The pathophysiology of malaria

The metabolism of platelet-activating factor in severe and cerebral malaria

In order to examine the effects of platelet-activating factor (PAF) in complicated Plasmodium falciparum infections, plasma concentrations of lyso-PAF, stable metabolite and principal precursor of PAF, were measured in 25 Vietnamese adults with severe malaria. The concentration of PAF in the cerebrospinal fluid (CSF) was determined in a sub-group of 23 comatose patients and, together with that of lyso-PAF, in the plasma of 20 patients on recovery of consciousness. The concentration of lyso-PAF in the plasma was depressed on admission to hospital (median [range]; 21 [8-143] vs. 293 [215-410] ng/ml in 10 controls; P < 0.001). There was, however, no change in plasma activity of acetylhydrolase which converts PAF to lyso-PAF (P > 0.01 vs. controls) while simultaneous reduction in the concentration of lipoproteins associated with lyso-PAF were less than those of lyso-PAF per se in the plasma. The plasma concentration of lyso-PAF on admission was associated with parasitaemia and the concentration of serum triglycerides (rs = -0.42, P = 0.04 in each case), the latter being consistent with hepatic effects of PAF reported in previous studies. CSF concentrations of PAF on admission were low (2.3 [0.5-7.7] vs. 0.9 [0-2.5] ng/ml after recovery, P < 0.01) compared with values reported previously in bacterial meningitis. Plasma concentrations of lyso-PAF after recovery lay between admission and control values. While increased availability of PAF may reflect parasite burden and may modulate liver-mediated metabolic disturbances such as hypoglycaemia and lactic acidosis, the role of PAF in cerebral malaria is uncertain.

Rosette formation by Plasmodium vivax

In contrast to Plasmodium falciparum, infections with P. vivax are seldom fatal. Red blood cells containing mature forms of P. falciparum sequester in the microvasculature of vital organs, and adhere to vascular endothelium (cytoadherence) and to uninfected red cells (rosetting). Rosetting of P. falciparum has been associated with the lethal syndrome of cerebral malaria. We have studied the rosetting properties of red blood cells infected with P. vivax obtained from adults with acute malaria in Thailand. Of 35 parasite isolates studied, 25 (71%) showed rosetting with a mean proportion of 41% of infected red cells (SD 34%, range 14-100%). Rosetting of P. vivax was related to maturation of the parasite; only cells containing parasites with visible malaria pigment rosetted. Rosetting and parasitaemia were not correlated. However, unlike P. falciparum, cells infected with P. vivax did not adhere to human umbilical vein endothelial cells, to C32 melanoma cells, to platelets, or to purified adhesion receptor molecule CD36. These findings suggest that thrombocytopenia in vivax malaria is not related to platelet-red cell attachment, and that rosetting alone is insufficient to cause the syndrome of cerebral malaria.

Acute lung injury complicating imported Plasmodium falciparum malaria

STUDY OBJECTIVE

To characterize adult patients with acute lung injury complicating severe imported Plasmodium falciparum malaria.

DESIGN AND SETTING

Retrospective study of patients with severe P falciparum malaria admitted to the medical ICU of a university hospital infectious diseases department.

PATIENTS

Forty adults with complicated malaria, with (group 1, 12 patients) or without (group 2, 28 patients) acute lung injury.

RESULTS

Patients with acute lung injury had a higher simplified acute physiology score on admission (24.2 +/- 3.2 vs 13.7 +/- 0.7 in group 2, p < 0.0001) and a longer time interval to adequate antimalarial therapy (8.8 +/- 2.5 vs 4.9 +/- 0.6 days in group 2, p = 0.046). Of the nine group 1 patients given mechanical ventilation, eight had a PaO2/FIO2 < or = 200 mm Hg. Two patients with moderate hypoxemia received oxygen through a nasal tube and one received continuous positive airway pressure via a face mask. Acute renal failure, unrousable coma, metabolic acidosis, and shock were significantly more common among group 1 patients. The number of complications of malaria was significantly higher in patients with acute lung injury (4.7 +/- 0.5 vs 1.6 +/- 0.1 in group 2, p < 0.0001). Five patients, including four with acute lung injury, had evidence of bacterial infection (pneumonia or primary bacteremia) at ICU admission. Four patients with acute lung injury died (33%) vs one patient without acute lung injury (4%, p = 0.022).

