Malaria remains one of the most significant parasitic diseases globally, caused by protozoan parasites of the genus Plasmodium. Five species infect humans: P. falciparum, P. vivax, P. ovale, P. malariae, and the zoonotic P. knowlesi. P. falciparum causes the most severe disease and is responsible for the majority of malaria-related deaths.
Life Cycle
The Plasmodium life cycle alternates between the mosquito vector (sexual reproduction) and the human host (asexual reproduction). Infection begins when an infected female Anopheles mosquito injects sporozoites into the bloodstream during a blood meal.
Sporozoites travel to the liver and invade hepatocytes, where they undergo exo-erythrocytic schizogony. In P. vivax and P. ovale, some parasites become dormant hypnozoites that can remain in the liver for months or years, causing relapses. After 5–16 days, the hepatocyte ruptures, releasing thousands of merozoites into the bloodstream.
Merozoites invade red blood cells and initiate the erythrocytic cycle. Within the erythrocyte, the parasite progresses through ring, trophozoite, and schizont stages. The schizont ruptures the red blood cell, releasing merozoites that invade new erythrocytes. This cyclic rupture causes the characteristic periodic fevers. Some merozoites differentiate into gametocytes, the sexual forms that are taken up by mosquitoes.
In the mosquito midgut, gametocytes form gametes that fuse to produce a zygote. The zygote develops into an ookinete that penetrates the gut wall and forms an oocyst. Within the oocyst, sporozoites develop and migrate to the salivary glands, ready to infect a new host.
Clinical Presentation
Symptoms typically appear 7–30 days after infection and include fever, chills, headache, myalgia, and fatigue. Uncomplicated malaria presents with paroxysms of fever every 48 hours (P. falciparum, P. vivax, P. ovale) or 72 hours (P. malariae). Severe P. falciparum malaria can progress to cerebral malaria, severe anemia, acute respiratory distress syndrome, acute kidney injury, and metabolic acidosis.
Laboratory Diagnosis
Microscopic examination of Giemsa-stained thick and thin blood films remains the gold standard for malaria diagnosis. Thick films concentrate parasites for detection and quantification, while thin films allow species identification based on morphological features. Parasitemia is reported as the percentage of infected red blood cells or the number of parasites per microliter of blood. Experienced microscopists can detect parasitemia as low as 5–10 parasites per microliter.
Rapid diagnostic tests (RDTs) detect parasite-specific antigens such as histidine-rich protein 2 (HRP2) for P. falciparum and lactate dehydrogenase (pLDH) or aldolase for pan-Plasmodium detection. RDTs are simple to perform and useful in resource-limited settings but cannot quantify parasitemia or distinguish past from current infection.
Molecular diagnosis by polymerase chain reaction (PCR) offers the highest sensitivity and specificity. Nested PCR and real-time PCR targeting the 18S rRNA gene can detect low-level parasitemia and identify mixed species infections. PCR is particularly valuable for confirming species in microscopically ambiguous cases and for monitoring treatment efficacy.
Serological tests detecting antibodies against Plasmodium antigens indicate past exposure and are useful for epidemiological surveys but not for acute diagnosis.
Antimalarial Treatment
Treatment depends on the Plasmodium species, parasite resistance patterns, and disease severity. Uncomplicated P. falciparum malaria is treated with artemisinin-based combination therapies (ACTs), such as artemether-lumefantrine or artesunate-amodiaquine. P. vivax and P. ovale infections require chloroquine in sensitive areas plus primaquine for hypnozoite eradication. Severe malaria requires intravenous artesunate followed by a full oral ACT course.
Prevention
Vector control measures include insecticide-treated bed nets, indoor residual spraying, and larval source management. Chemoprophylaxis with atovaquone-proguanil, doxycycline, or mefloquine is recommended for travelers to endemic areas. Vaccine development has yielded the RTS,S/AS01 vaccine, which provides partial protection against P. falciparum in young children.
