Malaria parasite clearance

Abstract

Following anti-malarial drug treatment asexual malaria parasite killing and clearance appear to be first order processes. Damaged malaria parasites in circulating erythrocytes are removed from the circulation mainly by the spleen. Splenic clearance functions increase markedly in acute malaria. Either the entire infected erythrocytes are removed because of their reduced deformability or increased antibody binding or, for the artemisinins which act on young ring stage parasites, splenic pitting of drug-damaged parasites is an important mechanism of clearance. The once-infected erythrocytes returned to the circulation have shortened survival. This contributes to post-artesunate haemolysis that may follow recovery in non-immune hyperparasitaemic patients. As the parasites mature Plasmodium vivax-infected erythrocytes become more deformable, whereas Plasmodium falciparum-infected erythrocytes become less deformable, but they escape splenic filtration by sequestering in venules and capillaries. Sequestered parasites are killed in situ by anti-malarial drugs and then disintegrate to be cleared by phagocytic leukocytes. After treatment with artemisinin derivatives some asexual parasites become temporarily dormant within their infected erythrocytes, and these may regrow after anti-malarial drug concentrations decline. Artemisinin resistance in P. falciparum reflects reduced ring stage susceptibility and manifests as slow parasite clearance. This is best assessed from the slope of the log-linear phase of parasitaemia reduction and is commonly measured as a parasite clearance half-life. Pharmacokinetic-pharmacodynamic modelling of anti-malarial drug effects on parasite clearance has proved useful in predicting therapeutic responses and in dose-optimization.

Fig. 1

A comparison of parasite dynamics in human malaria infections as illustrated by Fairley [5] following his classic studies of induced malaria in volunteers. The total numbers of parasites in the body of an adult are shown in the vertical axes, and time in days is shown in the horizontal axis

Fig. 2

Parasite clearance times in adult Thai patients with vivax malaria after different treatments [–30]. Parasite counts were determined at ≤6 h intervals on thin films, and at ≤12 h intervals on thick films. The open circles are individual asexual parasite clearance times, the closed circles are corresponding gametocyte clearance times, and the red diamonds denote failure to respond and administration of rescue treatment

Fig. 3

Parasite clearance in acute falciparum malaria. Parasite counts were determined at ≤6 h intervals. These data are taken from studies in severe malaria for choroquine (in fully chloroquine sensitive malaria), quinine and artesunate, and in uncomplicated malaria for cipargamin [28, 33, 42, 56]

Fig. 4

Parasite clearance following the start of anti-malarial drug treatment with an ACT in falciparum malaria. After an initial and variable lag phase, which depends on the stage of parasite development, the decline in parasitaemia is generally log linear [23, 31, 32, 37, 38, 40, 42, 56, 97, 100, 122]. The rate constant of this decline, or its derivative half-life, is the best metric for the assessment of resistance to drugs acting on ring stage parasites-notably artemisinin derivatives [31, 37, 38, 40]. The simpler measure- the proportion of patients who have microscopy detectable parasitaemia on day 3 [100, 101] whilst useful for screening, is heavily dependent on starting parasite density; two infections with the same clearance half-lives (3 h) typically associated with full susceptibility to artemisinin derivative are compared with a 50-fold difference in admission parasitaemia which results in an 18-h difference in parasite clearance time. An artemisinin resistant infection (parasite clearance half-life 6 h) is shown for comparison [38]

Fig. 5

A thin immunofluorescence blood smear showing three red blood cells which stain positive for the P. falciparum ring erythrocyte stage antigen. The two lower cells also contain ring stage parasites which stain with acridine orange, the upper cell has no intraerythrocytic parasite indicating that it has already been removed by “pitting”. This is the main mechanism of ring stage parasite clearance in non-immune patients following treatment with artemisinin derivatives [34, 82, 83]

Fig. 6

Individual parasite clearance half-lives in relation to presenting parasite density (shown on a log scale per µL) in 6975 patients with acute uncomplicated falciparum malaria treated with an artemisinin derivative (from reference [32]). The upper panel shows data from areas unaffected by artemisinin resistance, the lower panel shows data from areas where artemisinin resistance is prevalent. There is no evidence for density dependence in parasite clearance rates

Fig. 7

Recrudescent falciparum malaria following administration of a slowly eliminated drug such as mefloquine, showing an example of the changes in total parasite numbers (blue) in the body as anti-malarial drug levels (red) first rise then fall. As drug levels fall below the minimum parasiticidal concentration (MPC) the rate of parasitaemia declines until it reaches reaching a temporary plateau, at which time the corresponding drug level is a minimum inhibitory concentration (MIC) [23, 117, 124]. Meanwhile levels of any dormant forms remain unchanged, while gametocyte densities rise as stage 5 gametocytes enter the circulation from sequestered sites. All contribute to qPCR parasite DNA measurements. Dormant forms are either cleared or “awaken” to form either asexual or sexual stages. The top right inset shows an individual patient example with female gametocyte specific Pfs 25 mRNA transcript densities shown in green and Pf18s DNA shown in blue (data from reference [124]). There are multiple mRNA transcripts per cell, but the rising DNA densities at the time of falling transcript numbers clearly indicates recrudescence