Infectious Gastroenteritis: Infection of the Stomach and Bowel

Infectious gastroenteritis is an infection affecting the stomach, intestines or both. It commonly causes diarrhoea, vomiting and abdominal discomfort. Most cases improve without specific antimicrobial treatment, but fluid loss can cause dangerous dehydration, especially in babies, frail older adults and people with other health problems.

What infectious gastroenteritis is Gastroenteritis means inflammation affecting the stomach, intestines or both. When an organism causes this inflammation, it is called infectious gastroenteritis. The main features are usually a sudden change to loose or watery stools, vomiting, nausea and abdominal cramps. Fever, headache, body aches or reduced appetite may also occur. The word gastroenteritis does not identify the organism. Viruses, bacteria and parasites can produce overlapping illness patterns. Diarrhoea and vomiting also have non infectious causes. Medicines, inflammatory bowel disease, food intolerance, appendicitis and several other conditions can produce similar symptoms. Viral gastroenteritis Viruses cause many episodes of acute gastroenteritis. Norovirus is a common cause in people of all ages and frequently causes outbreaks in households, schools, hospitals, care homes and other shared settings. Norovirus often begins suddenly with vomiting, watery diarrhoea, nausea and abdominal discomfort. Most people start improving within two to three days, although they may remain infectious after symptoms stop. Rotavirus mainly causes gastroenteritis in babies and young children. Before routine infant vaccination, it was a major cause of severe childhood diarrhoea and hospital admission. Rotavirus vaccination reduces severe disease but does not prevent every episode of gastroenteritis. Other viruses, including adenovirus and some enteroviruses, can also affect the gut. Antibiotics do not treat viral gastroenteritis. Management mainly prevents or corrects dehydration while the immune system clears the infection. Bacterial gastroenteritis Bacterial causes include Campylobacter, Salmonella, Shigella and Shiga toxin producing Escherichia coli, known as STEC. Clostridioides difficile can cause diarrhoea when the normal gut bacteria have been disrupted, often after antibiotic exposure. Bacterial gastroenteritis may cause watery diarrhoea, fever, abdominal cramps or bloody stools. However, stool appearance and symptom severity cannot reliably identify the organism. Some bacterial infections remain limited to the gut and resolve without antibiotics. Others can cause invasive infection, sepsis or complications outside the bowel. STEC can cause severe abdominal cramps and bloody diarrhoea. A small proportion of people, particularly children, develop haemolytic uraemic syndrome, which can damage the kidneys and affect blood cells. Antibiotics are not recommended for suspected or confirmed STEC because they may increase the risk of haemolytic uraemic syndrome. Bloody diarrhoea therefore needs urgent assessment rather than automatic antibiotic treatment. Parasitic causes Parasites cause a smaller proportion of acute gastroenteritis in the UK. Giardia can cause prolonged diarrhoea, bloating, abdominal discomfort and weight loss. Cryptosporidium can spread through contaminated water, swimming pools, infected animals or close contact. Illness may be more prolonged or severe when immune function is impaired. Recent foreign travel, untreated water, camping, farm contact and persistent symptoms can change which organisms clinicians consider. Stool testing may be needed to identify a parasitic infection. Gastroenteritis and food poisoning are not identical terms Food poisoning describes illness caused by consuming contaminated food or water. Bacteria, viruses, parasites, microbial toxins or chemicals may be responsible. Some food poisoning is infectious gastroenteritis because a living organism infects the bowel. Other episodes result from a toxin already present in food and are not technically an infection. Symptoms may begin within hours when a preformed toxin is responsible. Other organisms have incubation periods lasting days or occasionally weeks. The most recently eaten food is not necessarily the source. Clinicians and public health teams consider the timing, foods eaten, travel, animal contact and whether other people are unwell. How infection spreads Many gastrointestinal organisms spread through the faecal oral route. Tiny amounts of infected stool reach another person's mouth through contaminated hands, food, water, objects or surfaces. Norovirus also spreads through droplets and surfaces contaminated during vomiting. It takes only a small amount of virus to cause infection. Food handlers can spread infection when hand hygiene is inadequate. Contaminated food may also arise during farming, processing, transport, storage or cooking. Animal contact is important for some bacterial infections. STEC can spread after contact with farm animals or environments contaminated by animal faeces. Careful handwashing with soap and water is particularly important. Alcohol hand gel should not be relied upon for norovirus because it is less effective against this virus. Fluid loss is the main immediate danger Vomiting and watery diarrhoea remove water and salts from the body. Dehydration develops when losses exceed the amount being absorbed. Early clues include thirst, a