Epidemiology and transmission routes
Malaria is characterized by high susceptibility—hundreds of millions of people fall ill worldwide. In tropical climates, the species P. falciparum is absolutely dominant, whereas in temperate regions P. vivax and P. malariae are more commonly diagnosed. Infection caused by P. ovale occurs in sporadic cases. In Russia, the disease is most often imported, although local foci may form in the south of the country.
The main transmission mechanism of the pathogen is vector-borne. Infection occurs through the bite of an infected female mosquito of the genus Anopheles. The parenteral route is significantly less common: parasite transmission is possible via blood transfusion (hemotransfusion) or as a result of using medical instruments that have not undergone proper sterilization.
Pathogenesis and malarial paroxysm
The incubation period is highly variable. It can last from several weeks to a year, and in the tertian form of malaria, it can stretch up to 20 months. Incubation ends when the parasites enter the bloodstream.
Clinical manifestations of the infection are closely related to the cycle of erythrocytic schizogony. Mass destruction of affected red blood cells is accompanied by the release of pyrogenic substances into the blood: cellular debris, merozoites, and metabolic waste products of the pathogen. This triggers the classic malarial attack (paroxysm), which features a strict paroxysmal phase alternation:
- Intense chills accompanied by a severe headache.
- A sudden temperature spike to 39–40 °C and higher.
- A rapid temperature drop (lysis) accompanied by profuse sweating and severe weakness.
Depending on the species of the pathogen, paroxysms can occur daily or repeat with an interval of 1–2 days. With a prolonged course of the disease, parenchymal organs—the liver, spleen, and kidneys—are affected.
Features of tropical malaria
The most dangerous form is tropical malaria, caused by P. falciparum. Its main feature is that the parasite infects red blood cells of any age, and its reproduction occurs predominantly in the small vessels of internal organs rather than in the peripheral blood.
The pathological process is accompanied by massive intravascular hemolysis and the formation of parasitic thrombi that clog capillaries. The situation is exacerbated by the immunopathological component: developing autoimmune reactions lead to the destruction of even those red blood cells that were not infected.
Severe complications include blackwater fever, acute renal failure, and CNS involvement (malarial coma due to microcirculation disorders in the brain). The mortality rate for this form is about 1%.
Immunity and Laboratory Diagnostics
Acquired immunity to malaria is non-sterile, unstable, and narrow-specific (to a specific species and stage of the parasite). It does not protect against reinfection. The protective function is performed by IgG class antibodies, which enhance phagocytosis. The pathogen actively evades the immune response by changing developmental stages and utilizing mechanisms of antigenic variation. At the same time, natural genetic resistance exists. Individuals with glucose-6-phosphate dehydrogenase deficiency, absence of Duffy antigens on red blood cells, or the presence of hemoglobinopathies (e.g., sickle cell anemia) are less susceptible to the disease.
The main diagnostic method is microscopic. The doctor examines blood preparations («thick drop» and smear) stained with Romanowsky-Giemsa or Wright stain (the parasite's nucleus turns red and the cytoplasm turns blue). In the «thick drop», red blood cells are destroyed, but the pathogen concentration is maximal. Feature of P. falciparum diagnostics: in uncomplicated cases, only ring-shaped trophozoites and crescent-shaped gamonts are found in the peripheral blood. Additionally, PCR, DNA hybridization, and serology (IFA, HAI, ELISA) are used.
Principles of treatment and prevention
Antimalarial drugs are classified by their site of action: they can act on sexual or asexual stages (schizontocidal, gamontotropic, sporozoitotropic). Quinine, mefloquine, chloroquine, primaquine, pyrimethamine, as well as artemisinin derivatives (artesunate, artemether) are actively used in treatment.
Prevention includes two areas:
- Public: destruction of mosquitoes and detection of patients (infected individuals and carriers).
