Cryptosporidium is another sporozoan organism with some similarities to Toxoplasma. It was originally found in cattle with diarrhea, where it caused diarrhea in calves (predominantly 7-14 days old but sometimes up to 30 days old). Other animals and some birds (including turkeys and chickens) also can become infected. Cryptosporidium was next reported to cause diarrhea in humans who were immunocompromised, particularly those with AIDS. Then, it was discovered that Cryptosporidium-associated diarrhea occurred in nonimmunocompromised persons, most often children, with a frequency in Western countries of 0.6%-7.3% and in developing countries of 5%-30% of patients with diarrhea. This incidence is similar to that of Giardia and the major bacterial GI pathogens. Cryptosporidium infection is also found in nonimmunocompromised persons in the cattle industry, male homosexuals, travelers in various parts of the world, and in day-care centers. The organism is rarely found in humans without diarrhea. The type found in cattle and humans (C. parvum) lives predominantly in the small intestine from which oocysts pass in the feces to act as sources of infection. Transmission is most frequently through contaminated water, although person-to-person spread has been reported in families, hospital personnel, and care centers. Cryptosporidium cysts (oocysts) are environmentally resistant and also resistant to standard water chlorination. The average incubation period is said to be about 7 days (range, 1-12 days). In nonimmunocompromised persons, illness and oocyst shedding are nearly always finished by 31 days after exposure. The most severe and persistent infections occur in human immunodeficiency virus 1 infections, particularly in AIDS and AIDS-related illnesses (about 6% of patients; range, 3%-28%). These patients have long-standing watery diarrhea, anorexia, abdominal pain, weight loss, and low-grade fever.

Diagnosis. Diagnosis is most commonly made through stool examination. Although the organisms can be seen in standard concentrated stool preparations, they are hard to identify, are about the same size as yeast cells (4-6 µm), and are most often overlooked. Also, cyst shedding varies from day to day, and there is some correlation between the number of fecal cysts and the presence and severity of diarrhea. Permanent stained slide preparations stained with a special modification of the mycobacterial acid-fast stain has been the most common reasonably effective approach. In two large proficiency test studies, detection rates varied between 75%-96% (in specimens where the participants were instructed to look for cryptosporidia). One study using a standard stool concentration method found that detection needed 5 times as many cysts in formed stools than in liquid stools. Also, various noncryptosporidial objects or organisms in stool specimens may appear acid-fast, requiring observer experience for accurate results. Fluorescent auramine-rhodamine staining has been reported by some (but not others) to be superior to acid-fast slide stain for screening patients. One commercial company markets a kit based on fluorescent monoclonal antibody against Cryptosporidium cyst antigen contained in smears of concentrated fecal specimens on glass slides. In three evaluations to date, this method detected 91% (range, 83%-100%) of cases while Ziehl-Neelsen acid-fast stain detected 85% (range, 76%-93%) of the same cases. Specimens cannot be fixed in PVA or microimmunofluorescent (MF or MIF) stool fixatives. Two companies have recently marketed very similar ELISA kits for combined Giardia and cryptosporidia that use small wells in plastic slides and is read by visual color change. PVA stool fixative cannot be used. In the only two evaluations to date, 93%-97% of cases were detected (in the study with 93% detection, the specimens were not concentrated). One additional company has a somewhat similar kit for cryptosporidia alone; the only published full evaluation to date reported 97% sensitivity.