Definition and aetiology of infectious diarrhoea

Diarrhoea is the passage of 3 or more loose or liquid stools in 24 hours, or more frequently than is usual for the patient.

Gastrointestinal infections often present with diarrhoea, but in some cases upper gastrointestinal symptoms such as nausea and vomiting are prominent. For assessment of acute gastroenteritis, including potential causes and complications, differential diagnoses and red flags, see Assessment of acute gastroenteritis.

Infectious diarrhoea can be categorised based on the severity and duration of symptoms – see Categorisation of infectious diarrhoea based on severity and duration of symptoms.

Table 1. Categorisation of infectious diarrhoea based on severity and duration of symptoms
[NB1]

DuPont, 2014 National Institute of Diabetes and Digestive and Kidney Diseases, 2016 Riddle, 2016
Severity of infectious diarrhoea [NB2]
Severity is related to the degree of incapacity caused by the diarrhoea: mild – the patient is able to undertake normal activities moderate – the patient is able to function but needs to modify normal activities severe – the patient is incapacitated and may require admission to hospital.
Duration of infectious diarrhoea
Acute diarrhoea usually lasts for 1 or 2 days but can persist for up to 14 days; most cases are viral and self-limiting. Persistent diarrhoea lasts between 2 to 4 weeks and most cases are due to persisting infection or post-infection changes such as mucosal inflammation. Chronic diarrhoea persists for longer than 4 weeks. Although pathogenic gastrointestinal protozoal parasites and, more rarely, bacteria can cause chronic infectious diarrhoea, most cases have a noninfectious cause.Kaiser, 2012 For differential diagnoses, see Definition and differential diagnosis of functional diarrhoea.
Note: NB1: Diarrhoea is the passage of 3 or more loose or liquid stools in 24 hours, or more frequently than is usual for the patient. NB2: For examples of red flags for potentially life-threatening differential diagnoses of acute gastroenteritis, see Examples of red flags for potentially life-threatening differential diagnoses of acute gastroenteritis.

Note:

Most cases of acute infectious diarrhoea are viral and self-limiting, and resolve without specific treatment.

Most cases of chronic diarrhoea have a noninfectious cause.

Infectious diarrhoea can have a viral, bacterial, protozoal parasite or toxin-mediated aetiology; for a summary of clinical clues to help differentiate between these causes, see Features of viral, bacterial, protozoal parasite and toxin-mediated diarrhoea.

Patients with immune compromise are susceptible to a wide range of diarrhoea-causing pathogens, including bacteria, viruses (eg cytomegalovirus (CMV)), and gastrointestinal protozoal parasites (eg Cyclospora cayetanensis, Cystoisospora belli, Cryptosporidium species and microsporidia species).

Consider infective proctitis caused by sexually transmissible pathogens (eg Neisseria gonorrhoeae, Chlamydia trachomatis, Treponema pallidum [syphilis], herpes simplex virus and monkeypox virus) in gay, bisexual and other men who have sex with men, transgender women and other people engaging in receptive anal sex who report a new sexual partner. In addition, oral–anal sex increases the risk for gastrointestinal infection, particularly for pathogens with a low infective dose, such as Shigella species, Cryptosporidium species, Giardia duodenalis and Entamoeba histolytica.

Table 2. Features of viral, bacterial, protozoal parasite and toxin-mediated diarrhoea
Printable table
Cause	Clinical features and risk factors	Examples
Viral	prominent upper gastrointestinal symptoms such as vomiting and nausea symptom onset is typically acute and resolves within 24 to 48 hours Risk factors: history of contact with a person who has acute infectious diarrhoea (person-to-person transmission) may be part of an outbreak with secondary cases	rotavirus norovirus adenovirus astrovirus
Bacterial	fever, tenesmus, severe abdominal cramps, mucus in stool, bloody stool symptom onset is typically acute with typical duration of 4 to 7 days Risk factors: recent travel immune compromise recent antibiotic use or hospital admission – test for Clostridioides difficile (formerly known as Clostridium difficile)	Campylobacter species Clostridioides difficile enterohaemorrhagic Escherichia coli nontyphoidal Salmonella species Shigella species Vibrio cholerae noncholera Vibrio species Yersinia enterocolitica
Protozoal parasite	abdominal distension, flatulence, nausea and anorexia symptom onset is typically gradual (over several weeks) but can be acute often less severe than other causes; some causes (eg amoebiasis) can be severe at onset and difficult to distinguish from bacterial diarrhoea Risk factors: recent travel immune compromise household contacts for giardiasis, close contact with animals Zajaczkowski, 2018	Blastocystis species Cryptosporidium species Cyslospora cayetanensis Cystoisospora belli Dientamoeba fragilis Entamoeba histolytica Giardia duodenalis microsporidia
Toxin-mediated [NB1]	vomiting, nausea and abdominal pain; diarrhoea, if present, occurs later in the course of illness short incubation period (typically several hours only) closely clustered cases; infections arise from a single point source Risk factors: close contacts	preformed toxins produced by Staphylococcus aureus and Bacillus cereus
Note: NB1: Several preformed bacterial toxins cause acute gastroenteritis, commonly referred to as food poisoning. Rarely, toxin-mediated gastroenteritis may be caused by nonbacterial toxins (eg ciguatoxins, tetrodotoxins).