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- Cryptosporidium: intracellular protozoan parasite. Most common species infecting humans are C. hominis, C. parvum, and C. meleagridis.
- Sporulated oocysts (4-6 micrometers in diameter) are ingested, then excyst and attach to intestinal epithelium; trophozoites mature to meronts, which release merozoites leading to zygote formation; oocysts then released through stool into environment.
- In those immunocompromised individuals, parasites develop intracellularly throughout GI tract epithelium and within epithelial cells of biliary tree and pancreatic ducts. Infection causes loss of villi, crypt hyperplasia, and reduction in brush border enzyme activity.
- Environmental oocysts are infectious when shed, can survive for at least 6 months in moist environment, and are resistant to chlorination.
- Sensitive to dessication, hydrogen peroxide, ozone, and UV radiation.
- Fecal-oral route transmission: person-to-person among family members or sexual contacts; tends to occur because infectious dose is small (as few as 10 oocysts); zoonotic and food-borne (raw oysters, raw milk) transmission also possible; most outbreaks due to contaminated water supply (Milwaukee, WI, 1993) or recreational water source.