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Arrange stages of Plasmodium life cycle in correct order:
A. Parasites reproduce asexually in RBCs, bursting the cells
B. Parasites reproduce asexually in liver cells, bursting them and releasing into blood
C. Gametocytes develop in RBCs
D. Sporozoites injected by infected female Anopheles mosquito
Options
1
D, B, A, C
2
A, B, C, D
3
B, D, A, C
4
C, A, B, D
Correct Answer
Option 1: D, B, A, C
Solution
1

D — Sporozoite injection: Infected female Anopheles bites → injects sporozoites into bloodstream.

B — Liver phase (exoerythrocytic): Sporozoites enter liver → multiply asexually → liver cells burst → merozoites released into blood.

2

A — RBC phase (erythrocytic): Merozoites enter RBCs → multiply → RBCs burst (causes fever, chills) → more merozoites released.

C — Gametocyte formation: Some merozoites differentiate into gametocytes in RBCs → taken up by mosquito → sexual reproduction continues in mosquito.

D (Sporozoites in) → B (Liver multiplication) → A (RBC multiplication/fever) → C (Gametocytes → mosquito)
Theory: Human Health and Disease
1. Plasmodium — The Malaria Parasite

Malaria is caused by protozoan parasites of the genus Plasmodium (phylum Apicomplexa). Four species infecting humans: Plasmodium falciparum: most dangerous, causes cerebral malaria (malignant/pernicious), irregular fever, highest mortality. P. vivax: most widespread, 48-hour fever cycle (benign tertian malaria), can cause relapses (hypnozoites in liver). P. ovale: similar to vivax, rarer. P. malariae: 72-hour cycle (quartan malaria), milder. P. knowlesi: simian malaria, can infect humans (Southeast Asia, emerging). Worldwide burden: ~250 million malaria cases/year, ~620,000 deaths (mostly children in sub-Saharan Africa, mostly P. falciparum). India: P. vivax and P. falciparum both common. Vector: female Anopheles mosquito. Males do not bite.

2. Life Cycle in Human — Exoerythrocytic Phase

Stage D: Infected female Anopheles bites human → injects SPOROZOITES from salivary glands into blood during blood meal. Sporozoites: infective stage, thin, elongated, motile. Stage B: Sporozoites reach liver (within 30 minutes via blood) → enter hepatocytes. Exoerythrocytic (pre-erythrocytic) cycle in liver: Each sporozoite → divides asexually → 10,000-30,000 merozoites (per hepatocyte). Duration: P. vivax 8 days, P. falciparum 6 days. Liver cells burst → release MEROZOITES into bloodstream. Hypnozoites (P. vivax, P. ovale only): some sporozoites remain dormant in liver for months/years → cause relapses. P. falciparum does NOT form hypnozoites → no relapses. No symptoms during liver phase. Liver phase undetected clinically.

3. Erythrocytic Phase — Cause of Symptoms

Stage A: Merozoites in bloodstream → invade RBCs (via specific red cell surface receptors — Duffy antigen for P. vivax, glycophorin A for P. falciparum). Inside RBC: Trophozoite → Schizont (asexual multiplication) → 8-24 merozoites per RBC → RBC ruptures → merozoites released. This rupture causes symptoms: Fever, chills, rigor: triggered by release of merozoites and haemozoin (malaria pigment = Fe3+porphyrin, toxic). Anaemia: RBC destruction. Splenomegaly: spleen enlarges from destroying parasitized RBCs. Fever periodicity: P. falciparum: irregular (all RBCs infected at different stages). P. vivax, P. ovale: 48 hours (synchronised). P. malariae: 72 hours. P. falciparum complications: Cerebral malaria (RBCs with parasites block brain capillaries). Severe anaemia. Blackwater fever (massive haemolysis → haemoglobinuria, black urine). Pulmonary oedema. Renal failure.

