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Balantidium coli is the largest protozoan parasite to cause infection in humans. Balantidiasis is a zoonotic infection found in pigs, rodents, cattle, reptiles, birds, fish, annelids, arthropods, and many simian hosts. Domestic pigs are the most important reservoir host and typically the source of human infection. Malnourished individuals and those suffering from concurrent infection are at greater risk of developing balantidiasis.
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PATHOGENESIS AND EPIDEMIOLOGY
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The parasite has 2 stages in its life cycle: the trophozoite and the cyst. The cyst is the resting, resistant stage and is the infectious ingested form. The trophozoite form causes manifestations of infection. The cysts are spherical or ovoid with a diameter of 40 to 60 μm and remain viable at room temperature for at least 2 weeks, particularly if kept moist and away from direct sunlight. The motile trophozoite is the form for division. Its shape and size vary with the amount of ingested food, from 30 to 300 μm in length and 30 to 100 μm in width.
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Transmission of the parasite occurs by ingestion of cysts. Following ingestion, excystation occurs in the small intestine with the resultant trophozoites colonizing the large intestine. The trophozoites replicate by binary fission and produce infectious cysts, which pass in the stool. Trophozoites may invade the wall of the large intestine or remain in the lumen and disintegrate. In most instances, they fail to cause any signs or symptoms. The parasite feeds upon bacteria and debris in the gut, but also releases enzymes (hyaluronidase) that attack the mucosal surface.
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Balantidiasis occurs worldwide but is most prevalent in tropical and subtropical regions and in resource-poor countries, which is a reflection of poor sanitation and inadequate protection of the water supply from sewage contamination. Domestic and wild swine represent a reservoir for human infections. Infection occurs when fecal material from swine contaminates drinking water or food. A relatively high prevalence has been found in New Guinea, southern Iran, South and Central America, central Asia, the Philippines, and some Pacific Islands.
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Several cofactors are involved in the pathogenesis of this disease, including the intrinsic virulence of the strain and the host’s susceptibility. Symptomatic infection may be observed in cases of malnutrition, alcoholism, hypochlorhydria, and immunodeficiency.
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CLINICAL MANIFESTATIONS
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Asymptomatic cyst excretion is the most common outcome of infection. In some patients, the trophozoites invade the mucosa and cause acute colitis, which results from the presence of large ulcerative lesions similar to those produced by Entamoeba histolytica. As the trophozoites multiply by binary fission in the mucosa and submucosa, adjacent lesions may anastomose with one another, and the ulcers often extend deeply into the muscularis. Fortunately, perforation or extraintestinal invasion rarely ensues.
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Acute colitis may manifest as mild to severe diarrhea that contains mucus and blood, abdominal pain, nausea, vomiting, and often tenesmus. Secondary ...