Cryptosporidium is a genus of apicomplexan protozoans that can cause gastrointestinal illness with diarrhea in humans. Cryptosporidium is the organism most commonly isolated in HIV-positive patients presenting with diarrhea. Treatment is symptomatic, with fluid rehydration, electrolyte correction and management of any pain. Cryptosporidium oocysts are 4-6 µm in diameter and exhibit partial acid-fast staining. They must be differentiated from other partially acid-fast organisms including Cyclospora cayetanensis.
General characteristics
Cryptosporidium causes the diarrheal illness cryptosporidiosis. Other apicomplexan pathogens include the malaria parasite Plasmodium, and Toxoplasma, the causative agent of toxoplasmosis. Unlike Plasmodium, which transmits via a mosquito vector, Cryptosporidium does not use an insect vector and is capable of completing its lifecycle within a single host, resulting in cyst stages that are excreted in feces and are capable of transmission to a new host.
A number of Cryptosporidium species infect mammals. In humans, the main causes of disease are C. parvum and C. hominis (previously C. parvum genotype 1). C. canis, C. felis, C. meleagridis, and C. muris can also cause disease in humans.
Cryptosporidiosis is typically an acute, short-term infection, but can become severe and nonresolving in children and immunocompromised individuals. In humans, it remains in the lower intestine and may remain for up to five weeks. The parasite is transmitted by environmentally hardy cysts (oocysts) that, once ingested, exist in the small intestine and result in an infection of intestinal epithelial tissue.
The genome of Cryptosporidium parvum, sequenced in 2004, was found to be unusual amongst eukaryotes in that the mitochondria seem not to contain DNA. A closely related species, C. hominis, also has its genome sequence available. The CryptoDB.org NIH-funded database provides access to the Cryptosporidium genomics data sets.
Life cycle
The Cryptosporidium spore phase (oocyst) can survive for lengthy periods outside a host. It can also resist many common disinfectants, notably chlorine-based disinfectants.
Treatment and detection
Many treatment plants that take raw water from rivers, lakes, and reservoirs for public drinking water production use conventional filtration technologies. Direct filtration, which is typically used to treat water with low particulate levels, includes coagulation and filtration but not sedimentation. Other common filtration processes including slow sand filters, diatomaceous earth filter, and membranes will remove 99% of Cryptosporidium. Membranes and bag- and cartridge-filter products remove Cryptosporidium specifically.
Cryptosporidium is highly resistant to chlorine disinfection; but with high enough concentrations and contact time, Cryptosporidium inactivation will occur with chlorine dioxide and ozone treatment. In general, the required levels of chlorine preclude the use of chlorine disinfection as a reliable method to control Cryptosporidium in drinking water. Ultraviolet light treatment at relatively low doses will inactivate Cryptosporidium. Water Research Foundation-funded research originally discovered UV's efficacy in inactivating Cryptosporidium.
One of the largest challenges in identifying outbreaks is the ability to verify the results in a laboratory. The oocytes may be seen by microscopic examination of a stool sample, but they may be confused with other objects or artifacts similar in appearance. Most cryptosporidia are 3-6 μm in size, although some reports have described larger cells. Real-time monitoring technology is now able to detect Cryptosporidium with online systems versus the spot testing and batch testing methods used in the past.
For the end consumer of drinking water believed to be contaminated by Cryptosporidium, the safest option is to boil all water used for drinking.
Exposure risks
The following groups have an elevated risk of being exposed to Cryptosporidium:
- People who swim regularly in pools with insufficient sanitation (Certain strains of Cryptosporidium are chlorine-resistant)
- Child-care workers
- Parents of infected children
- People caring for other people with cryptosporidiosis
- Backpackers, hikers, and campers who drink unfiltered, untreated water
- People, including swimmers, who swallow water from contaminated sources
- People handling infected cattle
- People exposed to human faeces
Cases of cryptosporidiosis can occur in a city with clean water; cases of cryptosporidiosis can have different origins. Like many fecal-oral pathogens, it can also be transmitted by contaminated food or poor hygiene. Testing of water, as well as epidemiological study, are necessary to determine the sources of specific infections. Cryptosporidium typically does not cause serious illness in healthy people. It may chronically sicken some children, as well as adults exposed and immunocompromised. A subset of the immunocompromised population is people with AIDS. Some sexual behaviours can transmit the parasite directly.
