Plasmodium berghei: deep vascular sequestration of young forms in the heart and kidney of the white rat

Ecology of asynchronous asexual replication: the intraerythrocytic development cycle of Plasmodium berghei is resistant to host rhythms

BACKGROUND

Daily periodicity in the diverse activities of parasites occurs across a broad taxonomic range. The rhythms exhibited by parasites are thought to be adaptations that allow parasites to cope with, or exploit, the consequences of host activities that follow daily rhythms. Malaria parasites (Plasmodium) are well-known for their synchronized cycles of replication within host red blood cells. Whilst most species of Plasmodium appear sensitive to the timing of the daily rhythms of hosts, and even vectors, some species present no detectable rhythms in blood-stage replication. Why the intraerythrocytic development cycle (IDC) of, for example Plasmodium chabaudi, is governed by host rhythms, yet seems completely independent of host rhythms in Plasmodium berghei, another rodent malaria species, is mysterious.

METHODS

This study reports a series of five experiments probing the relationships between the asynchronous IDC schedule of P. berghei and the rhythms of hosts and vectors by manipulating host time-of-day, photoperiod and feeding rhythms.

RESULTS

The results reveal that: (i) a lack coordination between host and parasite rhythms does not impose appreciable fitness costs on P. berghei; (ii) the IDC schedule of P. berghei is impervious to host rhythms, including altered photoperiod and host-feeding-related rhythms; (iii) there is weak evidence for daily rhythms in the density and activities of transmission stages; but (iv), these rhythms have little consequence for successful transmission to mosquitoes.

CONCLUSIONS

Overall, host rhythms do not affect the performance of P. berghei and its asynchronous IDC is resistant to the scheduling forces that underpin synchronous replication in closely related parasites. This suggests that natural variation in the IDC schedule across species represents different parasite strategies that maximize fitness. Thus, subtle differences in the ecological interactions between parasites and their hosts/vectors may select for the evolution of very different IDC schedules.

Unlike the synchronous Plasmodium falciparum and P. chabaudi infection, the P. berghei and P. yoelii asynchronous infections are not affected by melatonin

We have previously reported that Plasmodium chabaudi and P. falciparum sense the hormone melatonin and this could be responsible for the synchrony of malaria infection. In P. chabaudi and P. falciparum, melatonin induces calcium release from internal stores, and this response is abolished by U73122, a phospholipase C inhibitor, and luzindole, a melatonin-receptor competitive antagonist. Here we show that, in vitro, melatonin is not able to modulate cell cycle, nor to elicit an elevation in intracellular calcium concentration of the intraerythrocytic forms of P. berghei or P. yoelii, two rodent parasites that show an asynchrononous development in vivo. Interestingly, melatonin and its receptor do not seem to play a role during hepatic infection by P. berghei sporozoites either. These data strengthen the hypothesis that host-derived melatonin does not synchronize malaria infection caused by P. berghei and P. yoelii. Moreover, these data explain why infections by these parasites are asynchronous, contrary to what is observed in P. falciparum and P. chabaudi infections.

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.

Plasmodium berghei: malaria infection causes increased cardiac output in rats, Rattus rattus

Thirty-two 4-week-old male Wistar rats were infected with Plasmodium berghei malaria. On Days 12 through 14, blood volume, arterial blood pressure, right ventricular pressure, heart rate, cardiac output, stroke volume, hematocrit, and vascular resistances were determined. All of the cardiovascular parameters measured, with the exception of calculated pulmonary vascular resistance, changed progressively as the peripheral blood parasitemia increased. With a rising parasitemia, cardiac output increased, despite a reduced heart rate. The highest parasitemia of 63% was accompanied by a doubling of the normal cardiac output. The relationship between parasitemia and cardiac output can be described by the equation, cardiac output = (6.14) x % parasitemia + 452 ml/min/kg. The mean arterial blood pressure was lower than controls when parasitemia exceeded 20%, whereas systolic right ventricular pressure was elevated only at the highest parasitemias. When noninfected control rats were compared with those animals having parasitemias greater than 40%, in the infected animals, mean arterial pressure was 28% lower (P less than 0.01) and systolic right ventricular pressure rose by 21% (P less than 0.02). A 50% decline was observed in the total peripheral vascular resistance (P less than 0.01), although the pulmonary resistance was apparently unchanged. With P. berghei infection, there is also a marked anemia, an increase in plasma volume, and a 16% increase in blood volume (% body weight). It is concluded from these results that although the hemodynamic changes previously reported in the literature indicate that infection with malaria may result in focal blockages in microvessels and poor tissue perfusion, the total systemic effect, in the rat, is an increase in cardiac output secondary to a reduced peripheral resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Resistance to superinfection with Plasmodium berghei in rats fed a protein-free diet

The development of resistance to reinfection with Plasmodium berghei was studied in rats in which the primary infection had been almost totally suppressed by feeding a protein-free diet (peak parasitaemia 0.5%; patent for only the first four days after inoculation) On Days 5, 9, 15, 23 and 28 after primary inoculation groups of animals were challenged with the same strain of parasite. At the same time the diet was changed to that of a 17% casein formula. The development of resistance as judged by the level of parasitaemia following challenge reached a significant level nine days after the primary inoculation and almost complete protection by Day 23 of the study. The protective activity was immunological since it could be transferred to other animals by a single intravenous injection of a suspension of spleen cells from infected donors. The study illustrates that infected animals experiencing severe protein malnutrition are still capable of mounting a substantial immune response to malaria.

Studies on parasitic crisis in malaria: I. Signs of impending crisis in Plasmodium berghei infections of the white rat

White rats infected with Plasmodiam berghei were observed throughout the course of infection for indicators of impending crisis (penultimate stage). Two ratios, percentage young compact forms:percentage trophozoites and percentage reticulocytes:percentage parasitaemia, were reliable markers of the penultimate period and impending crisis. When the percentage young compact form:percentage trophozoite ratio exceeded 1.0 and was coincident with an increase of uninfected reticulocytes in the circulation (as given by a percentage reticulocyte:percentage parasitaemia ratio greater than 1.0) the parasitaemic decline of crisis consistently occurred 24-48 hours later.