The Organisms Cryptosporidium species, typically Cryptosporidium hominis, can infect the intestine in immunocompromised persons (eg, those with AIDS) and cause severe, intractable diarrhea. They have long been known as parasites of rodents, fowl, rhesus monkeys, cattle, and other herbivores and have prob ably been an unrecognized cause of self-limited, mild gastro enteritis and diarrhea in humans. Oocysts measuring 4–5 μm are passed in feces in enormous numbers and are immediately infectious. When oocysts in contaminated foods and water are ingested, sporozoites excyst and invade intestinal cells; the parasites multiply asexually within the apical portion of the intestinal cells, are released, and infect other intestinal cells to begin a new cycle. They also reproduce sexually, forming male microgamonts and female macrogamonts that fuse and develop into oocysts.
Pathology and Pathogenesis
Cryptosporidium inhabits the brush border of mucosal epithelial cells of the gastrointestinal tract, especially the surface of villi of the lower small bowel (Figure1A). The prominent clinical feature of cryptosporidiosis is watery diarrhea, which is mild and self-limited (1–2 weeks) in normal persons but may be severe and prolonged in immunocompromised or very young or old individuals. The small intestine is the most commonly infected site, but Cryptosporidium infections have also been found in other organs, including other digestive tract organs and the lungs.
Fig1. Cryptosporidium. A: Histologic section of intestine with organisms (arrows) at the apical portion of the epithelial cells. (Courtesy of Pathology, UCSF.) B: Oocysts (4–5 μm) stain pink in stool samples stained with a modified acid-fast stain. (Used with permission from Sullivan J: A Color Atlas of Parasitology, 8th ed. 2009.)
Diagnosis depends on detection of oocysts in fresh stool samples. Stool concentration techniques using a modified acid-fast stain are usually necessary (Figure 1B), and monoclonal antibody-based tests are available that can detect low levels of fecal antigen.
Epidemiology
The incubation period for cryptosporidiosis is from 1 to 12 days, and the disease is acquired from infected animal or human feces or from fecally contaminated food or water. For those at high risk (immunocompromised and very young or old persons), avoidance of animal feces and careful attention to sanitation are required. The organisms are widespread and probably infect asymptomatically a significant proportion of the human population. Occasional outbreaks, such as the one that occurred in Milwaukee in early 1993, affecting more than 400,000 people, can result from inadequate protection, treatment, or filtration of water supplies for large urban centers. In that instance, cattle manure from large dairy farms was the source of contamination of the water supply. As few as 30 organisms can initiate an infection—and the ability of the para site to complete its life cycle, including the sexual phase, within the same individual makes possible the fulminating infections frequently observed in immunosuppressed individuals.
