Resistance of Plasmodium falciparum in Tanganyika to pyrimethamine administered at weekly intervals

1) In a holoendmic area of Tanganyika, forms of Plasmodium falciparum resistant to pyrimethamine appeared 5 months after institution of weekly malaria prophylaxis with that drug, and in a further 5 months' time stabilized at a rate 20 per cent. lower than that normally encountered in the area before treatment. 2) The degree of resistance was eight to fifteenfold that of the effective dosage for susceptible parasites, and probably more, because symptoms of drug toxicity were reported by the ambulatory subjects whilst parasite densities were increasing. 3) The resistant parasite was spread markedly to untreated persons within the central treatment area, but only slightly to those two to four miles distant and not at all to those five to 12 miles away. 4) No remarkable difference in morphology or immunology was apparent between susceptible and resistant P. falciparum in the same area. Cross-resistance to proguanil did not appear to be of significant extent.