4. Gametocyte Formation and Mosquito Phase

Stage C: Some merozoites differentiate into GAMETOCYTES (sexual forms) inside RBCs. Gametocytes are not pathogenic in humans. They are taken up by female Anopheles during blood meal. In mosquito gut: Microgametocyte (male) → releases 4-8 microgametes (exflagellation). Macrogametocyte (female) → macrogamete. Fertilisation: micro + macrogamete → zygote. Zygote → Ookinete (motile, elongated) → penetrates stomach wall → Oocyst (under stomach wall). Oocyst matures → thousands of SPOROZOITES formed by sporogony. Oocyst ruptures → sporozoites migrate to salivary glands. Ready for injection into next human host. Temperature affects development: sporozoites do not form below 16°C — explains why malaria limited to tropical/subtropical regions. Duration in mosquito: 9-18 days (sporogonic period) depending on temperature and species.

5. Malaria Diagnosis and Treatment

Diagnosis: Gold standard: blood smear microscopy (Giemsa stain) — identifies species and stage. Rapid diagnostic tests (RDTs): detect Plasmodium antigens (HRP2 for P. falciparum, aldolase for all species). Quick, no microscope needed. Quantitative PCR: most sensitive, species identification. Treatment: Uncomplicated malaria (P. vivax): chloroquine (where sensitive) + primaquine (to clear liver hypnozoites and prevent relapse). Uncomplicated P. falciparum: artemisinin-based combination therapy (ACT) — artesunate + amodiaquine or lumefantrine. Severe malaria: IV artesunate (WHO recommended, replaced quinine). Chloroquine resistance: widespread in P. falciparum (most areas). P. vivax chloroquine resistance emerging (Papua New Guinea, India). Drug resistance mechanisms: pfcrt, pfmdr1 mutations (chloroquine resistance). Kelch13 mutations (artemisinin partial resistance — Southeast Asia).

6. Malaria Prevention and Vector Control

Prevention strategies: Personal protection: insecticide-treated bed nets (ITNs, LLINs — long-lasting insecticidal nets). Indoor residual spraying (IRS) with insecticides. Protective clothing, repellents (DEET). Chemoprophylaxis: travellers to endemic areas — atovaquone-proguanil, doxycycline, mefloquine, chloroquine (if sensitive area). Vector control: Elimination of breeding sites: drain stagnant water, oil on water. Biological control: Gambusia fish eat larvae. Larviciding: Bacillus thuringiensis var. israelensis (Bti). Sterile insect technique. Indoor residual spraying. Insecticide resistance in Anopheles: pyrethroid resistance widespread. Vaccines: RTS,S/AS01 (Mosquirix): first approved malaria vaccine (2021). Targets P. falciparum circumsporozoite protein. ~30-50% efficacy against clinical malaria. R21/Matrix-M: newer vaccine, ~75% efficacy in trials. WHO recommends for children in high-burden areas (sub-Saharan Africa).

7. Other Protozoan Diseases

Other important protozoan parasites: Entamoeba histolytica: causes amoebic dysentery and liver abscess. Transmitted faeco-orally. Trophozoite (active) and cyst (infective) stages. Metronidazole treatment. Giardia lamblia (intestinalis): Giardiasis (diarrhoea). Binucleate trophozoites (two faces appearance). Transmitted in contaminated water. Common in hikers (beaver fever). Leishmania donovani: Kala-azar (visceral leishmaniasis). Vector: sandfly (Phlebotomus). Fever, splenomegaly, anaemia. Amastigote stage in macrophages. Treated with sodium stibogluconate, liposomal amphotericin B. Trypanosoma: T. gambiense/rhodesiense — sleeping sickness (vector: tsetse fly). T. cruzi — Chagas disease (vector: kissing bug, Triatoma). Trichomonas vaginalis: sexually transmitted, causes vaginitis. Only trophozoite (no cyst). Plasmodium: malaria (vector: Anopheles).

8. Immunity to Malaria

Naturally acquired immunity: Residents of malaria-endemic areas develop partial immunity after repeated infections. This immunity: reduces severity (not infection) → clinical tolerance. Is species and stage-specific. Gradually acquired over years of exposure. Lost after leaving endemic area. Innate resistance factors: Duffy-negative RBCs (FY*Null): common in West Africa → resistant to P. vivax (Duffy antigen is the receptor P. vivax uses to invade RBCs). Sickle cell trait (HbAS): provides protection against P. falciparum severe malaria. HbC, HbE: also protective. G6PD deficiency: protective against falciparum malaria. Thalassaemia trait: protective in some studies. This explains the high frequency of these haemoglobin variants in malaria-endemic regions — they are maintained by balanced polymorphism (heterozygote advantage). Acquired immune responses: Antibodies against merozoite surface antigens prevent RBC invasion. Cellular immunity (CD4+ and CD8+ T cells) against liver stage. Regulatory T cells may limit excessive inflammation.

