Teaching Case

61-years-old male patient undergoing chronic highly active antiretroviral treatment (HAART) for long-standing HIV infection (CD4+ cell count 250/mmc).
Recurrent, progressively more severe episodes of abdominal pain and diarrhoea with blood-stained stools over the last three months, ultimately leading to hospitalisation.

Laboratory assays disclosed anaemia (Hb 7.1 g/dL) and mildly raised C-reactive Protein (20). Emergency CT examination yielded diffuse concentric wall thickening of the caecum, ascending colon and hepatic flexure showing the target sign consistent with mural oedema and inflammation. The terminal ileum was spared from pathologic changes. Associated findings included moderately ascites and inflammatory stranding of the right pericolonic fat. Significant adenopathies, focal liver or spleen lesions were not detected.
Microscopy and cultures on stools and blood were negative for bacteria (particularly Shigella and Clostridium difficile), mycobacteria, fungi and toxins. Positive serum antibodies for Entamoeba allowed a clinical diagnosis of amoebic colitis and antibiotic treatment was prescribed.
After 3 months, follow-up CT disclosed disappearance of both ascites and mural colonic abnormalities, thus confirming the clinical diagnosis.

Amoebiasis, defined by the WHO as infection by the protozoan Entamoeba histolytica regardless of symptoms, is transmitted through ingestion of contaminated food or water. It is endemic in countries with poor socio-economic and sanitary conditions. Most infected individuals are asymptomatic and harbour the non-pathogen variant Entamoeba dispar, whereas 4-10% develop invasive disease including colitis or liver abscess. In developed countries, high-risk groups include travellers, immigrants and HIV-infected people.
Pathologically, amebic colitis is characterised by intestinal wall invasion by trophozoites. Clinical presentation varies from mild, often intermittent episodes of watery or blood-tinged diarrhoea, to severe dysentery with fever, abdominal pain, weight loss, and fulminant acute abdomen.
Endoscopic findings range from sparse ulcerations to diffuse mucosal abnormalities, sometimes mimicking those observed in chronic inflammatory diseases. Diagnosis is obtained by the detection of trophozoites in the stools, by colonic biopsies, or with serological tests.
Amebiasis can be cured successfully by administration of metronidazole; surgical resection is necessary for rare necrotizing forms.
Opportunistic intestinal infections constitute a large source of morbidity in HIV patients. CT is the mainstay modality to image patients with suspected inflammatory conditions of the bowel in both elective and emergency conditions. Infectious colitides exhibit variable-degree wall thickening, often with the target sign consisting in concentric rings of different attenuation corresponding to mucosal hyperemia and submucosal oedema. This mural stratification is more pronounced when arterial phase images are acquired. It indicates acute non-malignant bowel injury from inflammatory, infectious or ischemic disease and virtually excludes malignancy. Ascites and pericolonic fat inflammatory stranding may be associated.
Considerable overlap exists between the imaging appearance of infectious colitides, usually diagnosed clinically using stool cultures, and laboratory and colonoscopy findings. Amebic colitis commonly occurs in the right colon or rectum and characteristically spares the ileum. In HIV-patients, Cytomegalovirus enteritis involves a variable colonic length usually on the right side, and often the terminal ileum too. Pseudomembranous colitis due to Clostridium difficile overgrowth appears as a pancolitis. Other differential diagnoses include ischemic colitis, tuberculosis, neutropenic thyphlitis and chronic inflammatory bowel diseases, the latter uncommonly associated with ascites.
Amebic colitis should be always kept in mind as a frequent cause of dysentery or other intestinal complaints in AIDS patients, particularly homosexual men. CT may suggest the diagnosis even in patient imaged for different reasons, and identify complications such as liver or lung abscess, ascites, megacolon and perforation.