Infectious Gastroenteritis: Causes and Management

Introduction

Infectious gastroenteritis is acute inflammation of the gastrointestinal tract caused by a viral, bacterial or parasitic pathogen, producing diarrhoea (three or more loose stools per day, or more frequently than is normal for that person) with or without vomiting, abdominal pain and fever.¹

It is extremely common and in the great majority of cases is self-limiting, resolving within a few days without specific treatment. The clinical priorities are therefore to assess and correct dehydration, identify the minority of patients who need investigation or antibiotics, exclude important mimics such as surgical pathology or inflammatory bowel disease, and give appropriate public health and infection control advice.

Causative organisms

Viral

Viruses cause the majority of gastroenteritis. Norovirus is the commonest cause in adults, is highly contagious, and characteristically causes outbreaks in hospitals, care homes and cruise ships, with prominent vomiting and a short illness of 1-3 days. Rotavirus was the leading cause of severe childhood gastroenteritis before the introduction of routine infant immunisation in the UK in 2013, which has substantially reduced its incidence. Adenovirus and astrovirus also cause illness in children.

Bacterial and parasitic

Clinical features and assessment

History

• Onset, duration, frequency and consistency of stools, and the presence of blood or mucus
• Vomiting - whether it dominates, and its timing relative to food
• Food history in the preceding days, including takeaways, reheated rice, poultry, eggs, shellfish and unpasteurised dairy
• Travel history, including destination and timing
• Recent antibiotics or hospital admission - consider C. difficile
• Contact with others who are unwell, or an institutional outbreak
• Occupation - food handlers, healthcare and childcare workers have exclusion requirements
• Immunosuppression, HIV status, pregnancy, age and comorbidity
• Fever, abdominal pain, and features suggesting an alternative diagnosis

Assessing dehydration

This is the single most important part of the examination and determines management. Look for tachycardia, hypotension or postural drop, prolonged capillary refill, dry mucous membranes, reduced skin turgor, sunken eyes, reduced urine output, weight loss and altered consciousness. In children, additional signs include a sunken fontanelle, absence of tears and lethargy or irritability.²

Abdominal examination should be performed to exclude peritonism, and to identify significant tenderness or distension that would suggest a surgical cause or a complication such as toxic megacolon.

Differential diagnosis

It is important not to assume that acute diarrhoea and vomiting is always gastroenteritis:

• Surgical causes: appendicitis (Campylobacter enteritis is a classic mimic), diverticulitis, bowel obstruction, mesenteric ischaemia
• Inflammatory bowel disease: a first presentation of ulcerative colitis or Crohn's disease can look identical, and infection can precipitate a flare
• Clostridioides difficile colitis: particularly after antibiotics or hospitalisation
• Overflow diarrhoea from faecal impaction, especially in older or immobile patients
• Systemic infection: urinary tract infection, pneumonia, sepsis and, in children, otitis media can all present with diarrhoea and vomiting
• Diabetic ketoacidosis, Addisonian crisis and hypercalcaemia
• Drug-related: many drugs, including metformin, laxatives, PPIs, colchicine and chemotherapy
• Coeliac disease, thyrotoxicosis and malabsorption in more chronic presentations

Investigations

Most patients with mild, self-limiting symptoms need no investigation at all.¹

Stool testing

Send stool for microscopy, culture and sensitivity if any of the following apply: the patient is systemically unwell or septic; there is blood or mucus in the stool; the patient is immunocompromised; symptoms have persisted beyond 7 days; there has been recent travel abroad; there is recent antibiotic use or hospital admission (also request C. difficile toxin); the patient works in a high-risk occupation such as food handling or healthcare; or a public health outbreak is suspected.

Request stool ova, cysts and parasites where there is relevant travel history or prolonged symptoms, and specify a request for Giardia or Cryptosporidium if suspected, as these are not always covered by routine culture.

