Coccidia (Coccidiasina) are a subclass of microscopic, spore-forming, single-celled obligate intracellular parasites belonging to the apicomplexan class Conoidasida. As obligate intracellular parasites, they must live and reproduce within an animal cell. Coccidian parasites infect the intestinal tracts of animals, and are the largest group of apicomplexan protozoa.
The class is divided into four orders which are distinguished by the presence or absence of various asexual and sexual stages:
- Order Agamococcidiorida Levine 1979
- Order Eucoccidiorida Léger & Duboscq 1910
- Order Ixorheorida Levine 1984
- Order Protococcidiorida Kheisin 1956
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.
Infected animals spread spores called oocysts in their stool. The oocysts mature, called sporulation. When another animal passes over the location where the feces were deposited, they may pick up the spores, which they then ingest when grooming themselves. The spores may also be ingested by mice; when another animal eats the mouse it becomes infected.
Inside the host, the sporulated oocyst opens, and eight sporozites are released. Each one finds a home in an intestinal cell and starts the process of reproduction. These offspring are called merozoites. When the cell is stuffed full of merozoites, it bursts open, and each merozoite finds its own intestinal cell to continue the cycle.
Symptoms of Infection
As the infection continues, thousands and thousands of intestinal cells may become infected. As they break open, they produce a bloody, watery diarrhea. This can cause dehydration, and can lead to death in young or small pets.
Diagnosis and Treatment
Coccidiosis can be diagnosed by finding oocysts in fecal smears. In early stages of the disease, there may be very few oocysts being shed, and a negative test does not rule out the disease.
Coccidiosis is most commonly treated through the administration of coccidiostats, a group of medications that stop coccidia from reproducing. In dogs and cats, the most commonly administered coccidiostat is sulfa-based antibiotics. Once reproduction stops, the animal can usually recover on their own, a process that can take a few weeks, depending on the severity of the infection and the strength of the animal's immune system. 
- Cryptosporidium (the organism) and Cryptosporidiosis (the resulting disease)
- Toxoplasma (the organism) and Toxoplasmosis (the resulting disease)
- Cyclospora cayetanensis (the organism) and Cyclosporiasis (the resulting disease)
- List of parasites (human)
- Zoalene, a fodder additive for poultry, used to prevent infections from coccidia.
- Ponazuril, a drug used to treat protozoan infection in large animals that has shown to be effective at treating coccidiosis in shelter puppies and kittens.
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- "Biodiversity explorer: Apicomplexa (apicomplexans, sporozoans)". Iziko Museums of Cape Town.
- Coccidia in Dogs
- "Coccidia". Mar Vista Animal Medical Center. Retrieved 28 September 2015.
- Coccidiosis treatment Coccidiosis treatment for Calves and Lambs
- Mar Vista Animal Medical Center.
- The Coccidia of the World, Donald W. Duszynski, Steve J. Upton, Lee Couch, Feb. 21, 2004.
- Life Cycle EIMERIA, Andreas Weck-Heimann, 1996–2005
- FarmingUK, Information about Coccidiosis
- Lillehoj, Hyun S. (October 1996). "Two Strategies for Protecting Poultry From Coccidia". Agricultural Research magazine (United States Department of Agriculture: Agricultural Research Service) (October 1996). Describes using live-parasite vaccine versus a monoclonal antibody to block the sporozoite from invading a host's cell.