TABLE OF CONTENTS
Tissue Cyst-Forming Coccidia (Sarcocystidae) and Toxoplasma: Classification & Life Cycle
Tissue cyst-forming coccidia are important protozoan parasites belonging to the family Sarcocystidae. Unlike Eimeria, these parasites have a heteroxenous life cycle involving both definitive and intermediate vertebrate hosts. Among them, Toxoplasma is one of the most significant genera because of its wide host range and veterinary and zoonotic importance.
The members of the family Sarcocystidae are closely related to Eimeriidae (monoxenous), except in their life cycle pattern. The members of Sarcocystidae are heteroxenous parasites; their life cycle largely depends on the existence of a predator-prey relationship.
In the life cycle, asexual reproduction (merogony) occurs in the vertebrate intermediate host (prey animals—cattle, sheep, and other herbivores), while sexual reproduction (gametogony) occurs in the definitive hosts, mainly carnivores such as cats, dogs, birds, and other felids (predators). Both the definitive and intermediate hosts are vertebrates.
There are two subfamilies in the family Sarcocystidae, namely:
- Sarcocystinae – Genus: Sarcocystis
- Toxoplasmatinae – Genus: Toxoplasma
Genus Toxoplasma (Toxo – Arc and Plasma – Form)
Toxoplasma was first described in the North African desert rodent Ctenodactylus gundi in 1908. Thereafter, it was recorded in more than 200 species of mammals and birds. Toxoplasma is a facultatively or obligatorily heteroxenous parasite.
In the life cycle, asexual reproduction occurs in the parenteral tissues of the intermediate host as well as the definitive host, whereas sexual reproduction occurs only in the intestinal epithelial cells of the definitive host. In Toxoplasma, two developmental stages, namely tachyzoites and bradyzoites, occur, but metrocytes are not formed.
Oocysts are formed from the zygote resulting from the fusion of male and female gametes. The mature oocysts are passed in the feces of the definitive host, where they undergo sporulation in the environment.
