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Malaria

Malaria

For medical students3 min readUpdated 2026-10-10

A vector-borne infection characterized by cyclic fever paroxysms and erythrocyte destruction. It is caused by intracellular parasites that destroy blood cells and can lead to severe organ damage.

Source of infectionInfected human or parasite carrier
VectorFemale mosquito of the genus *Anopheles* (about 30 species)
DiagnosticsThick blood smear and thin blood film
GeographyTropics (P. falciparum) and temperate climates

Epidemiology and Transmission Routes

Malaria is characterized by high susceptibility, with hundreds of millions of cases globally. In tropical climates, P. falciparum is absolutely dominant, whereas P. vivax and P. malariae are more frequently diagnosed in temperate regions. Infections caused by P. ovale occur sporadically. In non-endemic regions, cases are typically imported, though local transmission foci can occasionally emerge in southern latitudes.

The primary transmission mechanism is vector-borne. Infection occurs via the bite of an infected female Anopheles mosquito. A parenteral route is significantly less common, occurring via blood transfusion or the use of inadequately sterilized medical instruments.

Pathogenesis and Malaria Paroxysm

The incubation period is highly variable, ranging from several weeks to a year, and can extend up to 20 months in tertian malaria. Incubation ends when the parasites enter the bloodstream.

Clinical manifestations are closely linked to the cycle of erythrocytic schizogony. Massive destruction of infected erythrocytes releases pyrogenic substances into the blood: cellular debris, merozoites, and pathogen metabolites. This triggers the classic malaria paroxysm, which proceeds through a strict sequence of phases:

  1. Intense chills accompanied by severe headache.
  2. Sudden temperature spike to 39–40 °C (102–104 °F) or higher.
  3. Rapid temperature drop (lysis) accompanied by profuse sweating and severe weakness.

Depending on the species, paroxysms may occur daily or recur at 1- to 2-day intervals. Prolonged disease leads to damage of parenchymal organs, including the liver, spleen, and kidneys.

Features of Falciparum Malaria

The most dangerous form is falciparum malaria, caused by P. falciparum. Its key feature is that the parasite infects erythrocytes of any age, and its replication occurs predominantly in the microvasculature of internal organs rather than in peripheral blood.

Pathological processes involve massive intravascular hemolysis and the formation of parasitic thrombi that occlude capillaries. The situation is exacerbated by an immunopathogenic component: developing autoimmune reactions destroy even uninfected erythrocytes.

Severe complications include blackwater fever (hemoglobinuric fever), acute kidney injury, and central nervous system involvement (cerebral malaria due to microcirculatory impairment in the brain). Case fatality for this form is approximately 1%.

Immunity and Laboratory Diagnostics

Acquired immunity to malaria is non-sterile, unstable, and strain-specific (targeting a specific species and parasite stage), failing to protect against reinfection. IgG antibodies provide a protective role by enhancing phagocytosis. The parasite actively evades the immune response through developmental stage transitions and antigenic variation. However, natural genetic resistance exists. Individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency, Duffy blood group negativity, or hemoglobinopathies (such as sickle cell trait) have a lower susceptibility.

The primary diagnostic method is microscopy. Blood smears ("thick drops" and thin films) are stained with Romanowsky-Giemsa or Wright's stain (parasite nuclei stain red, cytoplasm blue). In a thick smear, erythrocytes lyse, but pathogen concentration is maximal. Diagnostic feature of P. falciparum: in uncomplicated infections, only ring-form trophozoites and crescent-shaped gametocytes are found in peripheral blood. Additional methods include PCR, DNA hybridization, and serology (IFA, indirect hemagglutination assay, ELISA).

Principles of Treatment and Prevention

Antimalarial drugs are classified by their site of action: they may target sexual or asexual stages (schizonticides, gametocytocides, sporozoiticides). Treatment relies on quinine, mefloquine, chloroquine, primaquine, pyrimethamine, and artemisinin derivatives (artesunate, artemether).

Prevention consists of two pillars:

Genetically engineered vaccines targeting specific parasite forms (sporozoites, merozoites, or gametocytes) are in active development.

Mnemonic

To remember the phases of a malaria paroxysm, use the three "C"s: Chill (with headache) — Conflagration (temperature spike up to 40°C) — Crashing sweat (rapid lysis with diaphoresis and weakness).

Frequently asked questions

Which stages of the plasmodium life cycle occur in the human body?

In the human body, the malaria parasite undergoes asexual reproduction (schizontogony) and initiates the preparation for sexual reproduction. The life cycle includes:

  • Exoerythrocytic schizogony — sporozoites invade hepatocytes, growing and dividing to form tissue merozoites.
  • Erythrocytic schizogony — merozoites invade erythrocytes, sequentially passing through trophozoite and schizont stages to form erythrocytic merozoites.
  • Initiation of gametogony — some erythrocytic merozoites transform into immature sex cells: microgametocytes and macrogametocytes.
How does the developmental cycle of the malaria parasite proceed in the mosquito (sporogony)?

The sexual cycle of the parasite takes place inside the female Anopheles mosquito, comprising gametogony and sporogony. The process consists of:

  • Gametogony — gametocytes mature into macro- and microgametes in the insect's stomach, fusing to form a diploid zygote.
  • Ookinetes formation — the zygote elongates, becomes motile, and penetrates the outer stomach wall.
  • Sporogony — the ookinete transforms into an oocyst, inside which thousands of sporozoites form via multiple divisions.
  • Migration — upon oocyst rupture, sporozoites enter the hemolymph and accumulate in the mosquito's salivary glands.
Which plasmodium species form hypnozoites and cause delayed relapses?

Only two species possess the ability to form hypnozoites and cause delayed exoerythrocytic relapses: Plasmodium vivax and Plasmodium ovale.

Upon infection with these species, a fraction of the liver-invading sporozoites enter a dormant, inactive state known as hypnozoites. They can persist in the liver without dividing for months to years. Subsequent reactivation, accompanied by division and merozoite production, leads to late relapses and a prolonged incubation period.

What changes in complete blood counts and biochemical panels are characteristic of malaria?

Laboratory tests for malaria assess markers of anemia, renal and hepatic impairment, hypoglycemia, and metabolic acidosis.

  • Complete blood count (CBC) — evaluates hemoglobin, RBC indices, platelet, leukocyte counts, and ESR; hemolytic anemia may develop.
  • Biochemical blood panel — measures bilirubin, AST, ALT, BUN, creatinine, and glucose to detect liver/kidney dysfunction and hypoglycemia.
What is the difference between a thick drop and a standard thin smear in diagnostics?

In a thick drop preparation, blood is unfixed, causing erythrocytes and parasites to disrupt and lyse. However, due to the larger blood volume, the pathogen concentration in the field of view is significantly higher, facilitating detection.

What is the tactical approach if a febrile patient has negative blood smears for plasmodia?

If microscopy at the peak of fever yields a negative result, blood sampling and testing must be repeated every 8 hours.

Why are not all developmental stages of the parasite visible in a thin smear during falciparum malaria?

The life cycle of P. falciparum occurs primarily in the microvessels of internal organs. Only ring trophozoites and crescent-shaped gametocytes enter the peripheral blood.

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