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Malaria microbiology

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Malaria - Microbiology

The Organism

Malaria is caused by Plasmodium species, which are coccidian/sporozoan (Apicomplexa) parasites of blood cells. Like other coccidia, they require two hosts: the mosquito for sexual reproduction and humans/animals for asexual reproduction. They account for ~216 million episodes with approximately 500,000 deaths annually, 90% of which occur in Africa.

Five Human Species

Species	Disease	Features
P. falciparum	Malignant tertian malaria	Most dangerous; 48-hr cycle
P. vivax	Benign tertian malaria	48-hr cycle; hypnozoites; Duffy antigen receptor
P. ovale	Benign tertian/ovale malaria	48-hr cycle; hypnozoites
P. malariae	Quartan malaria	72-hr cycle
P. knowlesi	Simian/quotidian malaria	24-hr cycle (shortest); zoonosis from macaques in SE Asia

Medical Microbiology 9e, Table 73.1

Life Cycle

Step 1 - Mosquito Bite (Sporozoite Inoculation)

An infected female Anopheles mosquito introduces sporozoites into the human bloodstream via its saliva. Within minutes, sporozoites travel to the liver.

Step 2 - Hepatic (Exoerythrocytic) Stage

Two sporozoite surface proteins - thrombospondin-related adhesive protein and circumsporozoite protein - bind to proteoglycans (heparan sulfate) on hepatocyte surfaces. The sporozoites enter hepatocytes and differentiate into merozoites over 8-25 days (varies by species).

Key feature - Hypnozoites: P. vivax and P. ovale can establish dormant hepatic forms called hypnozoites ("sleeping forms"). These can persist for months to years and cause relapse of infection long after the initial illness. P. malariae does not form hypnozoites but can cause recrudescence from persistent low-level blood-stage infection.

Step 3 - Erythrocytic Stage

After infected hepatocytes rupture, merozoites are released into the bloodstream. A lectin-like molecule on the merozoite surface binds to sialidated glycophorin (a red cell transmembrane protein), allowing invasion into RBCs within a "digestive vacuole."

Inside the RBC, the parasite progresses through:

Ring form (early trophozoite)
Trophozoite (mature; digests hemoglobin producing hemozoin/malarial pigment)
Schizont (divides to form up to 24 merozoites depending on species)
Merozoite release - rupture of the RBC liberates merozoites to infect new RBCs

Some trophozoites differentiate into male and female gametocytes instead, which are taken up by a feeding mosquito to restart the sexual cycle.

Step 4 - Mosquito Stage (Sexual Cycle)

In the mosquito midgut, male and female gametocytes fuse to form an oocyst on the gut wall. Sporozoites develop within the oocyst and migrate to the mosquito's salivary glands, ready to infect a new human host (~2 weeks).

Life cycle diagram (Robbins Pathology, Fig. 10.8 - P. falciparum):

Life cycle of Plasmodium falciparum showing hepatic stage, erythrocytic stage, and mosquito stages

Life cycle overview (Medical Microbiology 9e, Fig. 73.1 - showing hypnozoite relapse pathway):

Life cycle of Plasmodium species showing hypnozoite dormancy and relapse

Pathogenesis

P. falciparum - Mechanisms of Severe Disease

P. falciparum is uniquely dangerous because of cytoadherence:

Infected RBCs express PfEMP1 (Plasmodium falciparum Erythrocyte Membrane Protein 1), an adhesion molecule that concentrates in knob-like extensions on the RBC surface.
PfEMP1 binds to endothelial adhesion molecules - ICAM-1, VCAM-1, and CD36 - causing parasitized RBCs to arrest in capillary beds (sequestration).
This prevents the parasite from circulating through the spleen (where it would be destroyed) but causes microvascular obstruction.

In cerebral malaria, this process involves cerebral vessels, which become engorged and occluded - mainly affecting children in endemic areas, with high fatality.