- Individual: chemoprophylaxis in endemic zones. Mefloquine is used (if there is a risk of encountering chloroquine-resistant P. falciparum), savarine (proguanil + chloroquine), or a combination of chloroquine and primaquine.
Genetically engineered vaccines are in active development, targeting specific forms of the parasite (sporozoites, merozoites, or gamonts).
Briefly: answers to topical questions
What stages of the plasmodium life cycle occur in the human body?
In the human body, the malaria plasmodium goes through the stages of asexual reproduction (schizogony) and the beginning of preparation for the sexual process. The life cycle includes the following stages:
- Exoerythrocytic schizogony — sporozoites penetrate hepatocytes, where they grow and divide to form tissue merozoites.
- Erythrocytic schizogony — merozoites invade red blood cells, sequentially passing through the trophozoite and schizont stages with the formation of erythrocytic merozoites.
- Onset of gametogony — some erythrocytic merozoites transform into immature sex cells: microgametocytes and macrogametocytes.
How does the development cycle of the malaria plasmodium proceed in the mosquito's body (sporogony)?
In the body of a female mosquito of the genus Anopheles, the sexual development cycle of the plasmodium takes place, including gametogony and sporogony. The process consists of the following stages:
- Gametogony — in the insect's stomach, gametocytes mature into macro- and microgametes, fuse, and form a diploid zygote.
- Ookinete formation — the zygote elongates, becomes motile, and penetrates under the outer membrane of the stomach.
- Sporogony — the ookinete turns into an oocyst, inside which thousands of sporozoites are formed by multiple division.
- Migration — after the oocyst ruptures, sporozoites enter the hemolymph and accumulate in the mosquito's salivary glands.
Which plasmodium species form hypnozoites and cause delayed relapses?
Only two species of pathogens have the ability to form hypnozoites and cause delayed exoerythrocytic relapses: Plasmodium vivax and Plasmodium ovale.
When infected with these species, some of the sporozoites that have penetrated hepatocytes enter an inactive sleeping state. Such forms are called hypnozoites. They can persist in the liver without division from several months to several years. Their subsequent reactivation, accompanied by division and the formation of merozoites, leads to the development of late relapses of the disease and causes a prolonged incubation period.
What changes in clinical and biochemical blood tests are characteristic of malaria?
For malaria, laboratory tests evaluate signs of anemia, renal and hepatic impairment, hypoglycemia, and metabolic acidosis.
- Clinical blood test — hemoglobin, color index, red blood cell, platelet, white blood cell count, and ESR are examined; hemolytic anemia may develop in malaria.
- Biochemical blood test — bilirubin, AST, ALT, urea, creatinine, and glucose are evaluated to detect liver and kidney dysfunction, as well as hypoglycemia.
To memorize the phases of a malarial attack, use the rule of three «C»s: Chills (with headache) — Conflagration (temperature spike up to 40°C) — Convalescent sweating (rapid lysis with sweat and weakness).
Class and exam questions
What is the difference between a "thick smear" and a conventional smear in diagnostics?
In the "thick smear" preparation, blood is not fixed, which causes erythrocytes and the parasites themselves to become distorted. However, due to the larger blood volume, the pathogen concentration in the preparation is significantly higher, which simplifies its detection.
What is the appropriate management if a patient with fever has negative blood smears for Plasmodium?
If microscopy at the peak of fever gives a negative result, blood collection and examination must be repeated every 8 hours.
Why are not all stages of parasite development visible in the blood smear in falciparum malaria?
The life cycle of P. falciparum takes place predominantly in the small vessels of internal organs. Only ring-shaped trophozoites and crescent-shaped gamonts enter the peripheral blood.
What else to study on this topic
- Erythrocytic schizogony cycle: from merozoite to cell destruction
- Mechanism of blackwater fever formation
- Immunological evasion of Plasmodium
- Pharmacodynamics of antimalarial drugs (schizontocidal and gamontotropic)
- Natural genetic resistance to malaria
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