Blood tests and imaging

• Urea and electrolytes: to assess dehydration, acute kidney injury and electrolyte disturbance (hypokalaemia and hyponatraemia are common)
• Full blood count: anaemia and thrombocytopenia may signal haemolytic uraemic syndrome in the context of E. coli O157
• CRP and blood cultures if the patient is systemically unwell or febrile
• Blood glucose, and a venous gas with lactate in the acutely unwell
• Abdominal imaging: not routine, but indicated if there is significant abdominal tenderness, distension or suspicion of a surgical cause or toxic megacolon
• Faecal calprotectin and endoscopy: where symptoms persist and inflammatory bowel disease is suspected

Management

Rehydration and supportive care

Fluid replacement is the cornerstone of treatment. Most patients can be managed at home with oral rehydration solution, which contains a balanced combination of glucose and electrolytes that exploits sodium-glucose co-transport to maximise absorption. Encourage continued eating as tolerated; in children, breastfeeding and normal feeding should continue.²

Intravenous fluids are required if there is severe dehydration or shock, persistent vomiting preventing oral intake, or reduced consciousness. Electrolytes should be monitored and replaced, particularly potassium.

Antimotility agents and antiemetics

Loperamide may be used for symptomatic relief in mild, watery, non-bloody diarrhoea in adults, but should be avoided if there is bloody diarrhoea, fever, or suspected C. difficile or E. coli O157, since slowing transit retains toxin and increases the risk of toxic megacolon and haemolytic uraemic syndrome. It is not recommended in children. Antiemetics may help where vomiting prevents oral rehydration.

Antibiotics

Antibiotics are not indicated for most cases, since illness is usually viral or self-limiting, and treatment can prolong carriage of Salmonella and precipitate haemolytic uraemic syndrome in E. coli O157.¹ Consider antibiotics when:

• The patient is systemically unwell, septic or immunocompromised
• There is confirmed or strongly suspected Shigella or Campylobacter with severe illness (clarithromycin or azithromycin for Campylobacter)
• Giardiasis or amoebiasis is confirmed - metronidazole or tinidazole
• C. difficile - oral vancomycin first-line
• Enteric fever, cholera, or severe travellers' diarrhoea
• Never give antibiotics for suspected E. coli O157

Public health and infection control

Food poisoning and suspected infectious bloody diarrhoea are notifiable diseases in England, reportable to the UK Health Security Agency.³ Advise scrupulous hand hygiene with soap and water (alcohol gel is ineffective against norovirus and C. difficile spores), and that patients should stay away from work, school or nursery until 48 hours after the last episode of diarrhoea or vomiting. Food handlers, healthcare workers and those caring for vulnerable people may need longer exclusion and microbiological clearance. Advise avoiding swimming pools for 2 weeks after cryptosporidiosis.

Complications

• Dehydration, hypovolaemic shock and acute kidney injury - the commonest and most important complications
• Electrolyte disturbance: hypokalaemia, hyponatraemia and metabolic acidosis
• Haemolytic uraemic syndrome: the triad of microangiopathic haemolytic anaemia, thrombocytopenia and acute kidney injury, following Shiga toxin-producing E. coli O157 or Shigella - most common in young children
• Guillain-Barré syndrome following Campylobacter infection
• Reactive arthritis following Campylobacter, Salmonella, Shigella or Yersinia
• Post-infectious irritable bowel syndrome and transient secondary lactose intolerance
• Toxic megacolon and perforation, particularly with C. difficile, Shigella or if antimotility agents are misused
• Bacteraemia and metastatic infection (osteomyelitis, endocarditis, mycotic aneurysm) with Salmonella, especially in the immunosuppressed or those with sickle cell disease
• Amoebic liver abscess following Entamoeba histolytica infection
• Malnutrition and growth faltering in children with prolonged or recurrent illness

Red flags

Prognosis

The great majority of infectious gastroenteritis is self-limiting with an excellent prognosis, resolving within a few days for viral illness and within about a week for most bacterial causes, provided hydration is maintained.¹ Parasitic infections such as giardiasis run a more prolonged course but respond well to treatment.

Serious outcomes are concentrated in identifiable groups: infants, frail older people, pregnant women and the immunocompromised, in whom dehydration and invasive infection carry real mortality. Specific sequelae - haemolytic uraemic syndrome, Guillain-Barré syndrome and reactive arthritis - are uncommon but important, and a proportion of patients develop lasting post-infectious irritable bowel syndrome or temporary lactose intolerance after the acute illness has resolved. Globally, gastroenteritis remains a leading cause of childhood death, which is why oral rehydration therapy and rotavirus vaccination are among the highest-impact public health interventions available.