Walter T. Hughes

Cryptosporidiosis is a common enteric infection of humans and animals that is caused by the coccidian protozoan Cryptosporidium. The first human case was described in 1976. The infection came to prominence in the early 1980s because of its association with the acquired immunodeficiency syndrome (AIDS) and other immunocompromised states. Furthermore, by the mid-1980s, it had become recognized as a frequent cause of diarrhea in otherwise physiologically normal children, especially those attending day-care centers.


Cryptosporidium species are small, intracellular and extracytoplasmic protozoan parasites belonging to the protist phylum Apicomplexa, class Coccidia, family Cryptosporidiidae. The oocyst form of Cryptosporidium resides in the feces and is the infective stage. Each parasite completes its life cycle on intestinal surface epithelial cells. Sporozoites within the oocyst mature, undergo sporulation, and are released into the intestine, where they attach to the microvillar surface of epithelial cells. The organism is enveloped by the membrane of the host cell, thus forming a vacuole. Merogony, gametogony, and sporogony occur. Macrogametes and microgametes develop and on fertilization form an oocyst with four sporozoites. The oocyst is passed into the feces. At least ten species of Cryptosporidium have been identified. Cryptosporidium parvum causes most of the infections in humans, although C. felis, C. muris, and C. meleagridis have been found in a few cases.


Human-to-human, animal-to-human, and human-to-animal transmission of Cryptosporidium has been reported. Outbreaks have been traced to contaminated water and food supplies.

Cryptosporidium is a common cause of enteric infection worldwide. Immunocompromised hosts with impaired cell-mediated immunity are at increased risk for acquiring cryptosporidiosis and expression of severe disease. Surveys of selected populations reveal carrier prevalence rates ranging from 0.6% to 4.0% in North America and from 4% to 20% in developing countries. Approximately 3% to 4% of patients with AIDS in the United States and 50% of those in Haiti and Africa are infected. Cryptosporidium has been isolated from the stools of 4.1% of hospitalized, immunocompetent patients with diarrhea in Australia, from 1.4% of a similar population in the United Kingdom, from 4.3% of Costa Rican children, and from 6.1% of immunocompetent patients with diarrhea in developing countries. Estimates are that diarrhea caused by Cryptosporidium occurs in 20 to 30 million children annually in Latin America. Because the oocyst is highly stable in the environment, contaminated drinking water, apple cider, spas, waterparks, fountains, hot tubs, and swimming pools are sources of outbreaks of infection. A contaminated public water supply resulted in an epidemic of more than 400,000 cases of cryptosporidiosis in Milwaukee, Wisconsin. Some studies indicate that Cryptosporidium oocysts are present in 65% to 95% of surface water (i.e., rivers, lakes, and streams) tested throughout the United States. Between 1989 and 2000, more than 170 outbreaks were associated with recreational water venues, with almost half resulting in episodes of gastrointestinal illness.

In the United States, 13% of children younger than 5 years, 38% of those 5 to 13 years of age, and 58% of adolescents and young adults 14 to 21 years of age are seropositive for antibodies to C. parvum. Several outbreaks of diarrhea attributable to Cryptosporidium have occurred in day-care centers in the United States, which currently are the setting of highest risk for otherwise normal children. In these outbreaks, other organisms, such as Giardia lamblia, also may be found in stool samples. When this occurs, however, Cryptosporidium usually is the causative agent of the diarrhea. Almost one-third of household members of children infected in a child-care center outbreak reported diarrhea, as compared with only 3% of household members of uninfected control children.

Person-to-person transmission has been reported in the hospital environment, a finding suggesting the need for enteric isolation precautions to protect susceptible immunocompromised patients. Animal-to-human transmission may occur, especially from calves. C. parvum oocysts have been found in the feces of 3% to 5% of cats. Flies can carry oocysts from unsanitary sites to other surfaces.


The jejunum is the usual site of C. parvum infection, although in the severely immunocompromised host with AIDS, the entire
intestinal tract from mouth to rectum, including the gallbladder and biliary duct, may be affected. The histologic pattern is one of organisms adherent to the surface of the enterocytes between the microvilli and the parasitiferous vacuole. Varying degrees of villous atrophy, crypt hyperplasia, and infiltration of the lamina propria with neutrophils and monocytes may occur. With mucosal infection, intraepithelial CD4 lymphocytes participate in host responses to control the infection, in part through production of interferon-gamma, which inhibits development of the organism in enterocytes. Moreover, infection is associated with increases in local cytokines, such as interleukin-12 and tumor necrosis factor-alpha in addition to interferon-gamma. Patients with peripheral blood CD4 lymphocyte counts less than 200 cells/mm3 are at increased risk of cryptosporidiosis. Responses of specific immunoglobulins G, A, M, E, and secretory A have been demonstrated, but infection and diarrhea may persist in patients with high levels of both serum and secretory antibodies. Diarrhea is associated with impaired intestinal absorption and enhanced excretion.

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Jul 24, 2016 | Posted by in ORTHOPEDIC | Comments Off on Cryptosporidiosis
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