Notes on immunity in trypanosomiasis

Abstract

1. 1. The treatment of cattle suffering from trypanosomiasis with intravenous injections of potassium antimonyl tartrate was introduced into Southern Rhodesia in 1909 and has since been adopted with considerable success. 2. 2. In the course of 25 years' experience, it has frequently been observed that treated recovered animals may return to the tsetse-infested areas and remain there apparently in good health in spite of re-infection. This observation has been confirmed by veterinarians in other parts of Africa. 3. 3. Treated animals either are sterilised of the parasite and again become susceptible to re-infection, or are not completely sterilised and become tolerant. 4. 4. A method of inoculation and treatment based upon these observations was introduced in 1928. It consisted of the deliberate infection of cattle with known strains of trypanosomes—preferably the strains of the district in which the animals were to be exposed—and of treating them at the appropriate time. 5. 5. In this manner they were rendered tolerant or immune, but in practice it was found that their tolerance broke down when the animals were subjected to adverse conditions. To obviate breakdown would involve a revolution of present-day conditions of animal husbandry in tsetse-infested areas. 6. 6. Tolerant animals become reservoirs of infection; therefore a complete sterile immunity, rather than a tolerance, would be preferable. 7. 7. In nature the resistance of the wild animals in tsetse-infested areas is, in most cases, a tolerance. A similar condition is sometimes met with in indigenous domestic animals. 8. 8. Old hunters and explorers have attributed this resistance to the infection of young animals during their early days. 9. 9. Recent experiments with rats and mice indicate that young animals are susceptible to infection; but if well nourished and not unduly disturbed, they may survive an infection sufficiently virulent to kill weaklings or adults. If they recover, they again become susceptible; but if re-infected during the course of the original infection, they become tolerant. 10. 10. These observations indicate how tolerance may be acquired in nature and suggest that it is maintained by repeated re-infection. 11. 11. They explain the peculiar incidence of latent cases of sleeping sickness in Southern Rhodesia and suggest preventive measures. 12. 12. They also indicate that the tolerance of inoculated cattle should be maintained by repeated syringe or fly infection. 13. 13. The integral properties of the young which render it partly resistant have not been determined, but appear to be worthy of further research. 14. 14. A method of setting up a complete and lasting sterile immunity in man and animals against trypanosomiasis comparable to that commonly set up against bacterial and virus diseases appears to offer a solution to the tsetse fly problem.