Coccidiosis is a protozoal disease that causes diarrhea, weight loss, and decreased production in poultry. It is caused by protozoan parasites from the genus Eimeria, which are host-specific, and many species occupy a specific part of the intestine. Virtually every poultry flock raised on litter is affected by coccidiosis to some degree, and birds in high-density flocks or flocks where animals have direct contact with their feces are especially susceptible. The disease is transmitted via ingestion of food, water, and litter contaminated with oocysts. Birds love to peck the floor, and in doing so, they ingest the oocytes (parasite eggs) that are present in the manure of infected birds. Both clinically infected and recovered birds shed oocysts in feces, which contaminate feed, dust, water, litter, and soil.

The pathogenicity of coccidiosis is influenced by host genetics, nutritional factors, concurrent diseases, age of the host, and species of the coccidium. Eimeria necatrix and E tenella are the most pathogenic in chickens, causing extensive hemorrhage. Symptoms of coccidiosis include weight loss, paleness, ruffled feathers, depression, huddling, unwillingness to eat, and watery or bloody diarrhea. Under heavy infections, birds may appear depressed with ruffled feathers, and blood and/or clear to bright orange mucus may be present in the feces.

To manage coccidiosis, the level of exposure to the parasite can be modulated. A reduction in parasite load is normally all it takes to control coccidia in birds, and the ingestion of a few parasites is not a problem and actually helps birds to develop strong immunity against the parasite. Good hygiene practices, such as cleaning and disinfecting the coop and equipment, can also help prevent the spread of coccidiosis.