Differentials
Non-Shigella bacterial diarrhoea
SIGNS / SYMPTOMS
It is often not easy to differentiate between pathogens that cause bloody diarrhoea, as diagnostic yields in diarrhoeal stool samples are reported to be low.[31]
Bloody diarrhoea is suspicious for Shigella or enterohaemorrhagic Escherichia coli.
Other bacterial infections such as Campylobacter jejuni, Vibrio cholerae, Yersinia enterocolitica, and Salmonella may also cause bloody diarrhoea.[5]
Watery diarrhoea often occurs with V cholerae.
Clostridium difficile may rarely cause bloody diarrhoea. If risk factors such as previous antibiotic therapy or exposure are present, C difficile should be considered.
INVESTIGATIONS
Stool cultures are useful to differentiate bacterial causes of diarrhoea.
Viral gastroenteritis
SIGNS / SYMPTOMS
Viral causes of diarrhoea (such as norovirus, adenovirus, and rotavirus) do not usually cause bloody diarrhoea. In addition, vomiting tends to be a significant feature of these infections.[5]
INVESTIGATIONS
Negative stool microscopy and culture along with clinical suspicion.
Parasitic diarrhoea
SIGNS / SYMPTOMS
In low- and middle-income countries, Entamoeba histolytica, Giardia intestinalis, Cryptosporidium parvum, and Cyclospora cayetanensis commonly cause diarrhoeal disease.[5]
Parasitic diarrhoea in the high-income countries world is uncommon and may be related to exposure during travel.
Except in E histolytica infection, diarrhoea is not usually bloody.
INVESTIGATIONS
Stool microscopy is useful to detect parasites.
Idiopathic inflammatory bowel disease
SIGNS / SYMPTOMS
The course of idiopathic inflammatory bowel disease (Crohn's disease, ulcerative colitis, microscopic colitis) is often more protracted than that of shigellosis.[28] There may be a history or family history of the disease. Extra-intestinal manifestations of idiopathic inflammatory bowel disease may be present (e.g., skin lesions or fistulae).
INVESTIGATIONS
Negative stool cultures and positive sigmoidoscopy findings suggest inflammatory bowel disease.
Coeliac disease
SIGNS / SYMPTOMS
Persistent (>14 days) diarrhoea may prompt consideration of coeliac disease. However, the diarrhoea is not bloody.
INVESTIGATIONS
Positive coeliac serology is diagnostic. If there is clinical suspicion, duodenal biopsy may be indicated after a gluten challenge.
Malignancy
SIGNS / SYMPTOMS
May present with change in bowel habits, usually with loose stools.
There may be a history of anorexia or weight loss. Patients are usually >45 years old.
INVESTIGATIONS
Abnormal colonoscopy and evidence of a tumour or metastatic disease on CT studies characterise malignancy.
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