18.3
Malaria
Malaria is primarily caused by the protozoan parasite Plasmodium, transmitted through the bite of an infected Anopheles mosquito.
When the mosquito bites, it injects sporozoites into the human bloodstream.
These sporozoites travel to the liver, where they invade hepatocytes and divide asexually to form liver schizonts.
Each liver schizont produces numerous merozoites. Once the hepatocytes rupture, the merozoites are released into the bloodstream and begin invading erythrocytes.
Inside the erythrocytes, the merozoites develop into ring-shaped early trophozoites.
As they mature, these trophozoites develop into schizonts within the erythrocytes.
When the schizonts rupture, they lyse the erythrocytes, releasing more merozoites and hemozoin—a byproduct of hemoglobin digestion—into the bloodstream.
Macrophages engulf the merozoites and hemozoin, releasing proinflammatory cytokines, such as TNF-alpha. These cytokines contribute to the characteristic cyclic fever associated with malaria.
Repeated destruction of erythrocytes also leads to anemia and reduced oxygen delivery to tissues.
Malaria pathogenesis in humans reflects a delicate interplay between parasite biology and host response. Clinical illness reflects a host’s immune response to the parasite’s asexual replication cycle, which is often asymptomatic in individuals with partial immunity. From the parasite's perspective, transmission between mosquito and human with minimal host pathology is evolutionarily advantageous. Among the six Plasmodium species infecting humans, P. falciparum and P. vivax dominate in global prevalence and clinical relevance.
Parasite Life Cycle and Host Interaction
Within the human host, malaria parasites undergo a series of morphological changes during asexual replication, culminating in the rupture of schizonts and release of merozoites into circulation. This event initiates the host immune response, marked by the release of cytokines such as TNF-α and IFN-γ. These molecules mediate fever and systemic symptoms, defining uncomplicated malaria when organ function remains intact. Repeated exposures shape the immune system, producing antibodies that modulate future infections.
Immune Evasion and Sequestration
P. falciparum exhibits a unique cytoadherence mechanism via the PfEMP1 protein family encoded by var genes, allowing infected erythrocytes to bind endothelial receptors (e.g., ICAM-1, CD36). This sequestration removes parasites from circulation, complicating diagnosis and contributing to severe disease syndromes such as cerebral malaria. In contrast, P. vivax prefers reticulocytes and exhibits lower parasitemia, with minimal sequestration, and introduces relapse potential via dormant hypnozoites.
Determinants of Disease Severity
Severe malaria outcomes, including cerebral malaria, severe anemia, and metabolic acidosis, emerge from exaggerated host-pathogen interactions. Factors such as delayed treatment, comorbidities, parasite genetic diversity, and host polymorphisms modulate disease expression. In pediatric populations, rapid progression and overlapping syndromes amplify mortality risks. Placental malaria, mediated by specific PfEMP1 variants (e.g., VAR2CSA), exemplifies how parasite adaptation to novel host niches influences pathology.
Public Health Implications
Reducing malaria burden, especially cerebral malaria, hinges on early treatment access, stable drug infrastructure, and sustained immune exposure. While parasite biology remains conserved, antigenic variation and drug resistance influence disease patterns. Additionally, shifts in endemicity influence disease patterns, highlighting the impact of human interventions on malaria epidemiology.
Malaria is primarily caused by the protozoan parasite Plasmodium, transmitted through the bite of an infected Anopheles mosquito.
When the mosquito bites, it injects sporozoites into the human bloodstream.
These sporozoites travel to the liver, where they invade hepatocytes and divide asexually to form liver schizonts.
Each liver schizont produces numerous merozoites. Once the hepatocytes rupture, the merozoites are released into the bloodstream and begin invading erythrocytes.
Inside the erythrocytes, the merozoites develop into ring-shaped early trophozoites.
As they mature, these trophozoites develop into schizonts within the erythrocytes.
When the schizonts rupture, they lyse the erythrocytes, releasing more merozoites and hemozoin—a byproduct of hemoglobin digestion—into the bloodstream.
Macrophages engulf the merozoites and hemozoin, releasing proinflammatory cytokines, such as TNF-alpha. These cytokines contribute to the characteristic cyclic fever associated with malaria.
Repeated destruction of erythrocytes also leads to anemia and reduced oxygen delivery to tissues.
From Chapter 18:
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