CONCLUSIONS

Acute lung injury is more likely to occur in patients with extremely severe, multisystemic P falciparum malaria. In patients with acute lung injury and septic shock, bacterial coinfection should be suspected and treated empirically since it contributes substantially to early mortality.

In vivo sequestration of Plasmodium falciparum-infected human erythrocytes: a severe combined immunodeficiency mouse model for cerebral malaria

Cerebral malaria is a fatal complication of infection by Plasmodium falciparum in man. The neurological symptoms that characterize this form of malarial disease are accompanied by the adhesion of infected erythrocytes to the vasculature of the brain. To study this phenomenon in vivo, an acute phase severe combined immunodeficiency (SCID) mouse model was developed in which sequestration of P. falciparum-infected human erythrocytes took place. During acute cerebral malaria in humans, the expression of intercellular adhesion molecule-1 (ICAM-1) is induced in vascular endothelium by inflammatory reactions. Acute phase ICAM-1 expression can also be obtained in SCID mice. The endothelium of the midbrain region was the most responsive to such inflammatory stimulus. It is noteworthy that the reticular formation in the midbrain controls the level of consciousness, and loss of consciousness is a symptom of cerebral malaria. We found that infected human erythrocytes were retained 24 times more than normal erythrocytes in ICAM-1-positive mouse brain. Sequestration to the brain was reduced by anti-ICAM-1 antibodies. These in vivo results were confirmed by the binding of P. falciparum-infected erythrocytes to the ICAM-1-positive endothelium in tissue sections of mouse brain. We conclude that the SCID mouse serves as a versatile in vivo model that allows the study of P. falciparum-infected erythrocyte adhesion as it occurs in human cerebral malaria. Upregulation of ICAM-1 expression in the region of the midbrain correlates with increased retention of malaria-infected erythrocytes and with the symptoms of cerebral malaria.

Dichloroacetate for lactic acidosis in severe malaria: a pharmacokinetic and pharmacodynamic assessment

Lactic acidosis and hypoglycemia are potentially lethal complications of falciparum malaria. We have evaluated the pharmacokinetics and pharmacodynamics of dichloroacetate ([DCA], 46 mg/kg infused over 30 minutes), a stimulant of pyruvate dehydrogenase and a potential treatment for lactic acidosis, in 13 patients with severe malaria and compared the physiological and metabolic responses with those of a control group of patients (n = 32) of equivalent disease severity. The mean +/- SD peak postinfusion level of DCA was 78 +/- 23 mg/L, the total apparent volume of distribution was 0.75 +/- 0.35 L/kg, and systemic clearance was 0.32 +/- 0.16 L/kg/h. Geometric mean (range) venous lactate concentrations in control and DCA recipients before treatment were 4.5 (2.1 to 19.5) and 5.5 (2 to 15.4) mmol/L, respectively (P > .1). A single DCA infusion decreased lactate concentrations from baseline by a mean of 27% after 2 hours, 40% after 4 hours, and 41% after 8 hours, compared with decreases of 5%, 6%, and 16%, respectively, in controls (P = .032). These changes were preceded by rapid and marked decreases in pyruvate concentrations. Arterial pH increased from 7.328 to 7.374 (n = 10, P < .02) 2 hours after the infusion. Hypoglycemia was prevented by infusing glucose at 3 mg/kg/min. There was no clinical, electrocardiographic, or laboratory evidence of toxicity. These results suggest that DCA should be investigated further as an adjunctive therapy for severe malaria.

