Kendall G. Powers

Plasmodium knowlesi: Functional immunity and antimerozoite antibodies in rhesus monkeys after repeated infection

Abstract Three monkeys, after variable periods of infection with Plasmodium knowlesi , were cured and rechallenged with three strains of P. knowlesi . Functional immunity (a prolonged prepatent period and a modified course of parasitemia) was evident in only one of these monkeys. Sera obtained before each challenge were incubated with a mixture of normal and schizont-infected erythrocytes, and the inhibition of merozoite invasion of erythrocytes was evaluated in vitro . Some sera inhibited invasion. However, there was no correlation between functional immunity and this in vitro test. It appears from this study in monkeys and one in man that hosts with a high degree of functional immunity may not always have antibodies that inhibit merozoite invasion in vitro . Other methods for assessing functional immunity are needed.

Plasmodium vinckei: Production of chloroquine-resistant strain

Abstract To date only two species of malarial parasites, Plasmodium falciparum in man and P. berghei in mice, have demonstrated the ability to develop a high degree of resistance to chloroquine. A strain of P. vinckei which is resistant to the maximum tolerated dose (MTD) of chloroquine in mice has been selected from a pyrimethamine-resistant parent strain. Groups of five or more mice were treated with varying doses of drug for 3–4 days starting the day after parasite inoculation. Subpassages were made from mice treated with the highest dose of drug that allowed for some development of parasites. Resistance developed gradually during 37 successive chloroquine-treated passages over a period of about 44 weeks. In subsequent passages, the parasite was resistant to the MTD of chloroquine for mice (200 mg/kg/day). The dose of chloroquine which suppressed the parent strain by 99.9% on Day 7 was 5 mg/kg/day for 4 days. Sixty percent of mice treated with 10 mg/kg/day of chloroquine and all mice treated with doses of 20 mg/kg/day and greater were cured. Associated with resistance to chloroquine were the parasites failure to produce detectable pigment in treated mice and the appearance of multiple vacuoles. The resistant strain in treated mice is characterized by slower developing, less fatal parasitemias while in untreated mice fulminating infections often result in death. This new species of chloroquine-resistant malaria may be of value as a laboratory model for the study of acquired drug resistance.

Activity of Two Chlorinated Lincomycin Analogues Against Chloroquine-Resistant Falciparum Malaria in Owl Monkeys

The chloroquine-resistant Oak Knoll strain of Plasmodium falciparum, recently adapted to the owl monkey (Aotus trivirgatus), was insusceptible to chloroquine therapy. Two chlorinated lincomycin analogues tested in this host-parasite system cured blood-induced infections. Acute infections were treated orally for 7 consecutive days with either 15 or 75 mg of clindamycin hydrochloride (U-21) per kg per day, 10 or 50 mg of N-demethyl-4′-pentyl clindamycin hydrochloride (U-24) per kg per day, or 20 mg of chloroquine base per kg per day. These lincomycin analogues cleared trophozoites from the peripheral blood by the end of the 7-day treatment period. The speed of clearance of parasites was not dose-related, but curative activity appeared dependent upon the amount of drug given as well as the number of daily treatments. The efficacy of U-21 and U-24 is of particular interest since they represent major structural departures from compounds commonly used in the treatment of malaria.

Plasmodium cynomolgi: Influence of X-irradiation and sporozoite dilution on relapse patterns in infected rhesus monkeys

Abstract It has been possible to demonstrate a direct relationship between number of sporozoites in an inoculum and the number of subsequent relapses in experimental infections with Plasmodium cynomolgi. The reduction in Sporozoite numbers was accomplished through the use of two techniques; (1) X-irradiation and (2) volumetric dilution. The authors conclude that these observations tend to support a concept of latency as an explanation for relapse and delayed patency in malaria. It is difficult to reconcile these results with the generally accepted concept of a continuous cycle of liver schizogony in relapsing malarias.

Plasmodium knowlesi: morphology and course of infection in rhesus monkeys treated with clindamycin and its N-demethyl-4'-pentyl analog.

Abstract The clinical and morphologic effects of clindamycin and N -demethyl-4′-pentyl clindamycin were evaluated using Plasmodium knowlesi in rhesus monkeys. Both compounds cured blood-induced infections when administered daily for five consecutive days. When the rapidity of action of these antibiotics was compared with chloroquine it was evident that although they were able to control fulminating infections in all treated monkeys their effect was about 2 days slower than chloroquine in decreasing parasitemias and 3 to 4 days slower in clearing parasites from the blood. Morphologic changes within the parasite associated with drug action were studied in time sequences by light and electron microscopy. Changes were observed in the parasite ribosomes 24 hr after drug administration. Affected ribosomes showed electron-lucent zones measuring ~50 A in the center. During the following 24 hr these changes became more prominent with foci in which disintegrated ribosomes were replaced by fine fibrillar material and the cisternae of the endoplasmic reticulum became dilated. By 72 hr this dilation was more apparent and resulted in abundant coalescence of vacuoles in the cytoplasm. Mitochondria became dilated with fibrils in the matrix, although the degree of swelling was not a conspicuous and constant feature. The nucleus presented a fine fibrillar appearance and fewer granules were seen than in normal parasites. The latter two observed changes appear to be secondary to changes in the ribosomes and probably are not directly related to the action of the antibiotics. These studies indicate that clindamycin and its analog affect primarily the ribosomes and their mode of action is different from that of the commonly used antimalarials.

Tolerance to Type III Pneumococcal Polysaccharide in Monkeys

Rhesus and owl monkeys were injected intramuscularly with 10 to 10,000 γ of Type III pneumococcal polysaccharide. The monkeys were bled weekly, and then reinoculated with the polysaccharide at suitable intervals. The antibody content of their sera was measured by radioimmunoassay and the antigen content of selected sera estimated by the inhibitory effect of the serum upon the capacity of rabbit antisera to precipitate the polysaccharide. Ten γ of the polyeaccharide immunized the monkeys. Ten thousand γ did not immunize them but instead reduced their serum antibody to very low levels. One thousand γ also reduced their serum antibody to very low levels, but not uniformly as far as 10,000 γ. These findings indicate that 1,000 to 10,000 γ of this polysaccharide appear to cause immunologic tolerance in primates.