Balantidiasis is a protozoan infection caused by infection with Balantidium coli. Symptoms and signs Usually asymptomatic in immunocompetent individuals, but the symptoms of balantidiasis include: Intermittent diarrhea Constipation Vomiting Abdominal pain Anorexia Weight loss Headache Colitis Marked fluid loss The most common ones are intermittent diarrhea and constipation or inflammation of the colon combined with abdominal cramps and bloody stools. Transmission Balantidium is the only ciliated protozoan known to infect humans. Balantidiasis is a zoonotic disease and is acquired by humans via the feco-oral route from the normal host, the pig, where it is asymptomatic. Fecally contaminated food and water are the common sources of infection in humans. Morphology Balantidium coli exists in either of two developmental stages: trophozoites and cysts. In the trophozoite form, they can be oblong or spherical, and are typically 30 to 150 µm in length and 25 to 120 µm in width. It is its size at this stage that allows Balantidium coli to be characterized as the largest protozoan parasite of humans. Trophozoites possess both a macronucleus and a micronucleus, and both are usually visible. The macronucleus is large and sausage-shaped while the micronucleus is less prominent. At this stage, the organism is not infective but it can replicate by transverse binary fission. In its cyst stage, the parasite takes on a smaller, more spherical shape, with a diameter of around 40 to 60 µm. Unlike the trophozoite, whose surface is covered only with cilia, the cyst form has a tough wall made of one or more layers. The cyst form also differs from the trophozoite form in being non-motile and does not undergo reproduction. The parasite must be ingested as a cyst in order to cause infection. Diagnosis The diagnosis of balantidiasis can be an intricate process, partly because the related symptoms may or may not all be present at once. However, the diagnosis of balantidiasis can be considered when a patient has diarrhea combined with a probable history of current exposure to pigs (as pigs are the primary reservoir), contact with infected persons, or anal sexual contact. The diagnosis of balantidiasis can be made by microscopic examination of stools in search of trophozoites or cysts, or colonoscopy or sigmoidoscopy to obtain a biopsy specimen from the large intestine, which may provide evidence for the presence of trophozoites. Prevention Preventative measures require effective personal and community hygiene. Some specific safeguards include the following: