The parasite that causes cryptosporidiosis is generally referred to as Cryptosporidium parvum, one of a group of tiny parasites that infect the intestinal and respiratory systems of animals. The cryptosporidium life cycle is complicated and fascinating. For the unfortunate person (often called the host) who contracts this parasitic disease, it begins with swallowing a microscopic Cryptosporidium parvum oocyst:
- Oocysts, found in untreated surface water, contaminated water supplies, cattle manure, and on objects contaminated with human feces, pass through the stomach and enter the small intestine.
- Induced by the favorable environment in the intestine, oocysts open (excyst), each one releasing four crescent shaped sporozoites. Each sporozoite is a tiny parasite.
- The sporozoites push through the surface of intestinal cells, but they don’t go all the way inside. They stop between the layers of the cell wall and begin to multiply, producing the next parasitic stage— six to eight merozoites. This is asexual multiplication: male and female parasites don’t exist at this stage.
- Merozoites break out of the host’s cells, invade new cells and repeat the process.
- Merozoites can continue to invade new cells and multiply in a repeating cycle that results in vast numbers of cryptosporidium parvum parasites and infested intestinal cells. Eventually, however, some merozoites develop differently and become male or female gametocytes.
- Male and female gametocytes join to form gametes, developing between the layers of the cell wall as before. They become oocysts.
- Oocysts break out and are carried away with the intestinal contents. At this stage, the host is likely to have symptoms of cryptosporidiosis—diarrhea quickly carries oocysts out of the body.
- Some oocysts mature very quickly and are able to release their sporozoites before they are passed in the stool. These sporozoites immediately invade intestinal cells and begin to multiply, starting the process of cryptosporidium infection over again.
- When the host’s immune system is healthy, the body eventually fights off the parasite, the intestine heals and symptoms subside. For those whose immune system can’t kill off the parasite, cryptosporidium infection can be virtually endless.
- Oocysts passed into the environment are infective and resilient. They are unaffected by cold, chlorine, and other chemicals added to water to disinfect it. They survive a long time in moist conditions, but perish if they dry out. If they are swallowed by another suitable host, a new cryptosporidium infection ensues.
Note that there are several stages in the cryptosporidium life cycle where the parasite multiplies within the host: when the original sporozoites multiply asexually, when the merozoites continue asexual multiplication, and when oocysts mature quickly and excyst releasing more sporozoites. This is why a symptomatic cryptosporidium infection can develop after a person swallows just a handful of oocysts.
A person with cryptosporidiosis passes millions of infective Cryptosporidium parvum oocysts—it’s easy to see how even minimal contamination of a swimming pool or filtration problems at a water treatment facility can result in a cryptosporidium outbreak involving hundreds or even thousands of people. Large scale cryptosporidium outbreaks seem to be occurring more and more often.
Related content:
Cryptosporidia are coccidia, related to the parasites that cause malaria and toxoplasmosis.
Cryptosporidium parvum can be spread by migrating water birds such as the Canada Goose
Cryptosporidium parvum is a common cause of travelers diarrhea.
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.
Medical Parasitology 5th ed. Leventhal, Ruth and Russell F. Cheadle. Philadelphia: F.A. Davis, 2002.
