Malaria - A Complete Overview

1. Epidemiology

• Robbins & Kumar Basic Pathology, p. 392
• Murray & Nadel's Textbook of Respiratory Medicine

2. Causative Organisms

3. Life Cycle and Pathogenesis

Hepatic (Pre-erythrocytic) Phase - Clinically Silent

1. Sporozoites are injected into the bloodstream during a mosquito bite
2. They travel to the liver, where thrombospondin-related adhesive protein and circumsporozoite protein bind to heparan sulfate proteoglycans on hepatocytes
3. Sporozoites enter hepatocytes and differentiate into merozoites (incubation: 1-4 weeks)
4. Infected hepatocytes rupture, releasing thousands of merozoites into the bloodstream

In P. vivax and P. ovale, a subset of parasites remain dormant in the liver as hypnozoites, which can reactivate years later causing relapse.

Erythrocytic Phase - Clinically Apparent

1. A lectin-like molecule on the merozoite surface binds to sialidated glycophorin on red blood cells
2. The merozoite invaginates into the RBC inside a "digestive vacuole"
3. Intraerythrocytic organisms differentiate into trophozoites:
   • Some become gametocytes (restart the cycle in the mosquito)
   • Most become schizonts, which express PfEMP1 (P. falciparum erythrocyte membrane protein 1) - knob-like protrusions on the RBC surface
4. PfEMP1 binds endothelial adhesion molecules (ICAM-1, VCAM-1, CD36), causing sequestration of parasitized RBCs in capillary beds - the key pathogenic mechanism of severe malaria
5. Schizonts mature into merozoites, lyse the RBC, and the cycle repeats

4. Clinical Features

Uncomplicated Malaria

• Classic triad: Paroxysmal fever, rigors (shaking chills), and sweating
• Fever coincides with RBC lysis and merozoite release
• Associated symptoms: headache, nausea, myalgia, fatigue, mild anemia
• Periodic fever pattern (less reliable in P. falciparum):
  • P. knowlesi: every 24 hours
  • P. vivax, P. ovale, P. falciparum: every 48 hours
  • P. malariae: every 72 hours

Severe/Complicated Malaria (predominantly P. falciparum)

Chest X-ray in severe malaria:

5. Pathology / Morphology

• Hemolytic anemia with characteristic lab findings
• Hematin (malarial pigment) - brown pigment derived from hemoglobin digestion; deposited in the spleen, liver, lymph nodes, and bone marrow
• Massive splenomegaly (and hepatomegaly) from hyperplasia of mononuclear phagocytes
• Cerebral vessels congested with parasitized RBCs in fatal cases

6. Diagnosis

7. Treatment

Uncomplicated P. falciparum Malaria

• Artemisinin-based combination therapy (ACT) is the first-line treatment:
  • Artemether/lumefantrine
  • Dihydroartemisinin/piperaquine
  • Artesunate/amodiaquine
  • Artesunate/mefloquine

Uncomplicated P. vivax / P. ovale Malaria

• ACT or chloroquine (where chloroquine-sensitive)
• Primaquine is essential to eliminate hypnozoites (radical cure) - always check G6PD status first, as primaquine causes hemolysis in G6PD-deficient patients
• A single-dose regimen (tafenoquine) is now an alternative for patients with normal G6PD

Severe Malaria (all species)

• IV artesunate is first-line: 2.4 mg/kg by slow IV bolus at 0, 12, 24 hours, then every 24 hours
• Switch to oral ACT once patient is stable and parasitemia <1%
• If IV artesunate unavailable: IV quinine + doxycycline or clindamycin
• Careful fluid management to prevent pulmonary edema
• Treat concurrent hypoglycemia
• Artesunate resistance (slower parasite clearance) is an increasing problem in Southeast Asia

Drug Resistance

• Chloroquine resistance: widespread in P. falciparum, present in some P. vivax
• Artemisinin partial resistance: emerging in Southeast Asia (Greater Mekong Subregion) and increasingly detected in Africa, manifesting as delayed parasite clearance
• Partner drug resistance: piperaquine and mefloquine resistance complicates ACT options

8. Prevention

Personal Protection

• Insecticide-treated bed nets (ITNs/LLINs) - most effective public health intervention
• Pyrethroid-PBO nets or pyrethroid-chlorfenapyr nets in areas with pyrethroid resistance
• Indoor residual spraying (IRS)
• Repellents (DEET, picaridin)
• Protective clothing

Chemoprophylaxis for Travelers

• Atovaquone-proguanil (Malarone)
• Doxycycline
• Mefloquine
• Chloroquine (only in chloroquine-sensitive areas)

Preventive Treatment in High-Risk Groups (WHO 2025)

• Intermittent preventive treatment in pregnancy (IPTp): sulfadoxine-pyrimethamine
• Seasonal malaria chemoprevention (SMC): for children in high-transmission areas
• Perennial malaria chemoprevention (PMC): for infants in high-burden regions
• Mass drug administration (MDA): in selected low-transmission or emergency settings

Vaccines

• RTS,S/AS01 (Mosquirix) and R21/Matrix-M: approved malaria vaccines targeting the P. falciparum circumsporozoite protein. They offer partial protection and are now being deployed in high-burden African countries. Though protection is incomplete, widespread deployment is expected to prevent thousands of deaths in children annually.

9. Evolutionary and Genetic Context

• Sickle cell trait (HbS heterozygote) - protection from severe P. falciparum
• Thalassemia - partial protection
• G6PD deficiency - some protection
• Duffy antigen negativity - near-complete protection against P. vivax (common in West African populations)
• Pyruvate kinase deficiency - partial protection

Key References

• Robbins & Kumar Basic Pathology, pp. 391-393
• Murray & Nadel's Textbook of Respiratory Medicine, Chapter 58
• Brenner & Rector's The Kidney, Chapters 20, 42-44
• WHO Guidelines for Malaria, August 2025
• Recent systematic reviews: Fadilah et al. on primaquine/hypnozoite activity (PMID: 39331646), Sultanli et al. on neonatal malaria management (PMID: 41350822)