Glycerol metabolism in severe falciparum malaria

Gluconeogenesis and liver blood flow (LBF) in severe falciparum malaria were assessed from the clearance and metabolic response to intravenously administered glycerol (0.3 g/kg) and Indocyanine Green ([ICG] 0.4 mg/kg), respectively. Fasting baseline blood glycerol concentrations (mean +/- SD) were significantly higher in acute malaria (133 +/- 65 mumol/L, n = 14), than in convalescence (65 +/- 31 mumol/L, n = 9, P = .01), but basal triacylglycerol concentrations were similar. Estimated glycerol turnover was also more than twice as high in acute malaria compared with convalescence (1.36 +/- 0.87 v 0.54 +/- 0.15 mumol.min-1.kg-1, P = .015). The increment in plasma glucose (AUC0-55 min) following glycerol infusion was greater during acute malaria compared with convalescence (median [range], +31.6 [-0.9 to +107.6] v +14.5 [-103 to +27.1] mmol.min-L-1, P < .05), but the insulin increments were similar (P = .9), indicating reduced tissue insulin sensitivity. The increment in venous lactate (AUC0-55 min) was higher in severely ill patients (17.2 [-7.8 to +53.4] mmol.min.L-1, n = 10) compared with patients with moderately severe malaria (-3.1 [-8.7 to 3.2] mmol.min-L-1, n = 4, P = .01). LBF estimated from ICG clearance was lower during acute illness than in convalescence (mean +/- SD, 15.5 +/- 2.3 v 18.6 +/- 2.9 mL.min-1.kg-1, P = .007) and correlated inversely with the basal venous lactate concentration (rs = .53, P < .05). LBFs less than 15 mL.min-1.kg-1 were associated with hyperlactatemia, and all four fatal cases had LBFs of less than 12 mL.min-1.kg-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Convulsions in childhood malaria

A retrospective survey was conducted of all 2911 children admitted with malaria to 4 provincial hospitals in eastern Thailand between 1977 and 1987. 96 (3.3%) had cerebral malaria of whom 21 (22%) died, 225 (7.7%) had convulsions but were not comatose (4 died), and 2590 were conscious and had no fits (5 died). Thus the relative risk of a fatal outcome associated with convulsions, in the absence of cerebral malaria, was 9.2 (95% confidence interval [CI] = 2.5-34.1), P = 0.004. Overall, Plasmodium falciparum caused 81% of infections, P. vivax 16%, and 3% were mixed. Convulsions without cerebral malaria were more common in children under 3 years old (16%) compared with older children (3%): relative risk 5.6 (95% CI = 4.2-7.5), and were significantly associated with falciparum malaria (8.3%) compared with vivax malaria (4.7%): relative risk 1.7 (95% CI = 1.1-2.7). Convulsions are an important complication of malaria in young children, and are associated specifically with P. falciparum infection, even in otherwise uncomplicated malaria.

Brain swelling and ischaemia in Kenyans with cerebral malaria

Computed tomography was performed on 14 unconscious Kenyan children recovering from cerebral malaria (seven of whom had another scan 12-120 days later) to elucidate the cause of intracranial hypertension and neurological sequelae. Brain swelling, defined as a loss of cerebrospinal fluid spaces, was documented in six children, while a further two had conspicuously small ventricles only. There was severe intracranial hypertension in the two children with definite brain swelling in whom intracranial pressure was monitored. There was no evidence of acute hydrocephalus or vasogenic oedema. Four children with brain swelling also had widespread low density areas suggestive of ischaemic damage. The patterns of damage were not uniform but were consistent with a critical reduction in cerebral perfusion pressure (which was documented in the two in whom this was monitored), hypoglycaemia, or status epilepticus. All four had serious neurological sequelae. These data suggest that brain injury in cerebral malaria may be due in part to secondary systemic and intracranial factors as well as to the direct effect of intravascular sequestration.

