General Characteristics The life cycle of B. coli includes both the trophozoite and cyst stages (Figures 1 and 2). The cyst form is the infective stage. After ingestion of the cysts and excystation, trophozoites secrete hyaluronidase, which aids the invasion of the colonic tissue.

Fig1. A, Trophozoite of Balantidium coli. B, Cyst of  B. coli. (From Garcia LS: Diagnostic medical parasitology, ed 5,  Washington, DC, 2007, ASM Press.)

Fig2.  A, Balantidium coli trophozoite. B, B. coli trophozoite. (B courtesy Dr. Henry Travers, Sioux Falls, S.D.)

The trophozoite is quite large, oval, and covered with short cilia. It measures approximately 50 to 150 µm long and 40 to 70 µm wide. The organism can be seen in a wet preparation on lower power. The anterior end is somewhat pointed and has a cytostome (primitive mouth opening); in contrast, the posterior end is broadly rounded. The cytoplasm contains many vacuoles with ingested bacteria and debris. The trophozoite has two nuclei: one very large, bean-shaped macronucleus and a smaller, round micronucleus. The organisms live in the large intestine. The trophozoites have a rapid, rotatory, boring motion because of the movement of the cilia. The cyst is formed as the trophozoite moves down the intestine. Nuclear division does not occur in the cyst; therefore, only two nuclei are present, the macronucleus and the micronucleus. The cysts measure 50 to 70 µm in diameter (see Table 1).

Table1. Intestinal Protozoa—Ciliates

Epidemiology

B. coli is widely distributed in hogs, particularly in warm and temperate climates, and in monkeys in the tropics. Human infection is found in warmer climates, sporadically in cooler areas, and in institutionalized groups with low levels of personal hygiene.

Pathogenesis and Spectrum of Disease

 Some individuals with B. coli infection are asymptomatic, whereas others have severe dysentery, similar to that seen in patients with amebiasis. Symptoms include diarrhea or dysentery, tenesmus, nausea, vomiting, anorexia, and headache. Insomnia, muscular weakness, and weight loss also have been reported. Diarrhea may persist for weeks to months, with or without subsequent development of dysentery. Tremendous fluid loss may occur, with diarrhea similar to that seen in cholera or in some coccidial or microsporidial infections.

B. coli can invade tissue. It may penetrate the mucosa on contact, with cellular infiltration in the area of the developing ulcer. Some of the abscess formations may extend to the muscular layer. The ulcers may vary in shape, and the ulcer bed may be full of pus and necrotic debris. Although the number of cases is small, extraintestinal disease (peritonitis, urinary tract infection, inflammatory vaginitis) has been reported.

Laboratory Diagnosis

Routine stool examinations, particularly wet preparation examinations of fresh and concentrated material, demonstrate the presence of organisms. Organism recognition and identification on a permanent stained smear is usually difficult. These protozoa are large and stain very darkly, which obscures any internal morphology. B. coli organisms may be confused with helminth eggs or debris because of their size, particularly when the cilia are not visible. Recovery of B. coli from specimens in the United States is rare. However, laboratories should be able to identify these organisms and are required to do so in proficiency testing specimens.

Therapy

Tetracycline is the drug of choice for treating B. coli infection, although it is considered investigational for this infection. Iodoquinol or metronidazole may be used as an alternative. Nitazoxanide, a broad-spectrum antiparasitic drug, may be another alternative.

Prevention

 In areas where pigs are raised, the incidence of human infection can be quite high in pig farmers and slaughter house workers. Human infection is fairly rare in temper ate areas, although infections can develop into an epidemic, particularly in areas of poor environmental sanitation and personal hygiene. This situation has been seen in mental hospitals in the United States. Preventive measures involve increased attention to personal hygiene and sanitation measures, because the mode of transmission is ingestion of infective cysts through contaminated food or water.

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