Eukaryotes Genomes - CRYPTOSPORIDIUM PARVUM

The apicomplexan Cryptosporidium parvum is an obligate intracellular parasite and one of the most prevalent protozoan parasites of humans.

The organism was discovered early in the twentieth century, by Tyzzer in 1907, but its importance was only realised in the 1970s by veterinary workers and In 1976, it was identified at the Johns Hopkins School of Medicine as the causative agent of human cryptosporidiosis. In the early 1980s, the strong association between cases of cryptosporidiosis and immunodeficient individuals (such as those with AIDS--acquired immunodeficiency syndrome) brought Cryptosporidium to the forefront as a ubiquitous human pathogen.

Cryptosporidium parvum is a small parasite, measuring about 3-5 m. It lives on (or just under) the surface of the cells lining the small intestine, reproduces asexually, and oocysts are passed on via the fecal-oral transmission route. Transmission of the infection occurs via the oocysts and the most common methods of aquisition are from contaminated water (the source of most of the major recorded outbreaks), from animals, particularly lambs and calves, through contact with their infected faeces; person to person contact, which is considered especially relevant in children and from contaminated raw foods, e.g. raw meat, pasteurised milk, fruit and vegetables.

The parasite is protected by an outer shell that allows it to survive outside the body for long periods of time and makes it very resistant to chlorine disinfection. During the past two decades, Cryptosporidium parvum has become recognised as one of the most common causes of waterborne disease (drinking and recreational) in humans in the United States. The parasite is found in every region of the United States and throughout the world.

An essential issue in the infective potential of Cryptosporidium oocysts is their resistance to disinfection and environmental pressures. Oocysts can remain viable for about 18 months in a cool, damp or wet environment.

It is currently thought that the form infecting humans is the same species that causes disease in young calves. The forms that infect avian hosts and those that infect mice are not thought capable of infecting humans.

Intestinal cryptosporidiosis is characterised by severe watery diarrhea but may, alternatively, be asymptomatic. Pulmonary and tracheal cryptosporidiosis in humans is associated with coughing and frequently a low-grade fever; these symptoms are often accompanied by severe intestinal distress.

People with healthy immune systems are usually ill with cryptosporidiosis for several days but rarely more than two weeks. Some infected individuals may not even get sick. Some people with cryptosporidiosis seem to recover, then get worse again. Those who are infected may shed oocysts in their stool for months, even after they no longer appear to be ill. Cryptosporidiosis may cause complications for those with illnesses or conditions such as diabetes, alcoholism, or pregnancy. The effects of prolonged diarrhea and dehydration can be dangerous, especially for the very young, the elderly, and the frail. Cryptosporidiosis is most severe and long-lasting in immunocompromised individuals, such as people infected with HIV, cancer patients on chemotherapy, transplant patients, or others taking medications that suppress the immune system. This disease can be life-threatening for the immunocompromised.

Cryptosporidiosis cannot be diagnosed by symptoms alone. Watery diarrhea is a symptom of many intestinal diseases caused by bacteria, viruses, or parasites. There is currently no drug that can cure cryptosporidiosis. People with competent immune systems will recover on their own and appear to develop some immunity to subsequent infections. Anti-diarrheal medicines may ease some of the symptoms.

References:

http://www.biosci.ohio-state.edu/~parasite/cryptosporidium.html
http://vm.cfsan.fda.gov/~mow/chap24.html
http://www.cdc.gov/
http://www.cdfound.to.it/hTML/khan.htm
http://biology.kenyon.edu/slonc/bio38/hannahs/crypto.htm