• Non-neoplastic colonic mucosal fragment with ulceration, active (neutrophilic) inflammation, and increased chronic inflammatory cells
• The necrotic debris on the luminal surface contains aggregates of oval-shaped organisms that resemble macrophages
• The organisms contain abundant pale, vacuolated cytoplasm and small central round nuclei
• Erythrophagocytosis is evident (arrows)
• Colonoscopy shows an ulcer with yellowish exudates

In a colonic biopsy with ulceration, the main differential diagnoses are:

• Infection (e.g. bacterial, viral, parasitic/protozoal)
• Inflammatory bowel disease (look for features of chronicity: crypt distortion, epithelial metaplasia, basal plasmacytosis)
• Drug-induced injury (e.g. NSAIDs, mycophenolate, immune checkpoint inhibitors; correlate with medication history)
• Ischaemia (look for withered crypts, lamina propria hyalinization; consider in elderly patients or history of vascular compromise)
• Hence, it would be helpful to know of history of travel to endemic areas, underlying conditions or medication, as well as possible immunocompromised state

The following infective causes may show histological clues:

• PAS stain highlights round to oval trophozoites clustered within necrotic debris. The organisms show abundant magenta granular cytoplasm due to glycogen-rich cytoplasm, measuring approximately 15-30 μm.
• The findings are those of Entamoeba histolytica trophozoites
• CMV immunohistochemical stain was negative.
• PAS and CMV are helpful ancillary tests in colonic ulcers.

Diagnosis

• Colonic ulcer with Entamoeba histolytica trophozoites, consistent with amoebic colitis

Note: Serological testing detected the presence of amoebic antibody. The patient was treated with metronidazole and paromomycin, with complete resolution of the caecal ulcer on repeat colonoscopy 6 months later.

Learning Points

• Amoebic colitis may occur anywhere, being more common in tropical regions
• It is caused by Entamoeba histolytica
• It typically affects the  right-sided colon, followed by the rectosigmoid colon
• Patients may be asymptomatic, or may present with diarrhoea
• Endoscopically and radiologically, the affected colon shows ulcers with yellowish exudates and skip lesions, and the wall may be thickened, potentially mimicking malignancy (amoeboma)
• Histological diagnosis relies on the identification of Entamoeba histolytica trophozoites, usually found within ulcer slough or granulation tissue. PAS highlights the trophozoite cytoplasm but is not specific. Erythrophagocytosis (ingested RBCs) is the hallmark feature that distinguishes Entamoeba histolytica from non-pathogenic amoebae. Correlation with stool PCR, stool antigen testing, or serology can be helpful.
• Appropriate anti-amoebic therapy (e.g. metronidazole followed by a luminal agent) usually results in resolution