Malaria — Overview & Pathophysiology

Epidemiology

Causative Agents

Life Cycle of P. falciparum

1. Hepatic Stage (Pre-erythrocytic)

1. A feeding Anopheles mosquito injects sporozoites into the bloodstream.
2. Sporozoites travel to the liver within minutes. Two surface proteins — thrombospondin-related adhesive protein and circumsporozoite protein — bind to heparan sulfate proteoglycans on hepatocytes.
3. Inside hepatocytes, sporozoites differentiate into merozoites over 1–4 weeks.
4. Infected hepatocytes rupture and release merozoites into the blood.
   • P. vivax and P. ovale can form dormant hypnozoites in the liver, causing relapses weeks to months later.

2. Erythrocytic Stage

1. A lectin-like molecule on the merozoite surface binds to sialylated glycophorin (a red cell transmembrane protein), allowing the merozoite to invaginate into the RBC within a "digestive vacuole."
2. The intra-erythrocytic organism matures through: ring trophozoite → mature trophozoite → schizont → merozoites.
3. Upon RBC lysis, merozoites are released and infect fresh red cells — perpetuating the cycle.
4. The periodicity of this cycle (every 48 hours for P. falciparum/vivax/ovale, every 72 hours for P. malariae) drives the characteristic cyclical fevers.

3. Sexual Stage (Return to Mosquito)

• Some trophozoites differentiate into gametocytes rather than schizonts.
• When ingested by a feeding mosquito, gametocytes undergo sexual reproduction in the midgut → sporozoites migrate to salivary glands → cycle restarts.

Key Pathophysiologic Mechanisms

Cytoadherence & Sequestration (P. falciparum)

• Maturing schizonts express PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1), which forms knob-like protrusions on the RBC surface.
• PfEMP1 binds endothelial adhesion molecules — ICAM-1, VCAM-1, and CD36 — causing parasitized RBCs to adhere to capillary walls and sequester in microvascular beds.
• This cytoadherence is the primary reason P. falciparum is far more lethal than other species: it causes microvascular obstruction, tissue hypoxia, and organ dysfunction (brain → cerebral malaria; kidney → acute kidney injury; placenta → pregnancy complications).

Hemolytic Anemia

• Repeated cycles of RBC rupture cause hemolytic anemia, which worsens with high-grade parasitemia.
• Splenic clearance of both parasitized and innocent bystander RBCs amplifies the anemia.
• Massive intravascular hemolysis in severe P. falciparum can cause blackwater fever (hemoglobinuria → dark urine → renal failure).

Immune-Mediated Injury

• Rupture of schizonts releases merozoite antigens, hemozoin (malaria pigment), and GPI anchors → triggers TNF-α, IL-1, and IL-6 release from macrophages.
• This cytokine surge is responsible for the fever, rigors, headache, and systemic symptoms.
• TNF-α also contributes to cerebral malaria by upregulating ICAM-1 on brain endothelium, enhancing sequestration.

Splenomegaly

• Chronic antigen stimulation and removal of parasitized RBCs lead to progressive splenomegaly, which can be massive in endemic populations ("tropical splenomegaly").

Summary Table