Cryptosporidiosis in Humans
Cryptosporidiosis is a gastrointestinal disease caused by pathogens of the Cryptosporidium genus. In 1907, parasitologist Ernest Edward Tyzzer made observations on the genus and concluded that Cryptosporidium are intracellular protozoan parasites. The genus was named due to its absence of sporocytes within the oocytes, and of 16 species, C. parvum and C. hominis are the more important pathogens in humans (Tzipori 2008). Because Cryptosporidium oocytes are resistant to commercial disinfectants and microbial drugs, several precautions are to be taken in water sources over the world. Cryptosporidiosis in immunocompetent people causes ...view middle of the document...
When ingested, the oocytes travel through the gut lumen to the small intestine. There, the oocytes rupture, releasing the sporozoites which then adhere to and invade the epithelial lining of the gastrointestinal tract. Cryptosporidium disrupts the microvilli that cover the host cell and encloses itself in the host cell membrane. There, the parasite establishes an intracellular niche in which the parasite and the surrounding parasitophosphorus vacuole bulge into the gut lumen and are separated from the host cell cytoplasm (Clark 1999). The parasite then replicates into eight merozoites that rupture out of the host cell and infect other host cells. The merozoites differentiate to sexually reproduce in order to regenerate oocytes which are then excreted in the feces. (Clark 1999)
In both immunocompetent and immunocompromised people, diarrhea is the symptom that often leads to diagnosis since oocytes are found in stool samples. Usually in immunocompetent people with cryptosporidiosis, the diarrhea is abundant and watery, and also usually associated with weight loss. Other symptoms include abdominal pain, nausea and vomiting, and a low-grade fever. In most healthy, well-nourished individuals, diarrhea caused by C. parvum lasts from about 3 to 12 days. In poorly nourished children, oral and intravenous rehydration is often required due to excessive fluid loss. In infants with cryptosporidiosis, malnutrition often contributes to increased length in diarrhea, hospitalization, and even death. (Current 1991)
However, in immunocompromised individuals, such as those with AIDS, the symptoms are more severe and can involve organ systems other than the gastrointestinal tract (Clark 1999). In individuals with intestinal cryptosporidiosis, diarrhea becomes progressively worse over time and is often a cause of death due to excessive fluid loss (Current 1991). There are four clinical categories of AIDS-related cryptosporidiosis: a cholera-like illness, a chronic diarrheal illness, an intermittent diarrheal illness, and a transient diarrheal illness (Clark 1999). Other systems affected by C. parvum include respiratory cryptosporidium, gallbladder and biliary tree cryptosporidium, and pancreatic duct cryptosporidium (Current 1991).
There are several suggestions to why this parasite causes diarrhea in the host. The process of intestinal absorption can be impaired or the secretion can be enhanced through the regulation of the intestinal epithelial cells that are infected by Cryptosporidium. Some studies suggest that Cryptosporidium induces programmed cell death in the biliary epithelial cells. Other studies suggest that Cryptosporidium infection results in the release of cytoplasmic lactate dehydrogenase, which also results in cell death. (Current 1991)
Cryptosporidium is resistant to antimicrobial drugs and to most common disinfectants. Studies found that about 20% of the oocytes of C. parvum within host cells do not form a...