Abstract:
This study sought to measure medium-sized extracellular vesicles (mEVs) in plasma, when
patients have low Plasmodium falciparum early in infection. We aimed to define the relationship
between plasma mEVs and: (i) parasitaemia, (ii) period from onset of malaria symptoms until
seeking medical care (patient delay, PD), (iii) age and (iv) gender. In this cross-sectional study,
n = 434 patients were analysed and Nanosight Tracking Analysis (NTA) used to quantify mEVs
(vesicles of 150–500 nm diameter, isolated at 15,000 × g, β-tubulin-positive and staining for
annexin V, but weak or negative for CD81). Overall plasma mEV levels (1.69 × 1010 mEVs mL−1)
were 2.3-fold higher than for uninfected controls (0.51 × 1010 mEVs mL−1). Divided into four age
groups, we found a bimodal distribution with 2.5- and 2.1-fold higher mEVs in infected children
(<11 years old [yo]) (median:2.11 × 1010 mEVs mL−1) and the elderly (>45 yo) (median:1.92 × 1010
mEVs mL−1), respectively, compared to uninfected controls; parasite density varied similarly with
age groups. There was a positive association between mEVs and parasite density (r = 0.587,
p < 0.0001) and mEVs were strongly associated with PD (r = 0.919, p < 0.0001), but gender had no
effect on plasma mEV levels (p = 0.667). Parasite density was also exponentially related to patient
delay. Gender (p = 0.667) had no effect on plasma mEV levels. During periods of low parasitaemia
(PD = 72h), mEVs were 0.93-fold greater than in uninfected controls. As 75% (49/65) of patients
had low parasitaemia levels (20–500 parasites μL−1), close to the detection limits of microscopy of
Giemsa-stained thick blood films (5–150 parasites μL−1), mEV quantification by NTA could
potentially have early diagnostic value, and raises the potential of Pf markers in mEVs as early
diagnostic targets
