Acute Infectious Diarrhoea

Acute infectious diarrhoea is three or more loose or watery stools in 24 hours lasting less than 14 days, caused by viruses, bacteria or protozoa.

• Viral (norovirus, rotavirus, adenovirus) accounts for most cases in high income settings; bacterial (Campylobacter, Salmonella, Shigella, Escherichia coli, Vibrio cholerae) and protozoal (Giardia, Entamoeba, Cryptosporidium) agents dominate in low resource settings.
• It is a leading cause of death in children under 5 years in sub-Saharan Africa.
• Transmission: faecal-oral, contaminated water and food, poor sanitation, travel, close contact.

Watery diarrhoea with vomiting and cramps suggests viral or toxin-mediated disease; bloody mucoid stool with fever and tenesmus (dysentery) suggests Shigella, Campylobacter, enteroinvasive or enterohaemorrhagic E coli, or Entamoeba histolytica.

• Rice water stool with rapid dehydration suggests cholera.
• Dehydration: thirst, dry mucosa, reduced skin turgor, sunken eyes, tachycardia, orthostatic hypotension, oliguria, lethargy in severe cases.
• Differential diagnosis: inflammatory bowel disease, Clostridioides difficile infection, ischaemic colitis, drug-induced diarrhoea, thyrotoxicosis, overflow diarrhoea, HIV enteropathy and antibiotic-associated diarrhoea.
• Complications: hypovolaemic shock, acute kidney injury, hypokalaemia, metabolic acidosis, haemolytic uraemic syndrome, bacteraemia, toxic megacolon, reactive arthritis, Guillain-Barre syndrome, malnutrition and persistent diarrhoea in children.

Most cases need no tests.

• Stool culture, microscopy and molecular panels for dysentery, severe illness, immunosuppression, fever above 38.5 C, symptoms over 7 days, outbreaks or health care workers.
• Electrolytes, urea and creatinine, bicarbonate and glucose for moderate to severe dehydration.
• Stool for ova, cysts and parasites (three samples) if persistent; Clostridioides difficile toxin after antibiotics; HIV testing where status is unknown.
• Blood cultures if febrile and toxic or immunosuppressed.

Supportive

• Oral rehydration with low osmolarity solution: mild to moderate dehydration 50 to 100 mL/kg over 4 hours, then replace each loose stool with 10 mL/kg.
• Severe dehydration or shock: Ringer lactate 30 mL/kg (children, over 30 minutes for those over 12 months) then 70 mL/kg over 2.5 hours; adults 1 to 2 L over 1 hour then guided by response.
• Continue feeding; breastfeeding continues throughout.
• Zinc in children: 20 mg daily for 10 to 14 days (10 mg daily under 6 months).

Pharmacological

• Antibiotics only for dysentery, severe disease, immunosuppression or cholera: azithromycin 500 mg once daily for 3 days (children 10 mg/kg once daily for 3 days), or ciprofloxacin 500 mg twice daily for 3 days (children 15 mg/kg twice daily for 3 days).
• Giardia: metronidazole 400 mg three times daily for 5 days or tinidazole 2 g single dose. Amoebic dysentery: metronidazole 800 mg three times daily for 5 days (children 10 mg/kg three times daily) followed by paromomycin 25 to 35 mg/kg/day in 3 divided doses for 7 days.
• Loperamide 4 mg then 2 mg after each loose stool (maximum 16 mg daily) only in adults with non-bloody, afebrile diarrhoea. Avoid in dysentery, young children and suspected C difficile.
• Ondansetron 4 mg oral or IV (children 0.15 mg/kg) for vomiting.

Special groups: HIV with chronic diarrhoea (cryptosporidiosis: nitazoxanide 500 mg twice daily for 3 days, optimise antiretroviral therapy); pregnancy (avoid fluoroquinolones, prefer azithromycin); elderly (cautious fluids).

Red flags: blood and fever with toxicity, severe dehydration, altered consciousness, oliguria, abdominal distension, age under 6 months, diarrhoea over 14 days.

Follow-up: review at 48 hours if not improving; consider persistent diarrhoea work-up beyond 14 days; hand hygiene, safe water and vaccination (rotavirus, cholera in endemic areas).