Morphology and Structure
The parasite exists in two forms: the vegetative form (trophozoite) and the cyst.
Vegetative Form (Trophozoite) This is a large, oval cell measuring 30–100 × 30–150 µm. It is covered in cilia, which serve as organelles of locomotion. The trophozoite structure features complex organelle organization:
- Nuclear apparatus: exhibits nuclear dimorphism. The cell contains two nuclei—a large, bean-shaped macronucleus and a small micronucleus.
- Digestive system: the anterior end features a slit-like depression (peristome) leading to the cell mouth (cytostome) and gullet (cytopharynx). Nutrition occurs by engulfing bacteria, other cells, and erythrocytes (forming food vacuoles). Undigested waste is eliminated through the anal pore (cytoproct) located at the posterior end.
- Osmoregulation: carried out by two contractile vacuoles.
The trophozoite multiplies by transverse binary fission.
Cyst Form A round cell, 40–65 µm in diameter, surrounded by a thick protective wall. It contains a single nucleus. The cyst is designed for long-term survival in the external environment and serves as the infective stage for humans.
Epidemiology of Balantidiasis
Balantidiasis is a zoonotic infection. Pigs serve as the primary reservoir of the pathogen and the main source of environmental contamination, where balantidia frequently reside in the bowel as commensals. Other potential hosts include non-human primates, rodents, dogs, cattle, and humans.
The transmission mechanism is fecal-oral. Humans become infected by ingesting cysts via contaminated food or water (alimentary or waterborne routes).
Pathogenesis and Symptoms
Upon entering the human body, the parasite may reside in the intestinal lumen for a prolonged period, causing asymptomatic carriage.
When the disease develops (balantidial colitis), trophozoites invade the mucosa and submucosa of the large intestine. A key virulence factor of Balantidium coli is the production of the enzyme hyaluronidase, which dissolves tissue barriers. This leads to the formation of deep ulcers and abscesses. Clinically, balantidiasis resembles amebiasis, presenting with a systemic toxic syndrome and diarrhea.
Diagnosis, Treatment, and Prevention
The primary diagnostic method is microscopic examination of freshly passed stool. Preparations are made as a wet mount with normal saline (isotonic sodium chloride). Under low magnification, actively motile, large trophozoites can be observed. Cysts are rarely found in human stool.
Treatment of balantidiasis involves tetracycline-class antibiotics (e.g., doxycycline), as well as antiprotozoal agents (such as metronidazole) and other medications effective against amebiasis.
Prevention includes strict personal hygiene (especially for pig farm workers) and protecting water sources and soil from animal fecal contamination.