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Quiz 18

Presentation of Quiz #18

A 42-year-old hog farmer developed diarrhea six to eight times a day. He reported that occasionally, there was blood and mucus in the stool. He also complained of abdominal colic, tenesmus, nausea, intermittent vomiting and occasional headaches. After five days of dysentery, he went to see his physician. During the discussion regarding his medical history, he indicated he also had mild diarrhea for several months. His past medical history was unremarkable. He had no history of travel outside of the United States.

The examination revealed a well-developed, well-nourished man who appeared to be somewhat uncomfortable with some tenderness over the colon. Stool examinations revealed the following:

This object measured about 70 by 40 microns and was easily seen in the saline wet preparation examination.

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Answer and Discussion of Quiz #18

The image presented in Diagnostic Quiz #18 is the following:

  1. This image is of a Balantidium coli trophozoite, an infection that is commonly seen in hogs, particularly in warm and temperate climates. This organism is a protozoan ciliate with infective cysts; invasive disease can also occur. Extraintestinal infection has occasionally been reported, involving the peritoneum, urinary tract, or vagina. Although human infection is rare within the United States, it has been reported, often being associated with hogs, as in this case. Although B. coli have been reported from many simian hosts, domestic hogs probably serve as the most important reservoir host for human infection. NOTE: The cilia are easily seen in the trophozoite image above, as is the large macronucleus and one large, round vacuole.

Comment: Although some individuals are asymptomatic, the majority of patients report diarrhea or dysentery. Also, a long period of diarrhea may precede dysentery, as seen in this case. B. coli is the only pathogenic ciliate and is the largest of the protozoa that parasitize humans. The trophozoite is quite large, oval, and covered with short cilia; it measures approximately 50 – 100 (occasionally over 100 microns) by 40 – 70 microns.

The trophozoite can be seen in a wet preparation on lower power. The anterior end is somewhat pointed and has a cytostome (primitive mouth opening); while the posterior end is broadly rounded. The cytoplasm contains many vacuoles with ingested bacteria and debris. There are two nuclei within the trophozoite, one very large bean-shaped macronucleus and the smaller round micronucleus. The organisms normally live in the large intestine. 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 cysts measure from 50-70 microns in diameter.

In severe cases, there may be tremendous fluid loss, with a type of diarrhea similar to that seen in cholera or in some coccidial or microsporidial infections. B. coli have the potential to invade tissue. On contact with the mucosa, the organism may penetrate the mucosa with cellular infiltration in the area of the developing ulcer. Some of the abscess formations may extend to the muscular layer. The ulcers vary in shape, and the ulcer bed may be full of pus and necrotic debris. As indicated above, the number of cases is small, but extraintestinal disease has been reported (peritonitis, urinary tract, inflammatory vaginitis).

Although diagnosis can be made from examination of wet preparations, identification from the permanent stained smear may be very difficult. These protozoa are so large that they tend to stain very darkly, thus obscuring any internal morphology. B. coli organisms may even be confused with helminth eggs because of their size, particularly when the cilia are not visible.

Tetracycline is currently the drug of choice, although it is considered investigational for this infection. Iodoquinol or metronidazole may be used as alternatives.

Three images of Balantidium coli from a routine stool examination (wet preparation). The first image is the trophozoite seen above in a saline wet preparation (note the presence of the cilia). The middle image shows a cyst photographed in a saline wet mount, but at a somewhat lower magnification. The image on the far right shows a trophozoite photographed from an iodine wet mount; note the large bean-shaped macronucleus.

This is an image showing the cyst as it appears in a permanent stained smear. Occasionally, the cysts and/or trophozoites stain so darkly, they can be confused with helminth eggs. Therefore, the wet preparation examination is recommended for the identification of B. coli organisms.


    1. Garcia, LS, 2016. Diagnostic Medical Parasitology, 6th Ed., ASM Press, Washington, DC.
    2. Garcia, L.S. 2009. Practical Guide to Diagnostic Parasitology, 2nd Ed., ASM Press, Washington, D.C.


Each Quiz has a two section format: the first section will present the Quiz topic and the second section will provide a discussion of the answer and/or various options in response to the Quiz situation presented to the user. In some situations, there may be more than one correct response.

The content within this site is made possible through the extensive contribution of Lynne S. Garcia, M.S., MT(ASCP), CLS(NCA), BLM(AAB), F(AAM), Director, Consultantation and Training Services (Diagnostic Medical Parasitology and Health Care Administration). For additional information, she can be contacted at

Reference: Garcia, L.S. 2015. Diagnostic Medical Parasitology, 6th Ed., ASM Press, Washington, D.C.