been made to immunize rats and guinea-pigs against Tr. brucei but thus far these have been but partially successful. That is to say, there has been at most a survival for a few days of the treated as compared with the untreated animals. The failure to immunize with such cultures is attributable in part to the excessive susceptibility, of the animals employed, to infection with Tr. brucei, and in part to the existence of a negative phase following the injections.