Infectious Gastroenteritis

Infectious gastroenteritis is inflammation of the gastrointestinal tract caused by a wide range of viral, bacterial and parasitic pathogens. It presents with diarrhoea, vomiting or both. It is among the commonest reasons for both community presentation and hospital admission worldwide.

The great majority of cases are self limiting and managed supportively. The central clinical priorities are:

• assessment and correction of dehydration
• identification of the small proportion of presentations requiring specific antimicrobial or other targeted treatment
• recognition of severity markers warranting escalation

Aetiology

Viral pathogens, principally norovirus and rotavirus, account for the majority of cases across all age groups.

Bacterial causes include Salmonella, Campylobacter, Shigella and diarrhoeagenic Escherichia coli. Parasitic causes include Giardia and Cryptosporidium. Together they contribute a smaller but clinically important proportion. They are particularly relevant where specific treatment, public health notification or outbreak investigation is indicated.

Diarrhoea, with or without vomiting, abdominal cramping and, depending on the specific pathogen, fever, characterises the typical presentation.

The pattern and character of symptoms provide useful clues to the likely causative organism category:

• watery, high volume diarrhoea generally reflects a small bowel, toxin mediated process (commonly viral, or toxin producing bacteria)
• frequent, small volume, blood or mucus streaked stool with tenesmus reflects a colonic, invasive process (typically bacterial)

Assessment of dehydration severity is the central clinical priority in every presentation. It directly determines the appropriate rehydration approach and setting of care.

It is based on a combination of:

• history: frequency and volume of losses, oral intake tolerance, urine output
• examination findings: skin turgor, mucous membrane moisture, capillary refill time, heart rate, blood pressure, and, in infants, fontanelle appearance and general alertness

Red flag features warranting further investigation or escalation:

• high fever
• significant abdominal pain out of proportion to the vomiting and diarrhoea
• blood in the stool
• signs of significant or worsening dehydration despite oral rehydration attempts
• systemic features suggesting sepsis

Specific epidemiological features raise concern for a specific pathogen requiring targeted management. These are recent travel, antibiotic exposure, food source implicated in a wider outbreak, or immunocompromise.

Diagnosis is clinical in the great majority of presentations, based on the characteristic symptom pattern and a careful assessment of dehydration severity, without the routine need for stool microbiology in mild, self limiting, community managed illness.

Stool microscopy, culture and sensitivity are indicated for:

• more severe presentations
• bloody diarrhoea
• significant systemic upset
• prolonged symptoms beyond the expected self limiting course
• immunocompromised patients
• a specific public health or outbreak investigation indication

Molecular multiplex panel testing is used where available. It provides rapid simultaneous detection of a broad range of viral, bacterial and parasitic pathogens.

Blood tests (full blood count, renal function and electrolytes, and, where systemic infection is suspected, blood cultures) support assessment of severity and complications, particularly relevant for patients with significant dehydration, systemic features, or where admission for intravenous rehydration is being considered.

Oral rehydration therapy is the cornerstone of management for the great majority of presentations. Use a low osmolarity oral rehydration solution given in small, frequent volumes.

It is particularly important and effective in mild to moderate dehydration. It is as effective as intravenous rehydration in this group, while avoiding the risks and resource burden associated with intravenous access.

Intravenous rehydration is reserved for severe dehydration, inability to tolerate oral fluids due to persistent significant vomiting, or where oral rehydration has failed. Use isotonic crystalloid (0.9 percent sodium chloride, or a balanced solution such as Hartmann solution).

Calculate the initial rate and volume according to the estimated fluid deficit, ongoing losses and maintenance requirements. Adjust according to age, weight and clinical response.

Pay careful attention to electrolyte correction, particularly potassium, once adequate urine output is established.

Antibiotics are not indicated for the great majority of presumed viral or uncomplicated bacterial gastroenteritis. Reserve them for specific indications (severe shigellosis, invasive salmonellosis, confirmed typhoid, and other specific scenarios).

Reasons to avoid routine use:

• the risk of prolonging carriage state in some infections (notably non typhoidal Salmonella)
• promoting antimicrobial resistance
• in the specific case of Shiga toxin producing Escherichia coli, precipitating haemolytic uraemic syndrome

Haemolytic uraemic syndrome is a critical and specific reason to avoid antibiotic and antimotility agent use where this organism is suspected or confirmed.

Antiemetics, where used, are selected according to age and clinical context. Ondansetron has the best evidence for reducing vomiting and supporting successful oral rehydration.

Dose: 4 mg orally as a single dose for adults, or a weight based paediatric dose, typically 2 mg for children weighing 8 to 15 kg, 4 mg for 15 to 30 kg, and 8 mg above 30 kg.

It is particularly useful in the paediatric emergency department setting to facilitate oral rehydration and avoid the need for intravenous access.

Antimotility agents such as loperamide are generally avoided in children and in any patient with bloody diarrhoea, high fever or suspected invasive bacterial infection. There is a theoretical risk of prolonging illness by delaying clearance of the pathogen. With Shiga toxin producing organisms specifically, there is an increased risk of haemolytic uraemic syndrome.

They may be considered for symptomatic relief in adults with mild, non bloody, non febrile traveller's diarrhoea where rapid symptom control is a priority.

Dietary advice, once rehydration is established, favours early resumption of a normal, age appropriate diet as tolerated, rather than prolonged dietary restriction or the historically recommended graduated reintroduction approach, given evidence that early normal feeding supports gut mucosal recovery without worsening symptoms.

Strict hand hygiene and appropriate exclusion from work, school or childcare are important measures to limit onward transmission. Exclusion is generally until 48 hours after the last episode of vomiting or diarrhoea, according to local public health guidance. This is particularly relevant given the high infectivity of several common causative pathogens.

Any suspected outbreak or cluster of cases should prompt notification to local public health authorities for investigation.