Plasmodium vivax -Introduction, History, Habitat, Morphology, Culture

Introduction of Plasmodium vivax

The genus Plasmodium causes the vector-borne disease malaria. These parasites show an alternation of generation accompanied by an alternation of hosts. In the human host, the asexual cycle (schizogony) takes place inside the erythrocytes while the sexual cycle (sporogony) takes place in the mosquito host.

Typically, the infected erythrocytes produce pigments that are visualized by light microscopy.

Plasmodium vivax, P. ovale, and P. malariae belong to the subgenera Plasmodium while P. falciparum belongs to the subgenus Laverania.

Plasmodium vivax is the causative agent of benign tertian or vivax malaria. The characteristic of this type of malaria is the occurrence of true relapses.

History of Plasmodium vivax

The first description of Plasmodium vivax came in 1880 by Laveran but he failed to distinguish the species. Six years later, in 1886, Golgi described the parasite as a distinct species of Plasmodium. It was then given the name Haemamoeba vivax in 1890 by Grassi and Feletii.

Habitat of Plasmodium vivax

Plasmodium vivax at various stages is found inside the parenchymal cells of the liver and the red blood cells (erythrocytes). During the erythrocytic schizogony cycle, all the forms of the malaria parasite are found in the peripheral blood.

Morphology of Plasmodium vivax

The various morphological forms of Plasmodium vivax are dependent on the host the parasite is residing in i.e. the diagnostic form or the infective form.

Diagnostic form

The diagnostic forms of Plasmodium vivax found in the human host include

• Early trophozoite (ring form)

• Late trophozoite (trophozoite form)

• Schizont

• Gametocytes

Early trophozoite (ring form)

• are relatively large and occur in peripheral blood

• have a delicate blue-stained ring of cytoplasm with a red chromatin dot

• in some cases, two red chromatin dots can be found separated or closed together

• sometimes, two early trophozoites can be found in a single infected RBC

Late trophozoite (trophozoite form)

• are large and amoeboid

• consists of abundant chromatin, dark pigment granules, and a vacuole which is characteristic

• during this form, a mass of brown hemozoin almost fills the RBC

Schizont

• they are large, round, and irregular

• occupy the entire infected which has also been enlarged

• hemozoin which almost fills the erythrocytes are present in one or two clumps

• mature schizont contains around 12 to 24 merozoites

Gametocytes

There are two types of gametocytes in Plasmodium vivax - macrogametocytes, and microgametocytes.

Macrogametocytes

• measures 10μm in diameter

• they are round or oval, compact, and filled with enlarged erythrocytes

• smaller nucleus with a compact mass of chromatin

• the fine granules are arranged in small masses and occur near the periphery of Plasmodium vivax

• the cytoplasm stains blue while chromatin pigments deep red and violet

Microgametocytes

• smaller than macrogametocytes

• oval or round with a large nucleus

• chromatin granules are arranged to form a spindle

• cytoplasm, which is dark blue, contains dark, coarse hemozoin pigments distributed throughout the cytoplasm

• do not occupy the entire host RBC

* Infected host erythrocytes

• the infected host erythrocytes are enlarged, irregular in shape, and pale in color

• develops Schuffner’s dots, which is a characteristic stippling and can be demonstrated with Romanowky’s stain of blood smear

• electron microscopy reveals surface invaginations surrounded by small vesicles

Infective form

The infective form of Plasmodium vivax for humans is the sporozoites.

Sporozoites

• measures 10μm to 14μm

• sickle-shaped

• presence of a single nucleus

• found in the salivary gland of the mosquito

Culture of Plasmodium vivax

In-vitro

The culture of Plasmodium vivax for routine diagnosis is not done due to limited success as it is still in the experimental stage.

However, it has been cultured in in-vitro in human erythrocytes and is done in media with of following properties:

• consists of RPMI 1940 medium

• an overlay of medium with human serum

• a thin layer of stationary human blood cells

• maintained with 7% carbon dioxide in the atmosphere

• maintained with 1% to 5% oxygen in the atmosphere

Laboratory animals

The Plasmodium vivax malaria parasite has been cultured in several species of primates such as monkeys.