Plasmodium & Malaria

Malaria is a protozoan haemoprotozoal infection caused by Plasmodium species, transmitted by the bite of an infected female Anopheles mosquito. It remains a heavily tested topic in NEET PG across Microbiology, Medicine and Pharmacology — focusing on life-cycle stages, species differentiation on the blood film, drug of choice for each scenario, and complications of falciparum.

Classification & species

Five species infect humans. The classic four are tested most often; P. knowlesi (a simian/monkey malaria, "fifth species") is an emerging zoonosis in Southeast Asia.

Species	Clinical name	Fever periodicity	Erythrocyte preference	Relapse?	Key film clue
P. falciparum	Malignant tertian	~48 h (often irregular)	All ages of RBC (no size change)	No (recrudescence only)	Ring forms, multiple per RBC, appliqué/accolé forms, banana-shaped (crescentic) gametocytes, Maurer's clefts/dots
P. vivax	Benign tertian	48 h	Young RBC / reticulocytes (enlarged RBC)	Yes (hypnozoites)	Schüffner's dots, amoeboid trophozoite, enlarged pale RBC
P. ovale	Ovale tertian	48 h	Young RBC (enlarged, oval, fimbriated edge)	Yes (hypnozoites)	Schüffner's (James's) dots, oval RBC with ragged ends
P. malariae	Quartan	72 h	Old/mature RBC	No	Band-form trophozoite, "rosette/daisy-head" schizont (8–10 merozoites), no dots

High-yield: Only P. vivax and P. ovale form hypnozoites (dormant liver stage) → cause true relapse. P. falciparum and P. malariae have NO hypnozoite stage; recurrence in these is recrudescence (from persistent low-level blood-stage parasitaemia), not relapse.

High-yield: P. falciparum is the most dangerous (cerebral malaria, high parasitaemia, infects all RBC ages). P. malariae causes the longest latency and is classically associated with quartan malarial nephropathy (immune-complex nephrotic syndrome in children).

Life cycle (two hosts)

Man is the intermediate host (asexual cycle / schizogony); the female Anopheles is the definitive host (sexual cycle / sporogony, because gametes fuse here).

Sporozoite (infective form, from mosquito saliva) → enters human via mosquito bite → liver (exo-erythrocytic schizogony) → releases merozoites → invade RBCs → erythrocytic schizogony (ring → trophozoite → schizont → merozoites rupture RBC → fever) → some merozoites become gametocytes (male microgametocyte + female macrogametocyte) → taken up by mosquito → fertilisation → ookinete → oocyst → sporozoites migrate to salivary gland.

Stepwise summary:

1. Sporozoite injected → travels to liver within ~30 min.
2. Pre-erythrocytic schizogony in hepatocytes (asymptomatic incubation).
3. In vivax/ovale, some become hypnozoites (dormant → later relapse).
4. Merozoites released into blood → invade RBCs.
5. Erythrocytic cycle: ring → trophozoite → schizont → rupture → clinical fever paroxysm.
6. Gametocytes form → ingested by Anopheles → sexual cycle (sporogony) completes the loop.

High-yield: The fever paroxysm coincides with synchronous rupture of erythrocytic schizonts releasing merozoites and malarial pigment (haemozoin). The infective form to man is the sporozoite; the infective form to mosquito is the gametocyte. The diagnostic blood stages are rings/trophozoites/schizonts/gametocytes.

Receptors for RBC invasion (favourite MCQ)

• P. vivax uses the Duffy blood group antigen (Fy) → West Africans who are Duffy-negative are resistant to vivax.
• P. falciparum uses glycophorin A (and binds via EBA-175).

Pathophysiology

• Haemolysis of infected (and uninfected) RBCs → anaemia, jaundice, splenomegaly.
• Haemozoin (malarial pigment, a haem breakdown product) deposits in macrophages → grey/black spleen and liver.
• Cytoadherence & sequestration (unique to falciparum): infected RBCs express PfEMP-1 knob proteins → bind endothelium (ICAM-1 in brain) → microvascular obstruction → cerebral malaria, placental malaria, and absence of mature forms in peripheral blood.
• Rosetting (infected RBC clumps with uninfected RBCs) worsens obstruction.
• Cytokine surge (TNF-α) drives fever, hypoglycaemia and lactic acidosis.

High-yield: Only ring forms and gametocytes of P. falciparum are usually seen in peripheral blood; mature trophozoites/schizonts are sequestered in deep capillaries. Seeing schizonts of falciparum peripherally implies very heavy infection (poor prognosis).

Clinical features

Classic paroxysm has three stages — cold stage (chills, rigors) → hot stage (high fever, headache) → sweating stage (defervescence, fatigue). The interval defines tertian (48 h) vs quartan (72 h).