dry mouth, dark urine, reduced urination, dizziness, tiredness and sunken eyes. These findings provide a pattern rather than a precise measurement of fluid loss. More severe dehydration can reduce blood flow to vital organs. The person may become confused, unusually sleepy, cold, pale or mottled and may breathe quickly. Dehydration can contribute to acute kidney injury, disturbances in sodium or potassium, seizures and shock. These complications require urgent medical treatment. Babies and young children dehydrate more quickly Children younger than one year, particularly those younger than six months, have a higher risk of dehydration. Low birth weight, malnutrition, frequent stools, repeated vomiting and stopping breastfeeding increase that risk. Possible signs include fewer wet nappies, few or no tears, a dry mouth, sunken eyes or a sunken fontanelle. Irritability, unusual sleepiness and reduced responsiveness are more concerning. A baby may initially remain interested in drinking despite losing fluid. Clinicians assess the whole pattern, including feeding, urine output, alertness, breathing, circulation and change from usual behaviour. Breastfeeding should usually continue. Formula should remain at its normal strength and should not be diluted. Oral rehydration solution can replace water and salts in the proportions needed for absorption. It is given frequently in small amounts, particularly when vomiting makes larger drinks difficult. Older adults may deteriorate without dramatic symptoms Older adults are also vulnerable to dehydration. Thirst may be less noticeable, and some people depend on others to obtain or prepare drinks. Frailty, dementia, swallowing difficulty, reduced mobility, kidney disease and medicines such as diuretics can increase the consequences of fluid loss. Vomiting and diarrhoea may also disrupt the safe use of regular medicines. New confusion, falls, dizziness, reduced urine or sudden functional decline may reflect dehydration or another acute illness. Gastroenteritis should not be assumed without considering sepsis, medicine effects and other causes. Care home residents require close attention because infection can spread rapidly and multiple linked cases may represent an outbreak. When symptoms suggest another diagnosis Not every episode of vomiting or diarrhoea is gastroenteritis. Severe or localised abdominal pain, abdominal swelling or rebound tenderness can indicate a surgical abdominal condition. Green vomit can suggest intestinal obstruction and needs emergency assessment. Blood or material resembling coffee grounds in vomit may indicate gastrointestinal bleeding. A stiff neck, severe headache, non blanching rash, breathing difficulty or altered consciousness suggests illness outside uncomplicated gastroenteritis. In young children, a temperature of 38°C or higher under three months, or 39°C or higher from three months, may indicate another serious infection and requires assessment in context. Blood or mucus in a child's stool, persistent diarrhoea and marked systemic illness also increase the need for investigation. How clinicians assess gastroenteritis The clinician asks when symptoms began, how often vomiting and diarrhoea occur and whether stool contains blood or mucus. Fluid intake, urine output and ability to keep drinks down are central questions. They ask about recent travel, untreated water, food exposures, animal contact, antibiotics, hospital care and contact with other unwell people. Occupation matters when the person handles food or works with vulnerable people. Assessment includes alertness, temperature, pulse, breathing, blood pressure and peripheral circulation. The abdomen is examined for tenderness, swelling and signs suggesting another diagnosis. Babies and children are assessed for feeding, wet nappies, tears, fontanelle appearance, skin colour, warmth, breathing and responsiveness. No single sign determines the degree of dehydration. Older adults require review of their usual cognition, mobility, medicines, kidney function and ability to access fluids. A relative or carer may identify important changes from baseline. When stool tests are useful Many short lived, uncomplicated cases do not need a stool test. Testing everyone would rarely change management. Stool microbiology is more likely to be needed when diarrhoea is bloody, severe, prolonged or associated with suspected sepsis. Recent foreign travel, weakened immune function and uncertainty about the diagnosis are also relevant. Testing may be required when several linked people are unwell, when the person handles food or when a public health investigation is underway. Local laboratories advise how samples should be collected and transported. A stool result identifies an organism only if it is present and detected in that sample. Results still require interpretation alongside symptoms, timing and treatment history. Blood tests may assess kidney function, electrolytes and complications when dehydration is significant or intravenous fluids are required. They are not routinely needed in every mild illness. Rehydration is the main treatment Most uncomplicated gastroenteritis is managed with rest and regular fluids. Small, frequent sips may be easier to tolerate than a large drink. Oral rehydration solution contains water, glucose and salts in proportions that support absorption through the intestine. A pharmacist can advise which preparation