Characteristics and cross-resistance patterns of chloroquine-resistant Plasmodium berghei infections in mice.

Abstract Groups of mice were exposed intraperitoneally with graded doses of packed red cells or homogenate of spleens of mouse donors infected with a chloroquine-resistant strain of Plasmodium berghei. After challenge doses of 20 μl RBC or 20 mg spleen, patent parasitemia resulted by three days in virtually all animals. By seven days of infection, parasitemia by Giemsa stain, was approximately 3%. Parasitemia increased at a relatively slow rate to 15 and 70% by 13 and 18 days respectively. Mean mouse rectal temperatures were essentially nondistinguishable from 98 to 99 °F of controls, by seven days of infection. By 13 days, mean temperatures varied between 93 and 96 °F and 86.5 and 101.5 °F in groups and individual mice respectively. By seven days, the mean hematocrit in infected mice was about 50% and not distinguishable from that of noninfected controls. The mean hematocrit was 31 and range 25 to 36% at 13 days of infection. Splenomegaly was a clinical feature of mouse malaria infections. A splenic index of about 5 (mg spleen weight per gm mouse body weight) of noninfected controls was compared to about 13 and 31 in infected mice by 7 and 13 days respectively. Spleen weight responses tended to be proportional to increased mean parasitemia. The mean survival time of infected mice was 23.5 days. Biochemical study of pooled sera of infected mice by miniature agar electrophoresis showed the following results: (1) Albumin concentrations decreased as total globulins increased, and hypergammaglobulinemia increased with the duration of infection. (2) Lactic dehydrogenase isozymes appeared elevated. Mainly, the beta- and gamma-type LDH isozymes stood out as distinctive biochemical markers of severe malaria infections. A quantitative bioassay of live malaria parasites in infected mice was used to measure reliably and reproducibly the parasite burden in red cells and in homogenates of vascular organs in which malaria parasites sequestered in large numbers. Potency estimations were determined by means of biological statistical tables. The total viable and infectious parasite burden was determined in two groups of 15 mice each after 13 days of infection. The potency determinations revealed 8 × 102 and 5.1 × 103 infectious malaria units per circulating blood and spleen respectively per mouse. Thus, it was established that the viable parasite burden can be 5.4-fold greater in a mouse's spleen compared to its circulating blood after 13 days of infection. Acute mouse infections responded to antimalarial therapy as follows: The malaria parasites were susceptible to action by cycloguanil hydrochloride, cycloguanil pamoate, pyrimethamine, p,p′-diaminodiphenylsulfone, sulfadiazine, and primaquine. Infections in mice were cross-resistant to quinine, quinacrine, and amodiaquine. Several combinations of two and three of the drugs administered simultaneously effectively cleared the malaria parasites from the circulating blood. However, autopsy of these animals one day after the antimalarial therapy course indicated that splenomegaly was not remarkably affected. These results may contribute to a better understanding of the pathogenesis of a chloroquine-resistant strain of P. berghei in mouse infections.