The Rôle of the Natural Defence Forces in the Evolution of the Drug-Resistance of Trypanosomes

Immunity in trypanosomiasis

Antibody-resistant strains are less sensitive to suramin and antrycide than antibody-sensitive strains. When living trypanosomes were exposed to suramin and antrycide in vitro, antibody-resistant strains needed 50 times more drugs than antibody-sensitive trypanosomes in order to make them non-infectious to mice. In therapeutic experiments in mice the minimal therapeutic dose of drugs for antibody-sensitive strains was 0·1 mg. but for resistant strains it was 0·3 mg./20 g. mice. Rabbits treated prophylactically with suramin resisted infection with the antibody-sensitive strain for a period of 4 months, but failed to resist infection with the antibody-resistant strain after 2 months.

Rabbits treated prophylactically with antrycide pro-salt, resisted infection with antibody-sensitive strains for a period of 2 months, but failed to resist infection with the antibody-resistant strain even 1 month after injection with the drug. Although trypanosomes can become drug resistant without being antibody resistant it is suggested that, under natural conditions, drug-resistant strains in animals and man develop from antibody-resistant strains, particularly when trypanostatic drugs are used. It is suggested in conclusion from these experiments that strains of trypanosomes which are exposed for some time to antibodies and become antibody resistant after passage through animals like rabbits, as well as those strains frequently passaged through mice, should be used in all tests for the efficiency of chemotherapeutic drugs.

The rapid development of drug-resistance by Trypanosoma evansi in immunosuppressed mice

The effect of immunosuppression on the development of drug resistance by trypanosomes was investigated in mice infected with Trypanosoma evansi. As a result of frequent passage in immunosuppressed mice given subcurative drug treatments clones of T. evansi rapidly developed high levels of resistance to mel Cy, diminazene aceturate and isometamidium chloride. Similar protocols in normal immunocompetent mice infected with the same parent clones did not lead to the development of drug-resistance. The resistant populations developed in immunosuppressed mice maintained their high levels of resistance when tested in normal mice. The mel Cy resistant clone was tested for cross-resistance to other trypanocides and was found to be also highly resistant to diminazene and pentamidine. The results indicate that impairment of the host immune system may lead to the rapid development of drug-resistance by T. evansi under experimental conditions in mice and may possibly play a role in the development of drug resistance by trypanosomes in the field.

Hypothesis: impaired immunity as a factor which contributes to the spread of drug-resistance

Evidence has accrued to indicate that host defence mechanisms enhance the efficacy of many of the drugs used to treat infectious diseases. Because of this, and also because of the likelihood of increased pathogen loads in immunoincompetent hosts, some infections are less likely to be completely cured by normal regimens of chemotherapy in individuals with drastically impaired immune responsiveness. In such circumstances natural selection could result in the accelerated emergence of drug-resistance pathogens.

A study of resistance to antrycide in a strain of Trypanosoma equiperdum

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