Plasmodium falciparum - Introduction, Classification, History, Habitat, Morphology, Culture

Introduction of Plasmodium falciparum

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 falciparum causes falciparum malaria or malignant tertian and is the most virulent of Plasmodium species infecting humans. Almost all the serious forms of malaria, as well as the majority of malaria deaths, are caused by this parasite.

Classification of Plasmodium falciparum

Plasmodium falciparum can be classified by:

Kingdom: Chromista

Subkingdom: Harosa

Infrakingdom: Halvaria

Superphylum: Alveolata

Phylum: Apicomplexa

Class: Aconoidasida

Order: Haemospororida

Family: Plasmodiidae

Genus: Plasmodium

Species: P. falciparum

Habitat of Plasmodium falciparum

Plasmodium falciparum is found in various stages of malaria parasites in the human host. They habitats inside the parenchymal cells of the liver and the red blood cells.

Morphology of Plasmodium falciparum

The various morphological forms of Plasmodium falciparum 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 falciparum found in the human host include

• Early trophozoite (ring form)

• Late trophozoite (trophozoite form)

• Schizont

• Gametocytes

Early trophozoite (ring form)

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

• are relatively large and rarely 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

Late trophozoite (trophozoite form)

• are amoeboid, vacuolated, uninucleated, and delicate

• late trophozoite measures around 1.25μm to 1.5μm in size

• in a stained preparation, a thin ring of blue cytoplasm and the darkish stained nucleus is present

• the forms are compact in cases of heavy infections

• yellow to black colored hemozoin, which are large masses of pigment, almost fill the RBC

• hemozoin are insoluble polymers of haem which is an end product of ingested host hemoglobin

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

• rarely seen in peripheral blood

Schizont

• schizont are large, round, irregular, and asexual diving forms

• occupy two third of infected RBCs, which have also been enlarged

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

• mature schizont contains around 10 to 36 merozoites in a grape-like cluster while the immature ones have only 2 to 4 merozoites

• each merozoites measures 5μm to 10μm in length

Gametocytes

Gametocytes are the sexual and erythrocytic stages of Plasmodium falciparum and are infective forms for mosquitos. They two types of gametocytes- macrogametocytes, and microgametocytes.

Macrogametocytes

• Macrogametocytes measure 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 falciparum

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

Microgametocytes

• microgametocytes are 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 not enlarged and contains several merozoites

• ‘applique’ and ‘accole’ forms are merozoites that lie along the red cell membrane

• in late trophozoite forms, Maurer’s dots or clefts are common

*Infected host WBC

• atypical mononuclear cells are observed

• brown-black malaria pigments are found in monocytes and neutrophils

Infective form

The infective form of Plasmodium falciparum for humans is the sporozoites and is found in infected mosquitoes.

Sporozoites

• sporozoites measures 10μm to 15μm

• sickle-shaped with equally pointed ends and lack pigments

• presence of a single nucleus

• thick pellicle consists of a thin outer membrane, a two-layered membrane, and a layer of subpellicular microtubules

• the posterior end has three polar rings and mitochondrion

• organ of locomotion is the peripheral fibers

• ookinete and oocyst are also found in the female Anopheline mosquito

• found in the salivary gland of the mosquito

Culture of Plasmodium falciparum

The culture of Plasmodium falciparum can be done in-vitro or with laboratory animals.

In-vitro

The culture of Plasmodium falciparum is not done for routine diagnosis. It is rather used for

• study the mechanism of invasion of erythrocytes by merozoites

• biochemical and metabolic studies with each stage

• source of antigen for serological as well as epidemiological studies

• analysis of Plasmodium falciparum antigen

• isolation of antigens for the development of vaccines

Plasmodium falciparum was cultured in-vitro for the first time in 1912 by Bass and Johns. Trager and Jensen cultivated and maintained Plasmodium falciparum in human RBC. This media has the 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

All the in-vitro methods to culture the parasite are modified versions of Trager and Jensen culture media.

Plasmodium falciparum is the only parasite widely cultivated in its asexual cycle.

Laboratory animals

The malaria parasite has been cultured in several species of laboratory animals such as primates such as monkeys.