Parasite biomass-related inflammation, endothelial activation, microvascular dysfunction and disease severity in vivax malaria.

Bridget E Barber,Matthew J Grigg,Timothy William,Ric N Price,Uma Parameswaran,Kim A Piera,Nicholas M Anstey,Tsin W Yeo,Mary M Stevenson

doi:10.1371/journal.ppat.1004558

Abstract

Plasmodium vivax can cause severe malaria, however its pathogenesis is poorly understood. In contrast to P. falciparum, circulating vivax parasitemia is low, with minimal apparent sequestration in endothelium-lined microvasculature, and pathogenesis thought unrelated to parasite biomass. However, the relationships between vivax disease-severity and total parasite biomass, endothelial autocrine activation and microvascular dysfunction are unknown. We measured circulating parasitemia and markers of total parasite biomass (plasma parasite lactate dehydrogenase [pLDH] and PvLDH) in adults with severe (n = 9) and non-severe (n = 53) vivax malaria, and examined relationships with disease-severity, endothelial activation, and microvascular function. Healthy controls and adults with non-severe and severe falciparum malaria were enrolled for comparison. Median peripheral parasitemia, PvLDH and pLDH were 2.4-fold, 3.7-fold and 6.9-fold higher in severe compared to non-severe vivax malaria (p = 0.02, p = 0.02 and p = 0.015, respectively), suggesting that, as in falciparum malaria, peripheral P. vivax parasitemia underestimates total parasite biomass, particularly in severe disease. P. vivax schizonts were under-represented in peripheral blood. Severe vivax malaria was associated with increased angiopoietin-2 and impaired microvascular reactivity. Peripheral vivax parasitemia correlated with endothelial activation (angiopoietin-2, von-Willebrand-Factor [VWF], E-selectin), whereas markers of total vivax biomass correlated only with systemic inflammation (IL-6, IL-10). Activity of the VWF-cleaving-protease, ADAMTS13, was deficient in proportion to endothelial activation, IL-6, thrombocytopenia and vivax disease-severity, and associated with impaired microvascular reactivity in severe disease. Impaired microvascular reactivity correlated with lactate in severe vivax malaria. Findings suggest that tissue accumulation of P. vivax may occur, with the hidden biomass greatest in severe disease and capable of mediating systemic inflammatory pathology. The lack of association between total parasite biomass and endothelial activation is consistent with accumulation in parts of the circulation devoid of endothelium. Endothelial activation, associated with circulating parasites, and systemic inflammation may contribute to pathology in vivax malaria, with microvascular dysfunction likely contributing to impaired tissue perfusion.

Highlights

While P. falciparum accounts for a majority of severe and fatal malaria cases worldwide, P. vivax is a major cause of morbidity outside of Africa, causing an estimated 70–390 million malaria cases per year [1]
Total parasite burden and activation and dysfunction of the endothelial cells lining blood vessels all contribute to severe disease in falciparum malaria, but have not been evaluated in severe vivax malaria
We measured parasite lactate dehydrogenase and P. vivax-pLDH (PvLDH) as proxies of total parasite biomass and found that, as in falciparum malaria, the total biomass of vivax parasites is underestimated by counting parasites circulating in peripheral blood, suggesting a hidden burden of vivax parasites

Summary

Introduction

While P. falciparum accounts for a majority of severe and fatal malaria cases worldwide, P. vivax is a major cause of morbidity outside of Africa, causing an estimated 70–390 million malaria cases per year [1]. Severe and fatal disease is characterised by cytoadherence of parasitized red blood cells (RBCs) to activated and dysfunctional endothelium, leading to parasite sequestration with microvascular obstruction [11,12,13]. Total parasite biomass is underestimated by circulating parasitemia quantitated by microscopy of peripheral blood, and is more accurately quantitated by plasma P. falciparum histidine rich protein-2 (HRP2). Pathogenesis and disease severity in falciparum malaria are both biomassrelated: in contrast to peripheral parasitemia [15,16,17], plasma HRP2 is strongly and independently correlated with disease severity and mortality among both children [17,18,19,20] and adults [21,22]

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