a. Describe the life cycle and host range of Toxoplasma gondii.
b. How are humans infected?
c. What are the most serious outcomes of infection?
Toxoplasma gondii is a unicellular eukaryote that acts as obligate intracellular parasite in many organisms. It causes toxoplasmosis disease. Intracellular parasites are the ones that can grow and reproduce inside the host cells. They can be either facultative or obligate. Obligate parasites are the ones that rely completely on the resources of their host for reproduction.
a. The life cycle of Toxoplasma gondii includes two components, sexual component, and asexual component. The hosts in which the sexual reproduction occurs are known as the definitive host and the ones in which the asexual reproduction occurs are known as the intermediate hosts. The definitive hosts of Toxoplasma gondii are cats and the intermediate hosts are all other warm-blooded animals. Toxoplasma gondii can infect all the homoeothermic animals.
Once the cats are infected with the parasite, the parasite reaches the epithelial cells of the small intestine through the gastrointestinal tract and infects them. In the epithelial cells of the intestine, the oocytes are formed through sexual reproduction. Oocytes are the cysts that contain zygote. The infected cells containing oocytes are then ruptured and released into the intestinal lumen and ultimately in the environment through feces of the cat. Resources like soil, food, or water, contaminated with cat feces infect other hosts; when ingested.
In the intermediate host, the parasite generally has two stages, the tachyzoite stage, and the bradyzoite stage. Trachyzoite stage is the motile and rapidly dividing stage and is formed from the sporozoites freed from the dissolution of cyst walls in the stomach of intermediate hosts. Intestinal epithelial cells are initially infected by tachyzoites where it divides and on the rupture of the infected cell, enters the bloodstream and reaches other organs. When the immune system acts on these tachyzoite stages, it is converted into the bradyzoite stage which is a slow-growing semi-dormant stage. When the bradyzoites forms clusters in the host cell, it is known as tissue cyst. When tissue cysts are ingested by other animals through raw or undercooked meat, the animal gets infected too. These tissue cysts can persist in the organism for its lifetime. When the tissue cysts are ingested by the definitive hosts, the bradyzoites again undergoes sexual reproduction.
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