Fever Periodicity

Episodic shaking chills and fever coincide with synchronized RBC lysis releasing merozoites:

24 hours - P. knowlesi (quotidian fever)
48 hours - P. vivax, P. ovale, P. falciparum (tertian fever)
72 hours - P. malariae (quartan fever)

Anemia

Hemolytic anemia from direct RBC destruction. A characteristic brown pigment called hematin (hemozoin) - derived from hemoglobin digestion - accumulates in and discolors the spleen, liver, lymph nodes, and bone marrow.

Species-Specific Features

P. vivax

Invades only young, immature erythrocytes (reticulocytes)
Receptor: Duffy blood group antigen (though Duffy-negative individuals in Madagascar have recently been found to be susceptible via an unknown mechanism)
RBC features: enlarged RBCs, Schüffner dots (pink granules), amoeboid ring-shaped trophozoite, schizonts with up to 24 merozoites, round gametocytes
Causes hypnozoite-mediated relapse
Most geographically widespread species (tropics, subtropics, temperate zones); >80% of cases in South America and SE Asia
Despite the "benign" label, can cause severe complications: delirium, seizures, renal failure, pulmonary edema, ARDS - similar to falciparum malaria

P. falciparum

Invades RBCs of all ages
Causes highest parasite burden and most deaths
Only species with knobs on infected RBCs (PfEMP1)
Peripheral smear: multiple ring forms per RBC, "appliqué/accolé" forms, banana-shaped gametocytes (the only stage seen in peripheral blood normally)
No hypnozoites; no relapse (but can recrudesce)

P. malariae

Invades old erythrocytes
72-hour (quartan) cycle
Smear: "band forms" (bar forms), "rosette" schizonts with up to 8-10 merozoites
No hypnozoites; can persist for decades as low-level blood infection (recrudescence)
Associated with nephrotic syndrome in children (immune complex deposition)

P. ovale

Similar to P. vivax; forms hypnozoites
Schizonts with oval/fimbriated RBCs

P. knowlesi

Zoonosis from macaques in Southeast Asia
24-hour cycle - can cause rapid, severe disease
Can infect RBCs at all developmental stages
Resembles P. malariae on smear (causes diagnostic confusion)

Laboratory Diagnosis

Method	Details
Thick & thin blood smear	Gold standard; thick smear for detection, thin smear for species ID; Giemsa stain
RDT (Rapid Diagnostic Test)	Detects P. falciparum HRP2 antigen or pan-Plasmodium pLDH; fast, field-friendly
PCR	Most sensitive; useful for low parasitemia and mixed infections
Serology	Cross-reacts among species; used for epidemiology, not acute diagnosis

Note: RDTs are not recommended for P. malariae diagnosis specifically.

Morphology Summary (Blood Smear)

Feature	P. vivax	P. falciparum	P. malariae	P. ovale
RBC size	Enlarged	Normal	Normal/small	Oval, fimbriated
Schüffner dots	Yes	No (Maurer clefts)	No	Yes
Gametocyte shape	Round	Banana/crescent	Round	Round
Max merozoites in schizont	~24	~24	~8-10	~8
Distinctive feature	Amoeboid trophozoite	Multiple rings/cell; appliqué forms	Band forms	Oval RBCs

Key Virulence Factors

Circumsporozoite protein - hepatocyte adhesion; target of the RTS,S (Mosquirix) vaccine
PfEMP1 - cytoadherence and antigenic variation (encoded by ~60 var genes, allowing immune evasion)
Hemozoin (malaria pigment) - activates Toll-like receptor 9, driving systemic inflammation and fever
Rosetting - P. falciparum-infected RBCs bind uninfected RBCs, worsening microvascular obstruction

Recent evidence alert: A 2024 review on artemisinin-resistant malaria (White & Chotivanich, Clin Microbiol Rev) highlights emerging resistance to artemisinin in P. falciparum (kelch13 mutations), particularly in SE Asia and now Africa - a significant update to treatment considerations. A 2025 broad review "Malaria: past, present, and future" provides current epidemiology and vaccine/drug pipeline updates.

Medical Microbiology 9e (Murray), Ch. 73
Robbins & Kumar Basic Pathology, Ch. 10

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