Cryptosporidium was first described in the scientific literature in 1907; however, until the advent of AIDS brought this parasitic protozoan into focus, we thought it only infected animals—AIDS patients, with their seriously compromised immune systems, are particularly susceptible to the parasite. Now we know that cryptosporidium is a cause of parasitic disease in healthy people as well. Today, there are thousands of papers in the medical literature about cryptosporidium but the parasite remains a difficult problem for physicians, diagnostic laboratories, and health departments.
Cryptosporidium is a protozoan parasite
Cryptosporidium is a microscopic parasite, a member of the coccidia—rather distantly related to Plasmodium sp., the coccidians that causes malaria. A cryptosporidium oocyst from a human infection is spherical and about 5 – 7 one thousandths of a millimeter in diameter, about the size of a human red blood cell. Inside the glassy spherical oocyst, four banana shaped sporozoites lie curved against the outer wall. If a suitable host ingests the oocyst, the sporozoites will come out and begin multiplying in the intestine.
There is disagreement about the classification of Cryptosporidium sp.: various species infect birds, reptiles, fishes and mammals. Most strains from human infections are referred to as Cryptosporidium parvum, though authors may also refer to C. muris or C. hominis. Cryptosporidium parvum also infects many other species of mammals, notably cattle, but also dogs, cats, mice, horses, monkeys, domestic poultry and others.
Cryptosporidium is spread by the fecal oral route
An animal or human with cryptosporidiosis passes millions of infective oocysts in the feces. Oocysts are immediately infective, so fecal contamination of surfaces and food items can pass the parasite quickly to another host. Contamination of surface waters however, has the potential to spread the parasite to many more hosts simultaneously. People typically catch cryptosporidiosis in particular circumstances:
- In close living conditions where bathroom facilities are shared and one person is initially infected: hospitals, daycares, dormitories, institutions, family groups etc.
- In communities where a municipal water supply becomes contaminated with sewage or animal wastes and filtration is inadequate.
- In swimming pools where an infected person has contaminated the water.
- In agricultural settings, such as cattle-rearing operations, where workers are exposed to animal feces.
Symptoms of cryptosporidiosis
Cryptosporidiosis can be a very mild infection with few symptoms, or it can be prolonged and debilitating. The classic symptom of cryptosporidiosis is diarrhea—profuse, watery, mucous diarrhea that can result in the loss of litres of fluid in a single day. Other typical symptoms include nausea and loss of appetite, a slight fever, and abdominal pain.
A healthy individual whose immune system is functioning well generally suffers a milder course of this parasitic disease, while one whose immune system is compromised, like an AIDS patient, suffers the worst illness. When the immune system cannot respond, the parasites continue to multiply in the intestine indefinitely, causing prolonged diarrhea, damage to the intestinal lining, and an inability to absorb nutrients.
Controlling cryptosporidium
Cryptosporidium has proved a difficult parasite to control. It’s difficult to detect in water supplies because huge quantities of water must be filtered to recover oocysts. Cryptosporidium is difficult to remove from water supplies because it’s very tiny and resistant to chemicals such as chlorine. It’s difficult to detect in human stools because routine tests for parasites don’t work—very specific and expensive methods must be used to detect the elusive oocysts. And cryptosporidiosis is difficult to treat because most of the drugs we have don’t work, though recent advances have improved the situation somewhat.
To avoid cryptosporidiosis, practice good personal hygiene, never drink untreated water, and comply with boil water advisories.
Related content:
The Canada Goose and Parasites
Sources:
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
