Course of Infection and Life Cycle in Humans
Parasite development in the human body is divided into two main stages: exoerythrocytic (hepatic) and erythrocytic schizogony. As a general rule, once erythrocytic replication begins (corresponding to the clinical stage of the disease), tissue replication in the liver ceases. Exceptions to this rule are P. vivax and P. ovale.
Upon infection with these two species, a portion of the sporozoites entering hepatocytes do not develop immediately, entering a dormant state instead. These resting forms are called hypnozoites (or bradyzoites). They can persist in the liver from weeks to months, ensuring pathogen persistence. Activation of hypnozoites is responsible for late relapses. Early relapses, however, arise from parasite forms that persist in the blood during erythrocytic schizogony.
Inside erythrocytes, the pathogen goes through several sequential morphological stages:
- Ring-form trophozoites (young forms with a large vacuole).
- Growing trophozoites of various ages.
- Schizonts (dividing forms).
- Morulas (fully formed merozoites just prior to cell rupture).
- Gametocytes (male and female sexual forms).
Sexual Development Cycle in the Vector
The female mosquito becomes infected when it ingests immature sexual forms of the parasite (gametocytes) along with human blood.
Gamete formation (gametogony) takes place in the insect's midgut. Here, gametocytes mature, fertilization occurs, and a zygote is formed. Shortly after, the zygote transforms into a motile form called an ookinete.
This is followed by sporogony. The ookinete actively penetrates the mosquito gut wall and transforms into an oocyst. Inside this structure, multiple divisions occur with massive productivity: up to 10,000 sporozoites can form within a single oocyst.
In the final stage, sporozoites leave the oocyst and enter the mosquito's hemolymph. Out of this vast number, only a small fraction (about 2%) ultimately reaches the vector's salivary glands to be transmitted to a new host during the next bite.
Species Identification in Blood Smears
Peripheral blood smears (Giemsa staining) allow the differentiation of four Plasmodium species. Each has unique morphological and historical features.
1. P. vivax (discovered by B. Grassi and R. Feletti, 1890) Causes tertian malaria. Infected erythrocytes are enlarged and exhibit fine brick-red stippling (Schüffner dots). The young trophozoite appears as a ring: a large central vacuole, peripheral blue cytoplasm, and a ruby-red nucleus; occasionally 2–3 rings are found in a single erythrocyte. The semi-mature trophozoite is very motile (hence vivax — "lifelike"), displaying an amoeboid shape with pseudopodia. The dividing schizont produces 12–24 merozoites. Gametocytes appear on days 3–4 of the illness.
2. P. malariae (discovered by C. Laveran, 1880) Causes quartan malaria. Typically, only a single young ring-form trophozoite is present in the erythrocyte. The semi-mature form acquires a unique band form. Division yields 6–12 merozoites arranged neatly around the pigment, forming a "rosette" layout.
3. P. falciparum (discovered by W. Welch, 1897) Causes malignant tertian (falciparum) malaria. Infects erythrocytes of any age, inducing large pink-purple spots known as Maurer clefts. Young forms are small rings, often multiple (2–3 per cell). A key feature is that only ring-form trophozoites and gametocytes (characteristic crescent or banana shape) circulate in the peripheral blood. All other developmental stages are sequestered in the capillaries of internal organs.
4. P. ovale (discovered by J. Stephens, 1922) Causes ovale malaria (similar to tertian). Infected erythrocytes are enlarged and frequently acquire a specific oval shape with frayed edges, containing coarse stippling (James dots). The ring stage differs from P. vivax by having a larger nucleus. Schizogony produces 6–12 merozoites.