Rosetting

Why do some individuals get severe falciparum malaria while others don't? Rosetting (the binding of uninfected erythrocytes to Plasmodium falciparum-infected erythrocytes), together with endothelial cytoadherence, has been shown to play a crucial role in the obstruction of the microvosculoture in P. falciparum malaria. Here, Mats Wahlgren, Victor Fernandez, Carin Scholonder and Johan Carlson review the literature surrounding rosetting.

Cytokines and T-cell response in malaria

The intracellular protozoan Plasmodium sp induces a complex immune response which sometimes implies serious pathological effects for the host. According to in vitro studies and epidemiological surveys, several effector mechanisms are displayed against plasmodial blood stages and a large interaction between humoral and cell-mediated immunity is presumed to occur among protected individuals. The key role of T cells in the antiplasmodial immune response is now well established, but all the regulatory heterogenous mechanisms are not yet fully known. An increasing body of data shows a dual role during malaria attack for some cytokines released by monocytes and macrophages (TNF, IL-1, IL-6) or by T cells (IFN-gamma, lymphotoxin (LT), IL-4). The importance of some plasmodial proteins in the cytokine-induced pathology and the stimulation of a preferential TH1 or TH2 mediated immune response to achieve protective immunity against Plasmodium sp are discussed.

Lactic acidosis and hypoglycaemia in children with severe malaria: pathophysiological and prognostic significance

Serial clinical and metabolic changes were monitored in 115 Gambian children (1.5-12 years old) with severe malaria. Fifty-three children (46%) had cerebral malaria (coma score < or = 2) and 21 (18%) died. Admission geometric mean venous blood lactate concentrations were almost twice as high in fatal cases as in survivors (7.1 mmol/L vs. 3.6 mmol/L; P < 0.001) and were correlated with levels of tumour necrosis factor (r = 0.42, n = 79; P < 0.0001) and interleukin 1-alpha (r = 0.6, n = 34; P < 0.0001). Admission blood venous glucose concentrations were lower in fatal cases than survivors (3.2 mmol/L, vs. 5.8 mmol/L; P < 0.0001). Treatment with quinine was associated with significantly more episodes of post-admission hypoglycaemia when compared with artemether or chloroquine. After treatment, lactate concentrations fell rapidly in survivors but fell only slightly, or rose, in fatal cases. Plasma cytokine levels fluctuated widely after admission. Sustained hyperlactataemia (raised lactate concentrations, 4 h after admission) proved to be the best overall prognostic indicator of outcome in this series. Lactic acidosis is an important cause of death in severe malaria.

Neutralizing monoclonal antibodies to glycosylphosphatidylinositol, the dominant TNF-alpha-inducing toxin of Plasmodium falciparum: prospects for the immunotherapy of severe malaria

Tumour necrosis factor-alpha (TNF-alpha) is an endogenous mediator of shock and inflammation. Many of the life-threatening and severe pathologies associated with complicated and cerebral malaria are thought to result from the overproduction of this cytokine in response to agents of parasite origin. The identification and characterization of these agents may therefore provide the molecular basis for a detailed understanding of the disease process. Recently it has been shown that glycosylphosphatidylinositols are a novel class of glycolipid toxin produced by the parasite, which substitute for the endogenous inositolglycan-based signal transduction pathways of the host. Glycosylphosphatidylinositol stimulates high levels of TNF-alpha and interleukin-1 production by macrophages and induces hypoglycaemia through an insulin-mimetic activity, and may therefore contribute to the cerebral syndrome and other malarial pathophysiology. That monoclonal antibodies to parasite-derived glycosylphosphatidylinositol can neutralize the toxic activities of whole parasite extracts is also demonstrated here. These findings suggest a central role for glycosylphosphatidylinositol of parasite origin in the aetiology of severe malaria and suggest novel approaches for the immunotherapy or immunoprophylaxis of disease.

