Balantidiasis
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: 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. “Contaminated water is the most common mechanism of transmission. Equally dangerous, however, is the ingestion of contaminated  food.” 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 because it is non-motile and does not undergo reproduction. Instead, the cyst is the form that the parasite takes when it causes infection. Diagnosis. The diagnosis of balantidiasis can be an intricate process, partly because the related symptoms may or may not  be present. However, the diagnosis of balantidiasis can be considered when a patient has diarrhea combined with a probable history of current exposure to pigs (since pigs are the primary reservoir), contact with infected persons, or anal intercourse. In addition, 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 from the large intestines which may provide evidence for the presence of trophozoites. Prevention. Preventative measures require effective personal and community hygiene. Some specific safeguards include the following: Treatment. Balantidiasis can be treated with  tetracycline, metronidazole or iodoquinol. History. The first study to generate balantidiasis in humans was undertaken by Cassagrandi and Barnagallo in 1896. However, this experiment was not successful in creating an infection and it was unclear whether "Balantidium coli" was the actual parasite used. The first case of balantidiasis in the Philippines, where it is the most common, was reported in 1904. Currently, "Balantidium coli" is distributed worldwide but less than 1% of the human population is infected. Pigs are a major reservoir of the parasite, and infection of humans occurs more frequently in areas where pigs comingle with people. This  includes places like the Philippines, as previously mentioned, but also includes countries such as Bolivia and Papua New Guinea. But pigs are not the only animal where the parasite is found. For example, Balantidium coli also has a high rate of incidence in rats. In a Japanese study that analyzed the fecal samples in 56 mammalian species, "Balantidium coli" was found to be present not just in all the wild boars tested (with wild boars and pigs being considered the same species), it was also found in five species of non human primate: Chimpanzee ("Pan troglodytes"), White-handed gibbon ("Hylobates  lar"), Squirrelmonkey ("Saimiri sciurea"), Sacred baboon ("Comopithecus hamadryas"), and Japanese macaque ("Macaca fuscata"). In other studies, "Balantidium coli" was also found in species from the orders Rodentia and Carnivora.