Last updated on:May 26th, 2022
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Malaria is a mosquito-borne protozoan infection caused by Plasmodium spp.. These are transmitted to humans via species of the Anopheles mosquito. Malaria is most prevalent in Africa, Asia, South America, and Melanesia, with most deaths occurring in sub-Saharan Africa.
The incubation period is 9-14 days for P. falciparum and P. knowlesi, 12-17 days for P. vivax and P. ovale, and up to 40 days for P. malariae.
Febrile episodes are caused by the cyclic rupture (between 24-72 hours, depending on species) of mature schizonts in the blood. Schizonts release pyrogens, resulting in cytokine activation. Since parasite population dynamics within the host are not synchronous, the fevers are rarely as periodic as the erythrocytic cycles. Note that most patients seek medical care before fever cycles become fully established.
Patients with uncomplicated malaria may present with non-specific flu-like symptoms, such as headache, cough, myalgia, abdominal pain, and diarrhea. These are due to the release of proinflammatory mediators.
Complicated malaria can present with cerebral malaria, acute respiratory distress syndrome, severe anemia, jaundice, and acute renal failure. In children, severe malaria can also present with severe anemia and metabolic acidosis.
Patients with cerebral malaria due to P. falciparum or P. knowlesi can develop confusion, drowsiness, convulsions, and coma. This is due to cerebral tissue hypoxia secondary to microvascular obstruction and endothelial activation. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is expressed on the surface membrane of infected red blood cells. These bind to the endothelium of blood vessels, leading to microvascular obstruction, increased blood-brain barrier permeability, release of proinflammatory cytokines, and cerebral edema. Note that the shorter erythrocytic cycle of P. knowlesi (24 hours vs 48-72 hours for P. falciparum) predisposes to a daily increase in parasitemia, and thus, more severe symptoms.