Coccidia (Coccidiasina) are a subclass of microscopic, spore-forming, single-celled obligate intracellular parasites belonging to the apicomplexan class Conoidasida.[1] As obligate intracellular parasites, they must live and reproduce within an animal cell. Coccidian parasites infect the intestinal tracts of animals,[2] and are the largest group of apicomplexan protozoa.
Infection with these parasites is known as coccidiosis. Coccidia can infect all mammals, some birds, some fish, some reptiles, and some amphibians. Most species of coccidia are species-specific in their host. An exception is Toxoplasma gondii, which can infect all mammals, although it can only undergo sexual reproduction in cats. Depending on the species of coccidia, infection can cause fever, vomiting, diarrhea, muscle pain, and nervous system effects and changes to behavior, and may lead to death. Healthy adults may recover without medication—but those who are immunocompromised or young almost certainly require medication to prevent death. Humans generally become infected by eating under-cooked meat, but can contract infection with T. gondiiby poor hygiene when handling cat waste.
The class is divided into four orders, which are distinguished by the presence or absence of various asexual and sexual stages:
The order Eucoccidiorida is divided into two suborders. These two groups differ in their sexual development: syzygy for Adeleorina and independent gametes for Eimeriorina.
The first suborder, Adeleorina, comprises coccidia of invertebrates and the coccidia that alternate between blood-sucking invertebrates and various vertebrates; this group includes Haemogregarina and Hepatozoon. There are seven families in this suborder.
The second suborder, Eimeriorina, comprises coccidia of a variety of coccidia many of form cysts. A number of genera, including Toxoplasma and Sarcocystis, infect vertebrates.