Frequently Asked Questions
1. What is the sequence of Plasmodium stages in human body? ⌄
Complete sequence in human host: (1) D — Sporozoites injected by female Anopheles mosquito bite into bloodstream. (2) B — Sporozoites travel to liver → enter hepatocytes → divide asexually (exoerythrocytic schizogony) → liver cells burst → merozoites released into blood. (3) A — Merozoites invade RBCs → divide asexually (erythrocytic schizogony) → RBCs rupture → release more merozoites → fever, chills, anaemia. (4) C — Some merozoites in RBCs develop into gametocytes (sexual forms) → female Anopheles takes gametocytes during blood meal → sexual reproduction in mosquito → sporozoites formed → cycle continues.
2. Why does malaria cause fever with a regular periodicity? ⌄
Malaria fever is caused by the synchronised rupture of RBCs releasing merozoites and the toxic haemozoin pigment. The periodicity depends on the time taken for erythrocytic schizogony (from RBC invasion to rupture). P. vivax and P. ovale: erythrocytic cycle = 48 hours → fever every 48 hours = tertian (every third day counting both end days). P. malariae: erythrocytic cycle = 72 hours → quartan fever (every fourth day). P. falciparum: asynchronous → irregular fever (no fixed periodicity) → also called malignant tertian malaria because it can cause continuous high fever. Temperature pattern in malaria attack: cold stage (rigor, shivering as parasites being released) → hot stage (high fever 39-41 degrees C) → sweating stage (fever breaks). Duration of one attack: 6-10 hours.
3. What is the role of the liver phase in malaria? ⌄
The liver (exoerythrocytic) phase serves multiple purposes: Amplification: each sporozoite → up to 30,000 merozoites → exponential increase in parasite numbers before RBC phase begins. Latency/incubation: during liver phase there are NO symptoms (parasites not yet in blood). Incubation period from mosquito bite to first symptoms: P. falciparum: 9-14 days. P. vivax: 12-17 days (or months if hypnozoite). Dormancy (hypnozoites): P. vivax and P. ovale form dormant liver stages (hypnozoites) that can reactivate months to years later causing relapse. Drug target: primaquine (and tafenoquine) specifically target liver stage including hypnozoites → prevent relapse. G6PD deficiency is a contraindication to primaquine (causes haemolysis in G6PD deficient patients). P. falciparum has no liver dormant stage → no relapse.
4. Who is the definitive host and who is the intermediate host in malaria? ⌄
In parasitology: definitive host = where sexual reproduction occurs. Intermediate host = where asexual reproduction occurs. In Plasmodium malaria: Definitive host: FEMALE ANOPHELES MOSQUITO. Sexual reproduction occurs here: gametocytes → gametes → zygote → ookinete → oocyst → sporozoites. Intermediate host: HUMAN. Asexual reproduction occurs here: exoerythrocytic schizogony (liver) and erythrocytic schizogony (RBCs). No sexual reproduction in humans (only asexual). This is often confused: despite humans being the ones who get sick, humans are only the INTERMEDIATE HOST. The mosquito is the definitive host (and also the vector, transmitting the parasite).
5. How does P. falciparum cause cerebral malaria? ⌄
P. falciparum is uniquely dangerous because it causes cytoadherence — infected RBCs express PfEMP1 (P. falciparum erythrocyte membrane protein 1) on their surface. PfEMP1 binds to endothelial receptors (ICAM-1, CD36, PECAM-1) in small blood vessels. This causes: Sequestration: infected RBCs stick to endothelium of deep tissues (brain, lungs, placenta) rather than circulating to spleen for removal. Rosetting: infected RBCs also stick to uninfected RBCs. Knobs: protrusions on infected RBC surface mediate these adhesions. Cerebral malaria results: sequestration in brain capillaries → obstruction → reduced blood flow → ischaemia → neurological dysfunction → coma → death if untreated. 20-25% mortality even with treatment. Survivors often have neurological sequelae.
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