Plasmodium ovale - Introduction, Classification, History, Habitat, Morphology

Introduction of Plasmodium ovale

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 ovale is the causative agent of ovale or mild tertian malaria. They also cause relapse like P. vivax.

Classification of Plasmodium ovale

Kingdom: Chromista

Subkingdom: Harosa

Infrakingdom: Halvaria

Superphylum: Alveolata

Phylum: Apicomplexa

Class: Aconoidasida

Order: Haemospororida

Family: Plasmodiidae

Genus: Plasmodium

Species: P. ovale

History of Plasmodium ovale

MacFie and Ingram, in 1917, first described Plasmodium ovale in Ghana. In 1918, Stephens discovered the malaria parasite in a soldier from East Africa and termed the name Plasmodium ovale.

Habitat of Plasmodium ovale

Plasmodium ovale typically infects young Red Blood Cells.

Morphology of Plasmodium ovale

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

Diagnostic forms in humans

The diagnostic forms of Plasmodium ovale 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

• the cytoplasm of Plasmodium ovale stains deep blue than P. vivax

• also, the cytoplasm of Plasmodium ovale is smaller, thicker, and heavier than that of P. vivax

Late trophozoite (trophozoite form)

• smaller and compact

• consists of coarse pigment granules and an inconspicuous vacuole

• lacks amoeboid form

Schizont

• mature schizonts almost fill their host cells (three-fourths)

• young schizonts are small and compact with few chromatin masses and yellow-coarse coarse pigments

• mature schizont contains around 10 merozoites which are arranged in the form of an irregular rosette

• Plasmodium ovale merozoites are larger than P. malariae

Gametocytes

There are two types of gametocytes- macrogametocytes, and microgametocytes. These gametocytes are smaller and not abundant as P. vivax. They take a longer time to appear in peripheral blood than other malaria parasites.

Macrogametocytes

• 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

• 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 is enlarged, oval, fimbriated

• develops Schuffner’s dots or James’ 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 ovale for humans is the sporozoites.

Sporozoites

• measures 11μm to 12μm

• elongated with pointed ends