Toxoplasma gondii - Introduction, Classification, History, Habitat, Morphology, Culture

Introduction of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite causing toxoplasmosis in humans. They are the only protozoans that are infectious to man in all the stages- tachyzoite, tissue cyst, and oocyst.

The definitive host is the feline while the bird or other mammals are the intermediate hosts. Merogony occurs in both definitive hosts and intermediate hosts. The sporogony and schizogony take place in the intestine of the definitive host.

Classification of Toxoplasma gondii

The classification of Toxoplasma gondii is done as:

Kingdom: Chromista

Superphylum: Alveolata

Phylum: Apicomplexa

Class: Conoidasida

Order: Eucoccidiorida

Family: Sarcocystidae

Subfamily: Toxoplasmatinae

Genus: Toxoplasma

Species: T. gondii

History of Toxoplasma gondii

Historically, Toxoplasma gondii was first discovered in 1908 by Nicolle and Manceaux in the spleen, liver, and blood of the North American rodent Ctenodactylus gondii. It was named Toxoplasma gondii in 1909. The coccidia was found to be infecting a man from Darjeeling. It was later found to be infecting a child congenitally in 1937 by Paige, Wolf, and Cowan.

Only in 1970 was the Toxoplasma gondii life cycle fully described with confirmation that cats are definitive hosts are other warm-blooded animals such as birds and mammals are intermediate hosts.

Habitat of Toxoplasma gondii

Since Toxoplasma gondii is an obligate intracellular parasite, they habitat by infecting reticuloendothelial cells and other nucleated host cells such as muscle and intestinal epithelium.

Morphology of Toxoplasma gondii

The morphological forms of Toxoplasma gondii include:

Diagnostic forms

The diagnostic forms of Toxoplasma gondii include tachyzoites and tissue cysts which are important diagnostic forms.

Tachyzoites

• observed during the acute stage of infection

• tachyzoites are active, multiplying trophozoites

• oval to crescent shape with a rounded posterior end and pointed anterior end

• the ovoid nucleus lies in the posterior end of tachyzoites

• capable of invading all types of mammalian cells (except erythrocytes)

• can be found in any organ but most commonly in skeletal muscle, cardiac muscle, and brain

• once inside the host cell, they undergo endodyogeny which is multiplication within a vacuole

Tissue Cyst, Bradyzoites

• resting form of the parasite

• tissue cyst, bradyzoites occur during the chronic stage of Toxoplasma gondii infection

• spherical with size varying from 40mm to 50mm

• each cyst is surrounded by an eosinophilic, weakly PAS-positive cyst wall

• inside each cyst, hundreds of trophozoites called bradyzoites which are strongly PAS-positive are present

• the bradyzoites, which are smaller than tachyzoites, undergo multiplication by endodyogeny

• cysts extremely resistant to host defenses

• tissue cysts can be found in any organ of the host but are most common in brain, skeletal, and heart muscles

Oocyst

• oocyst stage occurs in cats and other felines

• oval measures 10 μm to 12 μm in diameter

• each oocyst is enclosed in a thick, resistant wall

• the unsporulated oocysts which are excreted in the cat feces are non-infectious

• after undergoing sporulation in the environment, the sporulated oocysts containing two sporocysts each containing four sporozoites, are infectious to humans

Culture of Toxoplasma gondii

Toxoplasma gondii can be cultured in-vitro as well as the laboratory animals.

In-vitro culture

• Toxoplasma gondii can be cultured in vitro in normal murine alveolar and peripheral cell lines in tissue culture

• they are yet to be successful to be cultured in a cell-free medium

Laboratory animals

• laboratory animals such as mice and hamsters are intraperitoneally inoculated with Toxoplasma gondii