# Acute Infectious Diarrhoea in Adults

> Acute infectious diarrhoea in adults for NEET-PG Medicine, covering ORS as the cornerstone, when antibiotics help and organism-specific stool clues.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/acute-gastroenteritis-in-adults
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: Davidson's Principles and Practice of Medicine; Harrison's Principles of Internal Medicine; Katzung's Basic and Clinical Pharmacology
- How to cite: "Acute Infectious Diarrhoea in Adults", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/acute-gastroenteritis-in-adults

## Direct answer
Acute infectious diarrhoea is a sudden increase in stool frequency or liquidity caused by bacterial, viral or protozoal infection, and oral rehydration is the cornerstone of management in most adults. Antibiotics are reserved for dysentery with high fever, severe dehydration, immunosuppression, or organisms such as Shigella and Vibrio cholerae in significant illness. The stool pattern, fever and presence of blood guide the differential.

## What you must remember
- **ORS:** Low-osmolarity oral rehydration solution is first-line in most adults; intravenous fluids are needed for shock, persistent vomiting or severe dehydration.
- **Watery diarrhoea:** Enterotoxigenic Escherichia coli, Vibrio cholerae and norovirus cause watery diarrhoea (cholera and ETEC are the large-volume ones), and rice-water stools suggest cholera.
- **Dysentery:** Blood and mucus with fever point to Shigella, Campylobacter, invasive E. coli or Entamoeba histolytica.
- **Entamoeba histolytica:** Metronidazole followed by a luminal agent such as diloxanide furoate clears tissue and luminal infection, and colonoscopy may show flask-shaped ulcers.
- **Campylobacter jejuni:** Antecedent infection is the classic trigger of Guillain-Barré syndrome, typically a few weeks after diarrhoea.
- **Clostridioides difficile:** Antibiotic-associated colitis, treated with oral vancomycin or fidaxomicin, and antimotility drugs such as loperamide are avoided.

## A stepwise approach to bloody diarrhoea

Consider a 40-year-old with three days of bloody diarrhoea, fever and cramping after eating outside. The approach below keeps hydration first and treats the cause second.

1. **Assess hydration:** ORS is the mainstay if the patient can drink and passes urine; intravenous fluid is kept for shock or persistent vomiting.
2. **Avoid loperamide:** antimotility agents can trap invasive bacteria and toxins in the colon.
3. **Send stool tests:** microscopy and culture are sent, and empirical antibiotics are considered given fever and blood, with azithromycin often used where fluoroquinolone resistance is high locally.
4. **Think of amoebiasis:** if there is no improvement within a few days, or the stool shows trophozoites, metronidazole is added with a luminal amoebicide, and colonoscopy may show flask-shaped ulcers.
5. **Look for complications and mimics:** severe pain, distension or toxicity needs urgent review for toxic megacolon, and recent antibiotic use raises the possibility of C difficile.
6. **Review and counsel:** the stool culture result guides antibiotic adjustment, and household advice covers hand hygiene and safe food handling. Diarrhoea lasting beyond several weeks widens the differential to giardiasis, coeliac disease and inflammatory bowel disease.

## Where the exam tries to catch you

Examiners frequently test whether candidates recognise that most diarrhoea does not need antibiotics.

- **Trap:** prescribing antibiotics for simple watery diarrhoea — **Correct:** it is managed with ORS (plus zinc in children), and antibiotics add little.
- **Trap:** giving loperamide in bloody diarrhoea or suspected C difficile — **Correct:** it risks toxic megacolon and is avoided.
- **Trap:** confusing giardiasis with amoebiasis — **Correct:** Giardia causes greasy, foul-smelling stools without blood and responds to metronidazole or tinidazole, whereas amoebic dysentery shows blood, flask-shaped ulcers and needs a luminal agent.
- **Trap:** treating cholera with an antibiotic alone — **Correct:** early ORS or intravenous fluid is the priority, and a single dose of doxycycline is a supplement in severe disease, not a substitute.

## Frequently asked questions

### Why is loperamide avoided in bloody diarrhoea?
Antimotility drugs such as loperamide slow colonic transit and can trap invasive bacteria and their toxins in the colon. This raises the risk of toxic megacolon, particularly in dysentery and suspected Clostridioides difficile infection, so they are withheld until the cause is clear.

### Which organism is classically linked to Guillain-Barré syndrome after diarrhoea?
Campylobacter jejuni is the classic infective trigger for Guillain-Barré syndrome, which typically follows an episode of diarrhoea by a few weeks. The mechanism is thought to involve molecular mimicry between bacterial lipo-oligosaccharide and peripheral nerve gangliosides.

### What is the drug of choice for amoebic colitis?
Metronidazole is the drug of choice for invasive amoebic colitis, given to kill tissue trophozoites, and it must be followed by a luminal amoebicide such as diloxanide furoate to eliminate cysts. Metronidazole alone leaves the patient at risk of relapse and continued cyst passage.

### When is intravenous fluid needed in acute diarrhoea?
Intravenous rehydration is needed when a patient has shock, severe dehydration, persistent vomiting that prevents oral rehydration, or altered consciousness. Oral ORS remains first-line for other adults with mild or moderate dehydration.