An analysis of the development of resistance to proguanil and pyrimethamine in Plasmodium gallinaceum
Authors: Bishop Ann 
Published: June 2009

1. The development of resistance to proguanil and pyrimethamine was studied in a clone of Plasmodium gallinaceum (strain A) derived from a single erythrocytic parasite and maintained by a standard method of serial inoculations in young chicks.

2. The rate of the development of resistance to 0·05 mg./20 g. doses of proguanil was compared in strains maintained by inocula of 5 × 107, 108 and 109 parasites. Some evidence was obtained that the rate of the development of resistance to the drug was related to the numbers of parasites in the inocula.

3. No correlation was observed between the rate of the development of resistance to pyrimethamine and the size of the population of parasites exposed to its action in strains treated with 0·001 g./20 g. doses of the drug and maintained by inocula of 5 × 107 parasites or 109 parasites; but a greater variability was observed in the rate in the strains maintained by the small inocula than in the strains maintained by the large inocula. Resistance appeared to develop sporadically.

4. Some evidence was obtained that strains treated with a high dose of pyrimethamine (0·05 mg./20 g.) might become more sensitive to the drug as a result of treatment. In three experiments the infections were eradicated by the second or third course of treatment with the drug. However, strains which withstood this dose, developed resistance as rapidly, or more rapidly than those treated with 0·001 mg./20 g. doses. There was therefore no evidence from these experiments with pyrimethamine that resistance to the drug could be produced more readily with small than with large doses of the drug.

5. Except for slight variations, resistance to proguanil developed concurrently with resistance to pyrimethamine in strains treated with that drug.

6. In many of the strains, whether treated with proguanil or with pyrimethamine, resistance appeared to develop quite suddenly, but in others it developed more gradually. The pattern of the development of resistance in strains treated with either of these drugs could be explained by the selection of spontaneously occurring mutants, though some of the data suggested that the mutants differed in degree of resistance to the drug.

7. The rate of development of resistance to pyrimethamine was studied in strains derived from primary inocula composed of known numbers of pyrimethamine resistant parasites and 5 × 107 parasites of the untreated strain A. The addition of at least 106 resistant parasites to the inoculum was required to produce an increase in parasitaemia during the subsequent course of treatment with the drug, but the addition of 102 resistant parasites or, in some experiments, fewer than 102 resistant parasites to the primary inoculum was sufficient to produce a heavy parasitaemia during the second serial course of treatment if the strain was maintained by the standard method (p. 496). When these results are compared with the pattern of the development of resistance to pyrimethamine in the strains treated with that drug, it can be concluded that only in one strain was there any evidence that resistant parasites were present in the primary inoculum when it consisted only of parasites of the untreated parent strain. The rate of development of resistance in many of the strains treated with either proguanil or pyrimethamine suggested a mutation rate of a low frequency.