Cystoisospora

Cystoisospora is a genus of intracellular protozoan parasites in the phylum Apicomplexa. Formerly grouped under Isospora, species of Cystoisospora cause cystoisosporiasis, an intestinal infection in humans and animals. The genus includes several species that parasitize different hosts, such as Cystoisospora belli in humans; Cystoisospora suis in pigs; and Cystoisospora felis and Cystoisospora rivolta in cats, with Cystoisospora canis described in dogs.

Cystoisospora has a direct life cycle. Oocysts are shed in feces and, in the environment, sporulate to become infective. Ingested sporulated oocysts release sporozoites that invade enterocytes and undergo asexual (schizogony) and sexual replication to produce new oocysts that are excreted. Oocysts are ellipsoidal, about 20–30 micrometers long, with two sporocysts, each containing four sporozoites.

In humans, infection causes cystoisosporiasis characterized by watery diarrhea, abdominal cramps, and weight loss. Illness is usually mild in immunocompetent individuals but can be severe and prolonged in immunocompromised patients, particularly those with HIV/AIDS. In animals, disease ranges from subclinical to diarrheal illness, depending on species, age, and immune status, with pigs (C. suis) being a notable reservoir in veterinary contexts.

Diagnosis relies on detection of oocysts in stool by modified acid-fast staining or immunofluorescence; molecular methods such as PCR may aid confirmation. First-line treatment is trimethoprim-sulfamethoxazole; alternatives exist for those with sulfa allergy, under medical guidance. Prevention focuses on good sanitation and hygiene to reduce fecal-oral transmission, with attention to animal sanitation and water quality in settings with at-risk populations.