• Incubation: falciparum ~12 days, vivax/ovale ~14 days, malariae ~28 days (longest).
• Anaemia, splenomegaly (risk of splenic rupture, especially vivax), hepatomegaly, jaundice.
• Severe / complicated malaria is almost always P. falciparum (occasionally vivax/knowlesi).

WHO criteria for severe falciparum malaria (any one)

Feature	Threshold / definition
Cerebral malaria	Unrousable coma (GCS < 11), not attributable to other cause
Severe anaemia	Hb < 7 g/dL (adults) / < 5 g/dL (children)
Hypoglycaemia	Blood glucose < 40 mg/dL (2.2 mmol/L)
Acidosis	Bicarbonate < 15 mmol/L or lactate > 5 mmol/L
Renal impairment	Creatinine > 3 mg/dL (acute kidney injury / "blackwater fever")
Hyperparasitaemia	> 10% parasitised RBCs (or > 2% in non-immune)
Pulmonary oedema / ARDS	SpO₂ < 92% with raised respiratory rate
Hyperbilirubinaemia	Bilirubin > 3 mg/dL with other organ dysfunction
Others	Shock (algid malaria), DIC/abnormal bleeding, convulsions ≥ 2 in 24 h, prostration

High-yield: Blackwater fever = massive intravascular haemolysis → haemoglobinuria (dark/cola-coloured urine) + AKI, classically with falciparum (often after irregular quinine use in G6PD-deficient or repeatedly infected individuals). Algid malaria = septic-shock-like presentation with peripheral circulatory collapse.

Diagnosis & investigation of choice

High-yield: Peripheral blood smear with Giemsa/Leishman stain is the GOLD STANDARD. Make both a thick and a thin film.

Film	Purpose	Strength
Thick film	Screening / detecting parasite (RBCs lysed → concentrates parasites; ~10–20× more sensitive)	Detects low parasitaemia, estimates density
Thin film	Species identification + parasite count (% parasitaemia)	Preserves RBC morphology and parasite detail

Mnemonic: "Thick to deTECT, thin to idenTIFY."

• Rapid diagnostic tests (RDTs): detect antigens — HRP-2 (P. falciparum specific) and pLDH / aldolase (pan-species). Useful in field/low-resource settings; HRP-2 may stay positive for weeks after cure and can give false negatives with hrp2 gene-deletion strains.
• QBC (Quantitative Buffy Coat): acridine-orange fluorescence — sensitive but no speciation, needs fluorescence microscope.
• PCR / nested PCR: most sensitive, detects mixed/low-grade infections and knowlesi (which morphologically mimics malariae); used for confirmation, not routine.
• Repeat smears every 6–12 hours if first is negative but suspicion is high.
• Supportive: anaemia, thrombocytopenia (very common, a useful early clue), raised LDH, indirect hyperbilirubinaemia.

Management — drug of choice

Treatment depends on species, severity, region (resistance), and pregnancy.

Uncomplicated P. falciparum (or unknown species) → Artemisinin-based Combination Therapy (ACT). In India, the programme uses Artesunate + Sulfadoxine-Pyrimethamine (AS+SP) for most of the country; the Northeast states use Artemether-Lumefantrine (because SP resistance is high there).

Uncomplicated P. vivax → Chloroquine (blood stage) + Primaquine × 14 days (radical cure of hypnozoites).

Severe / complicated malaria (any species) → IV Artesunate is the drug of choice (replaced IV quinine; lower mortality — SEAQUAMAT and AQUAMAT trials). Quinine/quinidine is the alternative if artesunate unavailable.

Scenario	Drug of choice
Uncomplicated falciparum	ACT (AS+SP, or Artemether-Lumefantrine in NE India)
Severe malaria (all species)	IV Artesunate
Uncomplicated vivax	Chloroquine + Primaquine (14 d)
Radical cure / anti-relapse (vivax, ovale)	Primaquine (kills hypnozoites + gametocytes)
Chemoprophylaxis (chloroquine-sensitive area)	Chloroquine weekly
Chemoprophylaxis (resistant area)	Mefloquine / Doxycycline / Atovaquone-Proguanil
Pregnancy, 1st trimester (uncomplicated falciparum)	Quinine + Clindamycin (ACT now also acceptable per recent WHO)
Pregnancy (severe)	IV Artesunate

High-yield: Primaquine is the only drug that kills hypnozoites (radical cure) and the gametocytes of falciparum (transmission-blocking). It causes haemolysis in G6PD deficiency — screen for G6PD before giving; contraindicated in pregnancy. Single low-dose (0.25 mg/kg) primaquine is added to falciparum treatment as a gametocidal/transmission-blocking dose.

