Drug-resistance transfer among rodent plasmodia. 1. Acquisition of resistance to pyrimethamine by a drug-sensitive strain of Plasmodium berghei in the course of its concomitant development with a pyrimethamine-resistant P. vinckei strain.

A study on transfer of drug resistance between species of malaria parasites was carried out in mixed infections of P. berghei and P. vinckei . The biological characters which served as contrasting markers for separation of the two species were the inability of the ‘old’ blood-passaged strain of P. vinckei to develop in suitable mosquito vectors and its failure to produce parasitaemia in the adult golden hamster. Facility of sporogonic development and cyclical transmission and a pronounced virulence for the adult hamster marked the N.K. 65 strain of P. berghei . Development of a maximal and stable resistance to 200 mg/kg pyrimethamine in the P. vinckei strain was achieved by a gradual increase in drug level in successive blood transfers. Mixed, simultaneous infections of pyrimethamine-resistant P. vinckei and sensitive P. berghei were experimentally induced in white mice. Subinoculations of the mixed infection at the height of parasitaemia was carried out in adult hamsters. Suppression of the P. vinckei infection in the hamster by innate resistance and the reappearance of P. berghei in the blood was noted. Tests with the maximal concentration of pyrimethamine (200 mg/kg) showed the acquisition of drug resistance in some of the P. berghei infected hamsters. Persistence of the acquired resistance was demonstrated following fifteen successive blood transfers in the absence of drug pressure and after five cyclical transmissions. Parallel control experiments with pyrimethamine-sensitive P. vinckei and P. berghei in mixed infections failed to reveal any enhancement in drug resistance. No spontaneous drug-resistant mutant was found in the sensitive P. berghei after exposure to a single course of treatment with the drug, and persistence of parasites in some treated animals was considered as individual variations in host reactions to the drug. This resistance never persisted after blood transfer or cyclical transmission. No loss in drug resistance had been found following transfer of the resistant R strain from mouse to hamster. A marked difference in the course of the plasmodial infection and in the length of survival of mice inoculated with the drug-resistant P. berghei was noted following treatment with pyrimethamine in various concentrations. It is suggested that these differences represent a selective action of the drug on a heterogenic P. berghei population, consisting of drug-resistant and drug-sensitive parasites in the blood of the host. Injection of sonified material from pyrimethamine-resistant P. vinckei and P. berghei into mice inoculated with a drug-sensitive P. berghei failed to induce enhancement of resistance to the drug. The phenomenon of drug-resistance transfer observed under experimental conditions in rodent malaria is viewed in the light of similar phenomena in micro-organisms. We gratefully acknowledge the assistance rendered by Mr Jerold Sklarsh. This paper is Contribution no. N 408 from the Army Research Programme on Malaria. The work was carried out under the sponsorship of the Commission on Malaria, Armed Forces Epidemiological Board, and supported in part by the United States Office of the Surgeon General, Department of the Army.