Estimation of the total parasite biomass in acute falciparum malaria from plasma PfHRP2

Abstract

Background: In falciparum malaria sequestration of erythrocytes containing mature forms of Plasmodium falciparum in the microvasculature of vital organs is central to pathology, but quantitation of this hidden sequestered parasite load in vivo has not previously been possible. The peripheral blood parasite count measures only the circulating, relatively non-pathogenic parasite numbers. P. falciparum releases a specific histidine-rich protein (PfHRP2) into plasma. Quantitative measurement of plasma PfHRP2 concentrations may reflect the total parasite biomass in falciparum malaria.

Methods and findings: We measured plasma concentrations of PfHRP2, using a quantitative antigen-capture enzyme-linked immunosorbent assay, in 337 adult patients with falciparum malaria of varying severity hospitalised on the Thai-Burmese border. Based on in vitro production rates, we constructed a model to link this measure to the total parasite burden in the patient. The estimated geometric mean parasite burden was 7 x 10(11) (95% confidence interval [CI] 5.8 x 10(11) to 8.5 x 10(11)) parasites per body, and was over six times higher in severe malaria (geometric mean 1.7 x 10(12), 95% CI 1.3 x 10(12) to 2.3 x 10(12)) than in patients hospitalised without signs of severity (geometric mean 2.8 x 10(11), 95% CI 2.3 x 10(11) to 3.5 x 10(11); p < 0.001). Parasite burden was highest in patients who died (geometric mean 3.4 x 10(12), 95% CI 1.9 x 10(12) to 6.3 x 10(12); p = 0.03). The calculated number of sequestered parasites increased with disease severity and was higher in patients with late developmental stages of P. falciparum present on peripheral blood smears. Comparing model and laboratory estimates of the time of sequestration suggested that admission to hospital with uncomplicated malaria often follows schizogony-but in severe malaria is unrelated to stage of parasite development.

Conclusion: Plasma PfHRP2 concentrations may be used to estimate the total body parasite biomass in acute falciparum malaria. Severe malaria results from extensive sequestration of parasitised erythrocytes.

Figure 1. Model Describing the PfHRP2 Concentration Derived from 10¹² Total Body Parasites in the Last Cycle

Total PfHRP2 derives from the amount produced by the generation present in that cycle and the amount produced at schizont rupture of the previous cycle (broken lines). The concentration accumulated from previous production is added to this (black lines). PfHRP2 produced in previous cycles will decline over time (dotted lines). Values chosen for the parameters in this illustration were as follows: Hct, 0.35; body weight, 50 kg; and parasite multiplication factor, eight.

Figure 2. Estimated Total Parasite Biomass as a Function of the Parasite Multiplication Rate, Using the Model Described in the Text

Values chosen for the parameters were as follows: PfHRP2 concentration, 1,000 μg/l; Hct, 0.35; and body weight, 50 kg.

Figure 3. Total (Log₁₀) Parasite Biomass Estimated from Plasma PfHRP2 Concentrations in a Peripheral Blood Sample in 337 Patients with Falciparum Malaria

Figure 4. Total (Log₁₀) Parasite Biomass Estimated from Plasma PfHRP2 Concentrations

Plasma PfHRP2 concentrations (squares) in peripheral blood samples from 170 patients with uncomplicated malaria, 139 patients with severe malaria who survived, and 28 patients who died. For comparison the calculated circulating parasite biomass is also displayed (circles); this was calculated from the parasitaemia (per 1,000 red blood cells) in a peripheral blood sample and the estimated total red blood cell mass (see text for formulas).