The effects of subcurative doses of chloroquine on Plasmodium vinckei petteri gametocytes and on their infectivity to mosquitoes.

The effects of subcurative doses of chloroquine on rodent and human Plasmodium transmission to the mosquito have been studied by several authors who showed a short-term (12 h) enhancement of gametocyte infectivity by the drug, restricted to chloroquine-resistant strains, and a long term (4-6 days) enhancement of gametocytogenesis of chloroquine-sensitive strains of Plasmodium chabaudi. We investigated both short- and long-term effects of chloroquine on Plasmodium vinckei petteri, a chloroquine-sensitive rodent Plasmodium strain. Chloroquine treatment reduced the index of gametocytogenesis to 73% (5 mg/kg) and 55% (2.5 mg/kg) of controls, on day 6 post-infection (p.i.). The reduction was statistically significant with 5 mg/kg chloroquine. However, the reduction of gametocyte numbers did not affect the transmission capabilities of the strain. Our experiments showed that doses of 1 mg/kg chloroquine had no effect on the oocyst counts, 12 h post-administration to mice. A statistically non-significant 61% reduction of oocyst numbers was observed in mosquitoes fed on mice treated with 5 mg/kg chloroquine. The effect of 5 mg/kg chloroquine administration on the infectivity of gametocytes to mosquitoes fed 1 h post-treatment was also investigated. An overall 41% reduction of oocyst numbers was observed. This immediate effect was statistically significant in 73% of the mice. These results are consistent with the hypothesis that the short-term enhancing effect of chloroquine on transmission is restricted to the drug-resistant strains of Plasmodium.

Final stage of maturation of the erythrocytic schizonts of rodent Plasmodium in the lungs.

Schizonts of all rodent Plasmodium studied (Plasmodium yoelii, P. chabaudi, P. vinckei) show a characteristic morphology when they are completely mature: rounded or slightly elongate merozoites, completely detached from the pigment mass. At this stage, they are localized principally in the spleen and the lungs but, in impression smears of these organs they show two different aspects. In the spleen, schizonts are either inside the host erythrocyte or extraglobular but still close to a pigment mass; free merozoites are rare. In the lungs, on the contrary, merozoites are often free and dispersed; electron microscopy showed them to lie against the endothelium. Work by physiologists has shown the blood circulation in the alveoli to be much slowed down. Free merozoites, lined against the endothelium of relatively rigid capillaries, are in the best possible conditions to make contact with the intact red blood cells. Lungs appear to be the privileged site for the invasion of erythrocytes by the merozoites.

Survival of rodent malaria merozoites in the lymphatic network: potential role in chronicity of the infection.

Experiments performed during the last few years, lead us to hypothesise the existence of latent asexual forms of murine Plasmodium. In the present report we examined the organs of infected animals and describe novel structures, which we call merophores, containing merozoites which have resisted lysis seen with other asexual stage parasites. We propose that these merozoites represent a latent form of the parasite. Merophores were also found in the lymphatic circulation, and were demonstrated by subinoculation to have retained their viability. Depending on the parasite species two types of merophores were observed. For P. yoelii nigeriensis merophore sacks, with the latent merozoites found inside vesicles, were usually observed. Merophore leucocytes, where latent merozoites dispersed in the cytoplasm of macrophages or neutrophils, were solely seen with P. vinckei petteri. Both structures were seen in P. chabaudi chabaudi infections. Merophores were found in lymph nodes of rodents after the asexual parasitaemia had apparently subsided. They were formed soon after schizogony, principally in the spleen, either by pitting or by macrophage phagocytosis. Merophore numbers appeared to be proportional to the number of maturing schizonts. We propose that merophore formation and their circulation in the lymphatics play an important role in the pattern of recrudescences and chronicity of rodent malaria infections. It is further suggested that the lymphatic network, a privileged pathway for many parasites, might play a similar role in human malaria infections.

Development of irreversible lesions in the brain, heart and kidney following acute and chronic murine malaria infection.

