Malaria

Parasite/Hosts

Plasmodium: P. falciparum, P. vivax, P. ovale, P. malariae (4 regular) + P. knowlesi (zoonotic, SE Asia, monkey malaria). MOST LETHAL parasitic disease of humans.

Definitive host = female Anopheles mosquito (sexual reproduction/sporogony). Intermediate host = human (asexual/schizogony only). [Commonly inverted fact — mosquito is NOT just mechanical vector, it’s the sexual-stage host]

Life Cycle

Mosquito: blood meal ingests gametocytes → gamete fusion → zygote → ookinete → oocyst (gut wall) → sporogony → sporozoites → salivary glands.

Human — Exoerythrocytic (liver): bite injects sporozoites → hepatocytes → schizogony → thousands merozoites/cell. P. vivax + P. ovale ONLY: dormant HYPNOZOITES → reactivate months-years later = TRUE RELAPSE. P. falciparum + P. malariae: NO hypnozoite. Recurrence = RECRUDESCENCE (surviving blood-stage), not relapse.

Human — Erythrocytic (blood): merozoites → RBC → ring→trophozoite→schizont→rupture→new merozoites. = symptom-causing stage. Some merozoites → gametocytes (for mosquito uptake).

Clinical Manifestations

Benign malaria: malarial paroxysm triad = cold stage(rigors) → hot stage(fever) → sweating stage(defervescence). Periodicity:

• TERTIAN (48hr/every 3rd day): P. vivax, P. ovale, P. falciparum
• QUARTAN (72hr/every 4th day): P. malariae ONLY

Between paroxysms: anemia (RBC destruction) + splenomegaly (repeated cycles).

Malignant tertian malaria = P. FALCIPARUM ONLY. Causes ~ALL malaria mortality. Mechanism: PfEMP1 surface protein → SEQUESTRATION (cytoadherence to endothelium in deep capillaries — brain, placenta) → evades spleen clearance → microvascular obstruction. Complications: Cerebral malaria (coma, cerebral sequestration), Blackwater fever (massive hemolysis → dark urine + AKI), severe anemia, ARDS, placental malaria (low birth weight, maternal mortality).

Laboratory Diagnosis

Peripheral smear microscopy = GOLD STANDARD. Thick film (screening/sensitivity) + Thin film (species ID/morphology). Giemsa stain. P. falciparum: ONLY ring forms + gametocytes seen (later stages sequester away from circulation). P. vivax: enlarged RBC + Schüffner’s dots.

RDT (antigen, immunochromatography):

• HRP-2 = P. FALCIPARUM SPECIFIC
• pLDH/aldolase = PAN-SPECIES

QBC (quantitative buffy coat): acridine orange, capillary tube, fluorescence. FASTER/MORE sensitive than smear for screening. Doesn’t reliably speciate.

PCR: highest sensitivity + definitive speciation. Low parasitemia/mixed infections. Reference/research use mainly.

Treatment

P. vivax/ovale/malariae: Chloroquine (mostly effective, resistant P. vivax emerging SE Asia/Oceania).

• PRIMAQUINE for P. vivax + P. ovale ONLY (eradicates hypnozoites, prevents relapse). NOT needed for falciparum/malariae (no hypnozoite). Primaquine needs G6PD testing first (hemolysis risk in deficiency).

P. falciparum: ACT (Artemisinin Combination Therapy) = standard first-line (widespread chloroquine resistance). Artemisinin (fast-acting) + longer-acting partner drug (e.g. artemether-lumefantrine).

Severe/complicated malaria (cerebral, severe anemia, AKI, ARDS): IV ARTESUNATE = drug of choice OVER IV quinine (better mortality, major trials) + supportive care.

Prevention

Vector control (ITN bed nets, IRS, larval source management), chemoprophylaxis (travelers, matched to local resistance), vaccines (RTS,S/AS01, R21/Matrix-M — WHO recommended, children, moderate-high transmission areas, target P. falciparum circumsporozoite protein).

The Parasite and Its Hosts

Malaria is caused by protozoan parasites of the genus Plasmodium — four species regularly infect humans (P. falciparum, P. vivax, P. ovale, P. malariae), with a fifth, P. knowlesi (a monkey malaria), increasingly recognized as a zoonotic human pathogen in Southeast Asia. It remains the most lethal parasitic disease of humans globally.

The female Anopheles mosquito is the definitive host — sexual reproduction (sporogony) occurs in the mosquito gut — while humans serve as the intermediate host, in whom only asexual multiplication (schizogony) occurs. This host assignment is worth fixing precisely, since it is exactly the kind of fact commonly inverted: the mosquito is not merely a vector carrying the parasite mechanically, it is the site of the parasite’s sexual life cycle.