Amplification of cytoadherence in cerebral malaria: towards a more rational explanation of disease pathophysiology

Cerebral malaria in man and in mice is the consequence of a cascade of events, involving the production of toxins by the parasite and cytokines by the host, and eventually leading to the amplification of the expression of the receptors for cytoadherence on brain capillary endothelial cells. Variations in the intrinsic characteristics of parasite isolates or the genetic make-up of the host and the degree of antimalarial immunity can modulate this sequence of events. A working hypothesis is proposed in which two features of the parasite, the ability to cytoadhere and to produce toxins, are clearly dissociated and where the amplification of cytoadherence receptors is considered crucial. This hypothesis, illustrated by new data from human malaria and rodent models, suggests that cerebral malaria may occur when these features occur together during an infection, while not necessarily within the same parasite clone.

Relation of the stage of parasite development in the peripheral blood to prognosis in severe falciparum malaria

Admission blood films from 72 patients who died of severe falciparum malaria (50 Thai adults, 22 Gambian children) were matched retrospectively for parasitaemia with equal numbers of survivors. The peripheral blood parasites from fatal cases were more mature than those from survivors. Tiny rings (TR) comprised > 50% of parasites in 47/72 (65%) survivors but only 12/72 (17%) of fatal cases (P < 0.001). Parasites containing visible pigment (MTS: mature trophozoites and schizonts) comprised < 20% of the total parasite count in 10/72 (14%) survivors compared with 31/72 (43%) fatal cases (P < 0.001). Of the 39 patients with > 10(4) MTS/microL, 30 (81%) died. These findings were confirmed in a prospective study of 279 adult Thai patients admitted sequentially with acute falciparum malaria. Only 4 of the 19 fatal cases (21%) had > 50% TR, compared with 130 of 260 (50%) survivors, whereas > 20% MTS were found in 10/19 (53%) fatal cases, compared with 28/108 (27%) severe malaria survivors, and 26/155 (17%) patients with moderately severe malaria (P = 0.001). As a predictor of fatal outcome, the finding of either > 10(4) MTS/microL or > 5 x 10(5) parasites/microL in severe malaria had a sensitivity of 90% (95% confidence interval [CI] = 75-97%) and a specificity of 72% (95% CI = 59-86%). These observations are consistent with the hypothesis that a predominance of mature parasites in the peripheral blood reflects a greater sequestered biomass, and thus more severe disease. Simple microscopical assessment of parasite maturity on an admission blood slide provides important pathophysiological and prognostic information in severe falciparum malaria.

The effects of multiplication and synchronicity on the vascular distribution of parasites in falciparum malaria

The sequestration of erythrocytes containing mature forms of Plasmodium falciparum in the microvasculature of vital organs may cause large discrepancies between the peripheral blood parasite count and the total body parasite burden in falciparum malaria. Despite this, parasitaemia is widely used as an indicator of prognosis and response to treatment. A simple mathematical model describing the changes in circulating and sequestered parasite numbers during acute falciparum malaria is presented. The model uses two parameters only; the standard deviation (SD) of parasite age since merogony (schizogony) as as a measure of synchronicity, and a multiplication factor each 48 h asexual life cycle. The model predicts that during the rising phase of the infection the ratio of circulating to sequestered parasites is dependent largely on the synchronicity of infection rather than multiplication rate, and that in synchronous infections parasitaemias will show considerable fluctuation when the mean stage of parasite development is in the second half of the asexual life cycle. The model fitted well to serial parasite counts from 4 patients with acute uncomplicated falciparum malaria whose infections failed to respond to ciprofloxacin. All four infections were synchronous (SD < or = 4 h), and showed large fluctuations in parasitaemia over short periods related to synchronous sequestration and subsequent reinvasion following merogony. The parasite multiplication rate was determined mainly by the efficiency of merogony or merozoite invasion rather than clearance of circulating parasitized erythrocytes. This suggests that the spleen is relatively inactive during the rising phase of the infection. Quinine treatment did not prevent sequestration but did stop subsequent multiplication.(ABSTRACT TRUNCATED AT 250 WORDS)