Studies on the lipids of Plasmodium knowlesi-infected rhesus monkeys (Macaca mulatta). II. Changes in serum non-esterified fatty acids

Serum lipids and lipoproteins in malaria--a systematic review and meta-analysis

BACKGROUND

Serum lipid profile changes have been observed during malaria infection. The underlying biological mechanisms remain unclear. The aim of this paper is to provide an overview on those serum lipid profile changes, and to discuss possible underlying biological mechanisms and the role of lipids in malaria pathogenesis.

METHODS

A systematic review and meta-analysis to determine lipid profile changes during malaria was conducted, following PRISMA guidelines. Without language restrictions, Medline/PubMed, Embase, Cochrane Central Register of Controlled Trials, Web of Science, LILACS, Biosis Previews and the African Index Medicus were searched for studies published up to 11 July, 2013, that measured serum lipid parameters in malaria patients. Also, major trial registries were searched. Mean differences in lipid profile parameters were combined in fixed and random effects meta-analysis, with a separate analysis for different groups of controls (healthy, other febrile illnesses or very low parasitaemia). These parameters were also compared between severe malaria and uncomplicated malaria. Funnel plots were used to test for publication bias.

RESULTS

Of 2,518 studies reviewed, 42 met the criteria for inclusion in the qualitative analysis, and of these, 15 reported the necessary data for inclusion in the meta-analysis for cholesterol; nine for high-density lipoprotein (HDL), eight for low-density lipoprotein (LDL), and nine for triglycerides, respectively. Total cholesterol, HDL and LDL concentrations were lower in malaria and other febrile diseases compared to healthy controls. The decline was more pronounced and statistically significant during malaria compared to other febrile diseases. These results were consistent across included studies. Triglycerides were raised compared to healthy controls, but not statistically significant when compared to symptomatic controls.

CONCLUSIONS

This meta-analysis suggests that the observed lipid profile changes are characteristic for malaria. Although a definite link with the pathogenesis of malaria cannot yet be demonstrated, plausible hypotheses of biological mechanisms involving host lipid alterations and the pathogenesis of malaria exist. An increased research effort to elucidate the precise pathways is warranted, since this could lead to better understanding of malaria pathophysiology and consequently to novel treatment approaches.

Role of fatty infiltration during malaria in rhesus monkey

P. cynomolgi B-rhesus monkey model of malarial infection has been used to study lipid infiltration in host tissues in early (exoerythrocytic) and late (chronic) stages of malaria infection. Histochemically we could demonstrate significant infiltration of neutral & total lipids in liver during the exoerythrocytic stage and in liver and kidney in the erythrocytic stage. The parasite used in the study closely resembles the human parasite P. vivax. It has a defined prepatent period, can be cyclically passaged with ease and non fatal in nature.

Altered plasma lipid pattern in falciparum malaria

Plasma levels of HDL, LDL, total cholesterol and triglycerides were measured in 60 patients with falciparum malaria (37 severe cases and 23 mild) and in 83 healthy individuals, to study malaria-induced changes in plasma lipids. Triglyceride levels were lower in the patients than in the controls but the difference was significant only for those with severe malaria (P < 0.001). In contrast, the levels of all the other plasma lipids were significantly higher (P < 0.001) in those with severe malaria than in those with mild malaria, and in the mild malaria cases compared with the controls. Initially LDL cholesterol was estimated by the Friedwald formula, but this gave negative values in a few cases of severe malaria. Plasma lipoproteins were therefore also measured by nephelometry; the estimated levels of S particles, corresponding to LDL, were then found to be lower in all malaria cases than in the controls (P < 0.001) but never negative. Interestingly, levels of L particles in the patients with severe malaria were significantly elevated compared with the other patients and controls (P < 0.001), indicating impaired metabolism of chylomicrons. Plasma albumin, considered a negative acute phase protein (i.e. its level decreases as a consequence of the acute phase response), was reduced significantly and was directly correlated to HDL cholesterol levels (r = 0.715 and r = 0.895, respectively) in both mild and severe malaria. Follow-up of 22 of the severe malaria cases three weeks after treatment indicated that, while triglycerides had returned to similar levels to those in the controls, total cholesterol levels were still elevated and could give misleading results if lipid profiles were used, immediately after malaria infection, to assess an individual's risk of developing atherosclerosis.

Uninfected red cells from malaria-infected blood: alteration of fatty acid composition involving a serum protein: an in vivo and in vitro study

Alteration of uninfected erythrocytes from Plasmodium (the malaria parasite)-infected blood remained an open question. In this study we compared the in vivo fatty acid compositions of control and uninfected monkey erythrocytes. A large (40%) increase in the linoleic acid level was observed, which was recovered mostly in neutral lipids. An in vitro system was developed to study medium-mediated alterations of cultured erythrocytes by Plasmodium falciparum. The increase in the linoleate level was reproduced in vitro and was also localized in the neutral lipid fraction, especially in triacylglycerols. Studies using proteolytic digestion and heat denaturation showed that a heat-labile serum protein is indispensable for the increase in the linoleate level of red cells treated with the supernatant of P. falciparum cultures. Both the function and the mechanism of this modification of uninfected erythrocytes still remain unknown.

Fatty acids, fibrinogen and blood flow: a general mechanism for hyperfibrinogenemia and its pathologic consequences

Plasma fibrinogen is elevated in various stressful states and conditions in which active mobilization of free fatty acids (FFA) occurs. Reduction of plasma FFA by an assortment of hypolipidemic drugs is consistently followed by a decrease in the accompanying hyperfibrinogenemia. A direct link between FFA and fibrinogen has been demonstrated in animals, and in experiments employing incubated liver slices. Based on these clinical and experimental observations, we postulate that hepatic fibrinogen synthesis is stimulated by FFA. Since fibrinogen is a major determinant of whole blood viscosity, erythrocyte aggregation, and sludging of red cells in terminal and pre-terminal blood vessels, we propose that microcirculatory blood flow may be impaired in the presence of chronically elevated plasma FFA levls. Consequently, hypolipidemic drugs may be effective in prevention of circulatory complications associated with FFA-induced hyperfibrinogenemia.

Octadecenoic fatty acids and their association with hemolysis in malaria

Octadecenoic fatty acids have been implicated in prehemolytic and hemolytic phenomena associated with malaria. Oleic [18:1 (n-9)] and cis-vaccenic [18:1 (n-7)] acids were found and quantified in the major neutral and phospholipids of the erythrocytes and plasmas of normal and Plasmodium lophurae-infected ducks, and in the parasite itself. The octadecenoic fatty acids were elevated over normal values in the major phospholipid classes of infected erythrocytes, in the erythrocyte-specific alkoxy phosphatidylethanolamine of infected erythrocytes, and in the plasma unesterified fatty acids, triacylglycerols, cholesterol esters and phosphatidlycholine of infected ducklings. Oleic acid was the major fatty acid of P. lophurae (33% total lipid fatty acid). Theoretical considerations of octadecenoic fatty acid modifications of erythrocyte membrane structure and function in malaria are discussed.