coccidia

Coccidia are a diverse group of obligate intracellular protozoan parasites in the phylum Apicomplexa. They form oocysts and complete a complex life cycle that alternates between sexual and asexual reproduction. The term “coccidia” is informal and encompasses several genera, including Eimeria, Isospora (syn. Cystoisospora), Cyclospora, and Cryptosporidium, which infect a wide range of animals and, in some species, humans.

Infection begins when hosts ingest sporulated oocysts from contaminated food, water, or environmental surfaces. Sporozoites released from the oocysts invade intestinal or other epithelial cells and multiply asexually (merogony), producing merozoites. Several rounds of asexual replication may occur, followed by sexual development that forms micro- and macro-gametes. Fertilization yields new oocysts, which are excreted in feces. Oocysts often require environmental sporulation to become infective and can persist for long periods under favorable conditions.

Disease, coccidiosis, is primarily an intestinal illness. In animals, it varies with species and dose, causing enteritis, villous atrophy, diarrhea, weight loss, and dehydration. Poultry and livestock are economically affected, with particular species of Eimeria causing site-specific lesions (e.g., jejunal, caecal). In humans, disease is generally caused by Cyclospora, Cystoisospora, or Cryptosporidium species and ranges from self-limiting to severe diarrhea in susceptible individuals.

Diagnosis relies on detection of oocysts in feces by microscopy, flotation methods, or molecular assays; lesion observation at necropsy can aid in animals. Control focuses on sanitation, clean water, proper litter management, and, in agriculture, anticoccidial drugs such as ionophores or synthetic coccidiostats, and in some poultry operations, vaccination against multiple Eimeria species.