Balantidium coli
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| Balantidium coli (Malmsten, 1857) |
Balantidium coli is a species of ciliate protozoan, and is the only one that is known to be pathogenic to humans. Hosts of this parasite include pigs, wild boars, rats, primates (including humans), horses, cattle and guinea pigs. The cysts are transmitted within or between these species by fecal-oral transmission. Pigs are the most significant reservoir hosts, though they show few if any symptoms.
Cysts are the infective stage of this parasite, responsible for transmission of balantidiasis. The host most often acquires the cyst through ingestion of contaminated food or water. Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine. Both cysts and trophozoites are identifiable by a large, "sausage shaped" macronucleus.
The trophozoites reside in the lumen of the large intestine of humans and animals, where they replicate by transverse binary fission, during which conjugation may occur. Some trophozoites invade the wall of the colon using proteolytic enzymes and multiply. Some return to lumen and disintegrate. Trophozoites undergo encystation which is triggered by dehydration of the feces as they pass into the rectum. Encystation usually occurs in the large intestine, but may also occur outside of the host. Symptoms can be local due to involvement of the intestinal mucosa, or systemic in nature and include diarrhea. Balantidiasis can be treated by carbarsone, tetracycline, or diiodohydroxyquin.
[edit] Geographic distribution
Worldwide. Because pigs are a reservoir, human infections occur more frequently in areas where pigs are raised, and in the Philippines, Mexico, South America and Papua New Guinea.
Less than 1% of the human population is infected worldwide.
