Pathogenesis and laboratory diagnosis of faciparum malariae.
Q35 marksShort Essays
Answer
PathogenesisPlasmodium falciparum is transmitted by the bite of an infective female Anopheles mosquito, which injects sporozoites into the bloodstream; these travel to the liver and multiply within hepatocytes (exoerythrocytic/hepatic schizogony), producing thousands of merozoites that are released into the blood and invade red blood cells, initiating the erythrocytic cycle. Within RBCs, the parasite matures through ring, trophozoite, and schizont stages, ultimately rupturing the cell to release more merozoites, which invade fresh RBCs — this cyclical rupture (occurring roughly every 48 hours, though often less synchronized in falciparum) produces the periodic fever, as rupture releases parasite antigens/toxins (notably GPI-anchored proteins) that trigger a pro-inflammatory cytokine response (TNF-α, IL-1, IL-6).
P. falciparum is distinguished by several features that account for its severe disease potential:
All erythrocyte stages, not just young RBCs, can be invaded, permitting very high parasitaemia.
Infected RBCs develop surface knob-like protrusions displaying the parasite protein PfEMP1, which mediates cytoadherence — infected cells stick to vascular endothelium in deep capillaries/venules (sequestration), particularly in the brain, placenta, and other organs — removing the most mature (and most cytoadherent) parasite stages from peripheral circulation and causing microvascular obstruction, tissue hypoxia, and organ dysfunction. This sequestration underlies the severe complications: cerebral malaria, placental malaria, renal impairment, and severe anaemia.
No persistent liver (hypnozoite) stage exists (unlike P. vivax/P. ovale), so relapse does not occur, but recrudescence from undertreated blood-stage parasites can.
Laboratory diagnosis
Peripheral blood smear microscopy — the gold standard; thick smear for screening/detecting low parasitaemia, thin smear for species identification and quantifying parasitaemia. P. falciparum is identified by delicate ring forms (often with double chromatin dots and multiple rings per cell), absence of larger trophozoite/schizont stages in peripheral blood (since these sequester in deep vessels), and characteristic crescent/banana-shaped gametocytes.
Rapid diagnostic tests (RDTs) — immunochromatographic tests detecting P. falciparum-specific histidine-rich protein-2 (HRP-2) or lactate dehydrogenase/aldolase antigens; rapid, useful in field settings without microscopy access.
QBC (Quantitative Buffy Coat) method — fluorescence-based concentration technique, more sensitive than routine smear but does not allow precise species identification.
PCR — highly sensitive/specific, useful for detecting low-level or mixed infections, though not routinely used for acute point-of-care diagnosis.
Serology — detects past exposure, not useful for diagnosing acute infection.
