What is Toxoplasma gondii?

Toxoplasma gondii is a protozoan parasite that infects humans as well as many other animals such as pig, sheep and cattle; it is the causative agent of an infectious disease known as Toxoplasmosis. Toxoplasma gondii is one of the organisms that cause the TORCH infections – Toxoplasmosis, Others (such as syphilis, varicella-zoster, parvovirus B19), Rubella, Cytomegalovirus (CMV) and Herpes (common infections that can cause congenital anomalies).

Toxoplasma gondii transmission

1. Humans are commonly infected by eating undercooked meat containing the cyst or by eating food or drinking water that is contaminated with the feces of an infected cat containing the oocyst.
2. A fetus in the womb can be infected when the parasite in an infected pregnant mother crosses the placenta to infect the baby. The earlier the infection in pregnancy, the worse the prognosis
3. Someone who has had toxoplasma gondii infection in the past and the immune system was able to control the infection can still get reinfected whenever the immune system is suppressed. This occurs by reactivation of the oocysts that were in the body issues
4. A rare mode of transmission of t. gondii is through organ transplants where an infected organ is transplanted to an uninfected recipient

Toxoplasma gondii Epidemiology

T. gondii infection occurs worldwide with about 5% to 50% of adults in the United States of America having toxoplasma antibodies. About 1% to 6% of domestic cats in Europe excrete oocyst of toxoplasma gondii in their feces and about 1% of cats in the United States shed the Toxoplasma cysts. Sheep, Goats, Cattle and Pigs are frequently infected. Toxoplasma infection is usually sporadic, but outbreaks associated with ingestion of raw meat or contaminated water supply.

Toxoplasma gondii Characteristics

1. The definitive host of this parasite is the domestic cat and other members of the feline family
2. Humans and other mammals are the intermediate hosts
3. The rapidly multiplying forms of Toxoplasma gondii are called Tachyzoites these reproduces by asexual reproduction inside macrophages and infects more cells
4. The slowly multiplying forms of Toxoplasma gondii are called Bradyzoites these form are cyst that have been controlled by the immune system and they remain in tissues without causing problems in a competent immune system but become reactivated and serve as source of infection when the immune system becomes suppressed by any disease such as tuberculosis, diabetes, HIV/AIDs, chemotherapy etc. The bradyzoites are important for diagnosis of the disease when seen in tissue specimens
5. The cyst of Toxoplasma gondii can still be viable for up to 3 weeks in meat when stored at a temperature of 48 ^oC and can still be infectious at this stage; but deep-freezing of the meat to 208 ^oC kills the bradyzoites within 3 days. Toxoplasma gondii oocysts are resistant to harsh environmental conditions but are rapidly killed when heated to 708 ^oC.

Anatomy of the different life cycle stages of Toxoplasma gondii

1. Tachyzoites these are the proliferative forms of toxoplasma gondii that reproduce rapidly within macrophages by endogenous budding. Tachyzoites are sickle-shaped cells that are about 4 to 7 micrometer long and 2 to 4mm wide.
2. Bradyzoites (also called cystozoites) these are the forms that undergo slow reproduction within the cysts. The cysts have relatively resistant walls and can grow as large as 150 micrometer containing up to several thousand bradyzoites.
3. Oocysts are the rounded and encysted stages of toxoplasma parasite that have resistant cyst walls measuring about 9 by 14 micrometer in size.

 

Toxoplasma gondii Life cycle

1. The Life cycle of Toxoplasma gondii begins within the intestine of the cat as it ingests cysts of toxoplasma gondii in raw meat of mice
2. After ingestion, bradyzoites are released from the cysts in the small intestine – the parasite gains entry into the mucosal cell of the epithelial tissue of the small intestine of the cat
3. In the mucosal cells, they bradyzoites differentiate into male and female gametocytes (the gametes fuse to form oocysts)
4. Oocyst are then excreted in cat feces
5. Development of the parasite in an intermediate host (mammals, birds, humans etc)
6. Infection of man occurs following ingestion of oocyst or consumption of meat of an infected animal containing the cyst.

Birth defects caused by Toxoplasma gondii

1. Cleft lip
2. Hydrocephalus (abnormally large head due to accumulation of cerebrospinal fluid
3. Microcephaly (some babies may have small head instead of large head)
4. Microphthalmia (abnormally smallish eyes)
5. Blindness

Toxoplasma gondii Tests for diagnosis

1. Immunofluorescence assay for IgM antibody is used
2. Culture of the organism can be done
3. Microscopic examination of Giemsa-stained specimens will identify presence oftrophozoites inacute infections. The trophozoites are crescent in shape.

Toxoplasma gondii Prevention

1. Avoid eating undercooked meat – properly cook meat before eat
2. Pregnant women should avoid disposing cat litter