Life Cycle

In the mosquito: a blood meal ingests gametocytes from an infected human; male and female gametocytes fuse in the mosquito gut to form a zygote, which develops into a motile ookinete, then an oocyst on the gut wall — sporogony within the oocyst produces thousands of sporozoites, which migrate to the salivary glands, ready for the next bite.

In the human — exoerythrocytic (liver) stage: an infective bite injects sporozoites, which travel to the liver and invade hepatocytes, multiplying asexually (schizogony) into thousands of merozoites per infected cell. P. vivax and P. ovale additionally form dormant liver-stage hypnozoites, capable of reactivating months to years later to cause relapse — a feature P. falciparum and P. malariae lack entirely (their recurrences are properly termed recrudescence, from surviving blood-stage parasites, not true relapse from the liver).

In the human — erythrocytic (blood) stage: released merozoites invade RBCs and cycle through ring → trophozoite → schizont stages, rupturing the cell to release new merozoites — the stage responsible for essentially all clinical symptoms. Some merozoites instead differentiate into sexual-stage gametocytes, which are what a feeding mosquito must pick up to continue transmission.

Clinical Manifestations

Benign malaria (any of the four “benign” species presentations) classically produces the malarial paroxysm — a triad of cold stage (rigors), hot stage (high fever), and sweating stage (defervescence) — synchronized to the erythrocytic cycle length: tertian periodicity (fever every 48 hours / every third day counting inclusively) for P. vivax, P. ovale, and P. falciparum; quartan periodicity (every 72 hours / fourth day) uniquely for P. malariae. Between paroxysms, anaemia (from RBC destruction) and splenomegaly develop with repeated cycles.

Malignant tertian malaria, caused exclusively by P. falciparum, is the dangerous form, responsible for essentially all malaria mortality. Its severity stems from a unique pathogenic feature: P. falciparum-infected RBCs express surface adhesion proteins (PfEMP1) causing sequestration — infected cells cytoadhere to vascular endothelium in deep capillary beds (brain, placenta, other organs) rather than circulating freely, evading splenic clearance and causing microvascular obstruction. This drives the feared complications: cerebral malaria (impaired consciousness/coma from cerebral microvascular sequestration), blackwater fever (massive intravascular haemolysis causing dark, haemoglobin-laden urine and acute kidney injury), severe anaemia, acute respiratory distress, and placental malaria in pregnancy (contributing to low birth weight and maternal mortality).

Laboratory Diagnosis

Peripheral blood smear microscopy (thick film for screening/sensitivity, thin film for species identification and morphology) remains the diagnostic gold standard — Giemsa staining reveals ring forms, trophozoites, schizonts, and gametocytes, with species-specific morphological clues (e.g. P. falciparum typically shows only ring forms and gametocytes in peripheral blood, since later stages sequester away from circulation; P. vivax-infected RBCs enlarge and show Schüffner’s dots).

Rapid diagnostic tests (RDTs) detect parasite antigen via immunochromatography: histidine-rich protein 2 (HRP-2) is P. falciparum-specific, while parasite lactate dehydrogenase (pLDH) and aldolase are pan-species markers, useful together to both confirm malaria and identify (or exclude) falciparum specifically.

QBC (quantitative buffy coat) technique concentrates parasites in a capillary tube with acridine orange staining for fluorescence microscopy — faster and more sensitive than conventional smear for screening, though it doesn’t reliably speciate. PCR offers the highest sensitivity and definitive speciation, particularly valuable for low-parasitaemia or mixed infections, but is generally reserved for reference/research settings rather than routine bedside diagnosis.

Treatment

Treatment choice depends critically on species and local resistance pattern:

• P. vivax, P. ovale, P. malariae: chloroquine remains effective in most regions (though chloroquine-resistant P. vivax is emerging in parts of Southeast Asia/Oceania). Primaquine is added specifically for P. vivax and P. ovale to eradicate dormant liver hypnozoites and prevent relapse — a step not needed for P. falciparum or P. malariae, which have no hypnozoite stage. Primaquine requires prior G6PD testing, since it can trigger severe haemolysis in G6PD-deficient patients.
• P. falciparum: given widespread chloroquine resistance, artemisinin-based combination therapy (ACT) is now standard first-line treatment nearly everywhere — an artemisinin derivative (fast-acting, rapidly clears parasitaemia) paired with a longer-acting partner drug (eliminating residual parasites and reducing resistance emergence), such as artemether-lumefantrine.
• Severe/complicated malaria (cerebral malaria, severe anaemia, AKI, ARDS): IV artesunate is now the treatment of choice over IV quinine, given superior mortality outcomes in major trials, alongside intensive supportive care.

Prevention

Vector control (insecticide-treated bed nets, indoor residual spraying, larval source management), chemoprophylaxis for travellers to endemic areas (matched to local resistance pattern), and — where deployed — the RTS,S/AS01 and R21/Matrix-M malaria vaccines, now recommended by WHO for children in moderate-to-high transmission settings, targeting the P. falciparum circumsporozoite protein.

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