cryptosporidium

Cryptosporidium is a genus of protozoan parasites in the phylum Apicomplexa that infect the intestinal epithelium of humans and animals. The most important human pathogens are Cryptosporidium parvum and Cryptosporidium hominis; several other species infect animals and can be zoonotic.

The parasite forms oocysts containing four sporozoites. Oocysts are acid-fast and highly resistant to environmental stresses, including standard chlorination, which facilitates waterborne transmission. Humans are infected by ingesting oocysts through contaminated water, food, or contact with infected people or animals, and zoonotic transmission from livestock is common.

In the small intestine, sporozoites invade enterocytes and undergo both asexual and sexual development, producing new infectious oocysts that are shed in feces. The life cycle can occur in a single host, and oocysts can persist in the environment for long periods.

Cryptosporidiosis typically presents with watery diarrhea, abdominal cramps, nausea, and low-grade fever. In immunocompetent individuals, the illness is usually self-limited, resolving within about one to two weeks. In immunocompromised persons, especially those with advanced HIV/AIDS, infection can be chronic and life-threatening with persistent diarrhea and weight loss.

Diagnosis is based on demonstration of oocysts in stool by acid-fast staining, immunofluorescence assays, antigen detection, or molecular methods such as PCR. Treatment is mainly supportive; nitazoxanide is approved for immunocompetent patients, but effectiveness is limited in severe cases or immunocompromised hosts. Restoration of immune function is important in HIV-related disease.

Prevention focuses on safe drinking water and proper filtration, good hygiene, and minimizing exposure in high-risk settings such as childcare facilities and swimming pools. The 1993 Milwaukee outbreak highlighted the public health impact of Cryptosporidium and its environmental persistence.