High-yield: Artemisinins act on the ring stage, are fast-acting and short half-life (hence combined with a long-acting partner to prevent resistance). Mechanism: free-radical generation via iron/haem. Resistance (delayed parasite clearance) is linked to Kelch 13 (K13) propeller gene mutations, first reported in the Greater Mekong subregion — a top current-topic MCQ.

Mechanism / toxicity quick notes

• Chloroquine: concentrates in food vacuole, inhibits haem polymerase → toxic haem accumulates. Toxicity: retinopathy (bull's-eye), pruritus, QT effects.
• Quinine: cinchonism (tinnitus, headache, nausea), hypoglycaemia (stimulates insulin), QT prolongation.
• Mefloquine: neuropsychiatric effects, vivid dreams.
• Primaquine/Tafenoquine: haemolysis in G6PD deficiency (tafenoquine = single-dose anti-relapse alternative).

Complications

• Cerebral malaria — leading cause of death; coma, seizures, malarial retinopathy (most specific sign on fundoscopy), high mortality even with treatment.
• Severe anaemia, hypoglycaemia (also worsened by quinine), lactic acidosis.
• Acute kidney injury / blackwater fever.
• ARDS / pulmonary oedema, often appearing after starting treatment.
• Hypoglycaemia (pregnancy + quinine = high risk).
• Tropical splenomegaly syndrome (hyperreactive malarial splenomegaly) — massive spleen, high IgM.
• Quartan malarial nephropathy (P. malariae) — childhood nephrotic syndrome.
• Algid malaria (shock), DIC, splenic rupture (vivax).
• Recurrent falciparum in endemic areas may contribute to Burkitt lymphoma (co-factor with EBV) and is implicated in tropical splenomegaly.

Key differentials

Fever with thrombocytopenia / splenomegaly in the tropics:

• Dengue / chikungunya (thrombocytopenia, no rigors paroxysm pattern).
• Enteric fever (typhoid) — stepladder fever, relative bradycardia.
• Leptospirosis — myalgia, conjunctival suffusion, jaundice + AKI.
• Babesiosis — Babesia mimics falciparum rings but has Maltese cross tetrads, no pigment, no gametocytes, history of tick bite / asplenia.
• Visceral leishmaniasis (kala-azar) — massive splenomegaly, pancytopenia.
• Viral hepatitis, scrub typhus, sepsis.

High-yield: Distinguish Babesia from falciparum: Babesia shows the "Maltese cross" (tetrad), has no haemozoin pigment, and no gametocytes — and is acquired via tick (Ixodes), not mosquito.

Recently asked / exam angle

• Species → blood-film morphology matching (Schüffner's dots = vivax/ovale; band form = malariae; banana-shaped gametocyte = falciparum) is a perennial image-based MCQ.
• Hypnozoite-forming species (vivax + ovale) and primaquine for radical cure — repeatedly asked.
• Drug of choice for severe malaria = IV artesunate (post-SEAQUAMAT/AQUAMAT), no longer quinine.
• K13 gene mutation → artemisinin resistance, Mekong region — a modern favourite.
• Duffy antigen negativity → resistance to P. vivax (and glycophorin A for falciparum).
• Thick film for detection vs thin film for speciation — definition-level question.
• Primaquine contraindication in G6PD deficiency and pregnancy.
• HRP-2 RDT is falciparum-specific; hrp2-deletion false negatives is a newer angle.
• P. knowlesi as the "fifth" species, mimics P. malariae on microscopy, needs PCR — emerging-pathogen MCQ.
• Quartan nephrotic syndrome ↔ P. malariae.

Rapid revision

1. Infective form to man = sporozoite; to mosquito = gametocyte.
2. Definitive host = female Anopheles (sexual cycle, sporogony); man = intermediate host.
3. Hypnozoites → relapse only in vivax & ovale; falciparum/malariae = recrudescence.
4. Schüffner's dots → vivax/ovale; band form & rosette schizont → malariae; banana-shaped gametocyte → falciparum.
5. P. falciparum infects RBCs of all ages → highest parasitaemia and severity.
6. Thick film = detect, thin film = identify; Giemsa stain is gold standard.
7. IV Artesunate = DOC for severe malaria (all species).
8. ACT for uncomplicated falciparum; chloroquine + primaquine for vivax.
9. Primaquine kills hypnozoites + gametocytes; avoid in G6PD deficiency & pregnancy.
10. Duffy-negative people are resistant to P. vivax; falciparum uses glycophorin A.
11. PfEMP-1 cytoadherence/sequestration → cerebral & placental malaria; mature falciparum forms absent peripherally.
12. K13 mutation = artemisinin resistance (Mekong); blackwater fever = haemoglobinuria + AKI; malarial retinopathy = most specific sign of cerebral malaria.