Cryptosporidium

Cryptosporidium is an intracellular protozoan whose main human pathogens are Cryptosporidium parvum, the species exams ask about, and C. hominis, now the more commonly identified species in human infections. Transmission is fecal-oral, usually through drinking or recreational water contaminated with oocysts. The oocysts are highly chlorine-resistant, which is why municipal water treatment relies on filtration to remove them and why outbreaks are associated with swimming pools and water parks. The infectious dose is low, so person-to-person and animal-to-human spread also occur readily.

After ingestion, oocysts excyst in the small intestine and the parasite invades the brush border of intestinal epithelial cells, where it sits just under the cell membrane but outside the cytoplasm. The resulting villous blunting produces a non-bloody, watery, secretory diarrhea. In an immunocompetent host the illness is self-limited over one to two weeks. In patients with advanced immunosuppression — classically AIDS with a CD4 count below about 100 cells/µL — infection can become chronic and severe, with profound fluid loss and, less commonly, biliary tract involvement.

Diagnosis rests on identifying oocysts in stool. The classic technique is a modified acid-fast stain, in which the oocysts appear as red spheres against a blue background; stool antigen assays and PCR are also used. Restoring immune function is the definitive management in immunosuppressed patients, and nitazoxanide is the agent approved for treatment.

USMLE Step 1 tests Cryptosporidium in both the parasitology and gastrointestinal pathology sections. High-yield associations to lock in are the acid-fast oocysts on stool microscopy, chlorine-resistant waterborne transmission, non-bloody watery diarrhea, and severe chronic disease in an HIV-positive patient with a low CD4 count.