Resistance to diamino-diphenylsulphone in Plasmodium gallinaceum

A fourfold enhancement of resistance to diamino-diphenylsulphone has been produced over a period of 15 months in strain A of P. gallinaceum by subjecting the strain, maintained in a state of patent infection, to increasing doses of the drug. The DDS-resistant strain was cross-resistant to the minimum effective dose of sulphadiazine and slightly resistant to twice this dose, but it showed only a slight enhancement of resistance to pyrimethamine and proguanil. The antimalarial action of DDS was antagonized by p –A.B. in the ratio of 100 to 1. DDS, in the minimum effective dose, was antagonized completely by folic acid if given in equal doses, but not by smaller doses of the antagonist; the antagonism was not competitive. The relationship of cross-resistance between p –A.B.-inhibited sulphonamides and proguanil and pyrimethamine is discussed.