Irreversible pathological lesions were noted in the organs of mice infected with 1 of 3 rodent malaria species: Plasmodium chabaudi chabaudi, P. vinckei petteri and P. yoelii nigeriensis at different times during the course of the primary parasitaemia and long after microscopical clearance of the parasites. Moreover, similar lesions were also obtained when parasite levels were kept below 1% by subcurative drug treatment. The frequency and severity of the lesions correlated with the duration of the infection. Accumulation of tissue damage during chronic low-grade malaria infections has implications for the design of control measures.

Rodent malaria prophylaxis by transdermal delivery of primaquine.

The efficacy of a new transdermal delivery system of primaquine in order to obtain causal prophylaxis against sporozoite-induced Plasmodium yoelii infection was evaluated. A single administration of a 1.0 cm2 transdermal delivery system containing 5.0 mg of primaquine was able to protect 100% of treated mice. This result suggests that the transdermal route may be a very interesting approach for malaria prophylaxis and should encourage further studies in order to determine the absolute bioavailability of the drug as well as its dose-effect relationship.

Chronosexuality of Plasmodium species of Central African Muridae.

A host harbouring many parasite species of the same genus is a phenomenon frequently observed in numerous parasitic infections. This is the case for the Plasmodium parasites of Muridae in Central Africa, where three different parasite species are found in the same rodent host species. It is highly likely that these three Plasmodium species are transmitted simultaneously by the same vector. We and others have shown that the maturation periods of the various asexual and sexual stages in the rodent, differ amongst the three parasites. In this article we propose that these differences are the product of complex adaptations which result, for all three Plasmodium species, in a maximum peak of infectivity to the insect vector occurring around 3 a.m., the period of highest activity of the nocturnal host rodent.

Role of macrophages as possible transporters of Plasmodium yoelii nigeriensis merozoites through the lymphatic system. Preliminary note.

Vesicles containing apparently healthy merozoites from mature schizonts were observed in the spleen and lymph nodes of mice parasitized by Plasmodium yoelii nigeriensis. They differed from all parasitic stages undergoing digestion by the macrophage and from mature schizonts of the blood. Up to 40 merozoites from three mature schizonts may be seen in the same compartment. They are thought to be accumulations of latent merozoites.

Modifications in the rhythm of schizogony in Plasmodium chabaudi chabaudi associated with the selection of chloroquine resistance.

A high level of drug resistance was obtained with a line of Plasmodium chabaudi maintained under intense chloroquine selection pressure according to the protocols established for P. berghei. The main objective of this work was to verify if the characteristic asynchronous schizogonic rhythm of naturally resistant rodent malaria parasites was also found when the drug resistance was induced experimentally. The degree of resistance was evaluated through the use of the "2% delay test" (DT) and the schizogonic rhythm, by reference to the synchronicity index (SI). The strain had originally a DT of 4.26 and an SI of 0.52. Following the application of 80 mg/kg chloroquine at each passage, as early as at the 8th passage the parasites rapidly became resistant and asynchronous. At the 17th passage the DT was 3.32 and the SI, 0.20. In the drug-resistant line the original indices, both the DT and the SI, were restored after deep-freezing, sporogony, or passage through a Percoll gradient, or simply by repeated intravenous subinoculations of blood. The clear correlation between asynchronicity and drug resistance is easily explained by the action of chloroquine, which favours the schizogonic cycles initiated by latent merozoites.

The erythrocytic schizogony of two synchronized strains of plasmodium berghei, NK65 and ANKA, in normocytes and reticulocytes.

By a modified Percoll-glucose centrifugation technique the rings and young trophozoites of two strains of Plasmodium berghei, NK65 and ANKA, were separated from the other erythrocytic stages and inoculated into mice. The subsequent infection was followed for ANKA in normal mice and for NK65 in normal mice and in mice with high-grade reticulocytosis induced by injections of phenylhydrazine. The duration of the erythrocytic schizogony of the NK65 strain was shown to be independent of the age of the host cell, and the hour of inoculation did not influence the cycle of the ANKA strain.

Postprandial lymphatic flux and malaria.

24 hours after an intra-peritoneal inoculation of frozen blood infected with Plasmodium yoelii nigeriensis, the malarial infection is non detectable in most starved mice while it is patent in mice fed 2 hours before the inoculation. It is assumed that the post feeding lymphatic flow brings to the blood the latent merozoites.