Balantidium coli are mainly asymptomatically harbored by pigs in their large intestines, but other mammals such as nonhuman primates and boars can also serve as reservoirs. It has worldwide distribution but is more commonly reported in tropical and subtropical developing countries, in particular the Philippines, Papua New Guinea (reported prevalence of 28% among pig farmers), the Middle East, and several Latin American countries (prevalence 6%-29% Altiplano region of Bolivia). It is very seldom found and reported in developed countries.
The life cycle is very simple with only 2 stages: 1) a cystic infective stage and 2) an invasive trophozoite that infects the entire large intestine from cecum to rectum. The cyst has a thick wall that protects it from desiccation and allows its survival for at least 2 weeks at room temperature. Trophozoites are very sensitive to temperature and can only be observed in readily processed stool samples or in tissue sections.
Humans get the infection by ingesting cysts, usually from contaminated water sources or less commonly from contaminated food. Human-to-human transmission has been anecdotally reported, but it is possible in the setting of asylums, orphanages, mental health institutions, and prisons where poor hygiene may spread the infection easily.
Three clinical manifestations can occur:
- Asymptomatic infections are the most predominant form of infection. Repeated infections since childhood may protect individuals living in endemic areas from developing symptomatic disease. Interestingly, attempts at infecting humans have not been successful, indicating that even among exposed persons, symptomatic disease is infrequent.
- Chronic nonbloody diarrhea accompanied by nonspecific abdominal symptoms.
- Acute dysentery with tenesmus and abdominal pain is the classic yet infrequent form of presentation. Rarely, patients with this presentation may progress to a fulminant clinical course with intestinal bleeding and/or intestinal perforation, both associated with mortality above 30%. Extra-intestinal manifestations have been very rarely reported, including necrotizing pulmonary involvement, which may result from bloodstream dissemination or trans-diaphragmatic spread; genitourinary infections, including vaginitis, uterine infections and cystitis, which may result from direct spread from the anal area or from rectovaginal fistula; and vertebral osteomyelitis and myelitis, which result from hematogenous spread. Liver abscess is not a common manifestation of extra-intestinal involvement in contrast to what occurs in patients infected with Entamoeba histolytica.