is suitable. Breastfeeding and normal strength milk feeds should usually continue. After rehydration, children can return to their usual food as tolerated. Fruit juice and fizzy drinks can worsen diarrhoea, particularly in children, because their sugar concentration can draw more water into the bowel. Baby formula should never be deliberately weakened. Intravenous fluids may be needed when shock is suspected, dehydration worsens despite oral rehydration or vomiting prevents adequate oral intake. Some children can receive oral rehydration through a nasogastric tube before intravenous treatment is required. Antibiotics are not routine treatment Antibiotics do not work against viruses such as norovirus or rotavirus. They are also unnecessary for many uncomplicated bacterial infections that resolve without them. Unnecessary antibiotics can cause adverse effects, disturb normal gut bacteria and promote antibiotic resistance. They can also make some infections more dangerous. Antibiotics may be appropriate for selected organisms or clinical patterns. Examples include invasive bacterial infection, sepsis, severe shigellosis, cholera and some infections in very young or immunocompromised people. The choice depends on the organism, severity, age, immune status, travel history and local resistance information. Stool or blood results may alter treatment. Suspected STEC is a critical exception because antibiotics are generally avoided. A person with bloody diarrhoea should not take leftover antibiotics or another person's medicine. Medicines that slow diarrhoea are not suitable for children under 12. Adults should seek pharmacy or clinical advice, particularly when there is blood, fever or significant illness. Public health and notifiable disease responsibilities In England, registered medical practitioners must notify UKHSA when they suspect food poisoning. Notification is urgent when cases appear to form a cluster or outbreak. Infectious bloody diarrhoea, cholera, enteric fever and haemolytic uraemic syndrome are among the conditions requiring statutory notification. Clinicians should not wait for laboratory confirmation before reporting a suspected notifiable disease. Diagnostic laboratories separately report specified organisms to UKHSA. Health protection teams use these reports to identify outbreaks, investigate food or water sources and prevent further transmission. Rules and reporting routes differ across England, Scotland, Wales and Northern Ireland. The treating clinician or local public health team follows the relevant national system. Members of the public do not need to decide whether an illness meets a legal notification definition. They should tell a clinician about suspected food sources, travel, occupation and other linked cases. Preventing spread at home and in shared settings Anyone with diarrhoea or vomiting should stay away from work, school or nursery until at least 48 hours after both symptoms have stopped. Some specific infections require longer exclusion or microbiological clearance. Food should not be prepared for other people during illness or for at least 48 hours afterwards. Food handlers may receive additional occupational health or public health instructions. Hands should be washed thoroughly with soap and running water after using the toilet, changing nappies and cleaning vomit. Hands should also be washed before preparing or eating food. Toilets, taps, flush handles and frequently touched surfaces should be cleaned carefully. Clothing and bedding contaminated with stool or vomit should be washed separately on a hot wash. Do not share towels, flannels or eating utensils during illness. Avoid visiting hospitals and care homes until at least 48 hours after symptoms stop. Children and adults should avoid swimming pools for at least 48 hours after diarrhoea stops. Some infections, including cryptosporidiosis, may require a longer period away from swimming. Expected recovery Vomiting from uncomplicated gastroenteritis often settles within one or two days. Diarrhoea commonly lasts five to seven days and may occasionally continue longer. Norovirus usually improves within two to three days. Bacterial and parasitic infections vary more widely in duration. Improvement means the person is drinking, urinating and becoming more alert, even if stools have not returned fully to normal. Ongoing fluid loss still requires replacement. Medical advice is needed when vomiting continues beyond two days, diarrhoea continues beyond seven days or symptoms worsen rather than improve. Earlier assessment is required for dehydration, bloody diarrhoea or higher risk people. What this lesson should not be used for This lesson cannot determine the organism from the colour, smell or frequency of diarrhoea. It cannot distinguish food poisoning from every other cause of vomiting or abdominal pain. Do not use it to select antibiotics, anti diarrhoeal medicines or exact fluid volumes for a sick child. Seek professional advice when dehydration, blood in stool, severe pain, persistent vomiting or significant deterioration is present.

Most infectious gastroenteritis is viral or self limiting and is managed by preventing dehydration rather than using antibiotics. Babies and older adults can lose fluid quickly, while bloody diarrhoea, severe illness and linked food poisoning cases may require urgent testing, public health action and organism specific management.