Balantidiasis

Balantidiasis is an intestinal infection caused by Balantidium coli, the largest protozoan parasite known to infect humans. Pigs are the principal animal reservoir, and transmission is faecal oral, through ingestion of infective cysts from contaminated water or food. Colonisation is generally asymptomatic in the great majority of infected individuals; a minority develop symptomatic colitis that can, in severe cases, closely mimic amoebic dysentery both clinically and in potential for invasive complications.

Pathophysiology and epidemiology

• Following ingestion of cysts, the organism excysts within the small intestine, and trophozoites subsequently colonise the large intestine.
• Invasion of the colonic mucosa (facilitated by production of hyaluronidase, which breaks down intercellular matrix) can occur in a proportion of infected individuals. It produces ulceration similar in principle, though generally less extensive, to that seen in amoebic colitis.
• Risk factors include close contact with pigs (occupational or through proximity to pig farming) and residence in or travel to regions with poor sanitation. The infection is recognised worldwide though generally uncommon as a cause of clinically significant human disease relative to its animal reservoir prevalence.

• The majority of infections are asymptomatic, with the organism identified incidentally on stool examination performed for another indication.
• Symptomatic disease presents with diarrhoea, which may be watery or, in more significant colitis, bloody and mucoid, together with abdominal pain, tenesmus and, in more severe cases, weight loss and fever, closely mimicking the presentation of amoebic dysentery.
• Rarely, invasive disease can produce colonic perforation or, through haematogenous spread, extraintestinal involvement, though this is uncommon and generally reflects more severe, longstanding untreated infection or a significantly immunocompromised host.
• A relevant exposure history to pigs or pig farming environments, or travel or residence in a region with limited sanitation infrastructure, supports clinical suspicion in a patient with a compatible dysenteric presentation.

• Stool microscopy identifying the characteristic large, ciliated trophozoites (readily visible given their substantial size relative to other intestinal protozoa) or cysts confirms the diagnosis, with examination of a fresh, warm stool sample optimising detection of motile trophozoites.
• Colonoscopy, where performed for investigation of unexplained colitis, may identify characteristic ulceration, with biopsy demonstrating the organism within the colonic mucosa supporting diagnosis where stool examination has been inconclusive.
• Given the clinical overlap with amoebic dysentery, pursue specific stool examination and, where relevant, serological testing for Entamoeba histolytica alongside Balantidium specific testing. This applies to any patient presenting with a compatible dysenteric illness and relevant travel or exposure history. The epidemiological associations differ, and establishing the correct specific diagnosis guides targeted treatment.

• Tetracycline 500 mg orally four times daily for ten days is considered first line treatment, with metronidazole 750 mg orally three times daily for five days representing an effective and more widely available alternative, particularly relevant given tetracycline contraindication in young children and pregnancy.
• Iodoquinol 650 mg orally three times daily for twenty days is a further alternative option where the preferred first line agents are unavailable or not tolerated.
• Asymptomatic infection identified incidentally is generally treated given the organism's invasive potential and the theoretical, albeit low, risk of progression to symptomatic or invasive disease, in contrast to the more selective treatment approach sometimes applied to certain other incidentally identified, generally non pathogenic intestinal protozoa.
• Prevention through improved sanitation, safe food and water practices, and appropriate hygiene measures when in contact with pigs or pig farming environments represents the primary preventive strategy, given the absence of any specific vaccine for this infection.