Malaria causative agents

General characteristics and epidemiology

Malaria pathogens belong to protozoa. Biologically, they are single-celled organisms that function physiologically as completely independent living beings. In nature, protozoa are extremely widespread, inhabiting water and soil. Science knows about thirty thousand species of these microorganisms, of which more than a thousand are pathogenic.

The epidemiology of malaria is truly global in scale. To date, there are more than five hundred million infected people worldwide, making this disease one of the major problems in modern medicine.

Etiology: four main species

Despite the diversity of protozoa, malaria in humans is caused by four main species of plasmodia. Each of them has its own clinical features:

• Plasmodium falciparum is the causative agent of falciparum malaria. It is this species that is associated with the most severe course of the infection.
• Plasmodium vivax is the classic causative agent of vivax malaria.
• Plasmodium ovale also causes vivax malaria, which clinically proceeds similarly to an infection caused by P. vivax.
• Plasmodium malariae is the specific causative agent of quartan malaria.

Extra-human phase (sporogony in the vector)

The life cycle of Plasmodium is impossible without a specific vector, which is represented by mosquitoes of the genus Anopheles. The insect itself becomes infected at the moment of biting a sick person: along with a portion of blood, the mosquito ingests the sexual forms of the parasite called gametocytes.

Once inside the mosquito's body, Plasmodium undergoes the stage of sporogony:

1. Inside the insect, the ingested male and female gametocytes fuse.
2. As a result of this fusion, a zygote is formed.
3. The zygote transforms, and the active formation of sporozoites begins.
4. At the final stage, the sporozoites migrate to the mosquito's salivary glands.

It is the sporozoites that represent the invasive stage. Located in the salivary glands, they are fully prepared to invade the next human's body during the next bite.

Human phase: tissue schizogony

The development of the parasite in the human body is strictly divided into two consecutive stages: tissue (hepatic) and erythrocytic.

The first stage is tissue (extraerythrocytic) schizogony. The invasion process begins at the moment of the bite, when an infected mosquito injects saliva containing sporozoites directly into the human bloodstream. The parasites do not linger in the blood for long: within just thirty minutes, the pathogen purposefully migrates and invades hepatocytes (liver cells).

Inside the liver, a process of massive plasmodia reproduction is triggered, resulting in the formation of tissue schizonts. This stage corresponds to the incubation period of the disease and lasts from one to twelve weeks. Its main clinical feature is that it proceeds completely asymptomatically. However, the scale of reproduction is staggering: from a single sporozoite in the hepatocytes, more than thirty thousand new forms—merozoites—are produced.

Human phase: erythrocytic schizogony

The second stage of development in the human body is erythrocytic schizogony. It begins at the moment when tens of thousands of merozoites rupture the exhausted liver cells and massively re-enter the systemic circulation.

Once in the blood, the merozoites immediately invade red blood cells. Inside the red blood cells, a new cycle of asexual reproduction starts, leading to the formation of blood schizonts.

The medical significance of this stage is enormous: it is the process of erythrocytic schizogony and the subsequent destruction of erythrocytes that fully correspond to the clinical manifestations of malaria. All symptoms of the disease faced by the patient are associated exclusively with this phase of the parasite's life cycle.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphological differences between tissue and blood schizonts
• Mechanisms of hepatocyte destruction upon merozoite release
• Pathogenesis features of severe falciparum malaria (P. falciparum)
• The process of fusion of male and female gametocytes in the mosquito's body
• Pharmacological targets at the stages of tissue and erythrocytic schizogony

Other topics in the "Antivirals" section

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