Gastroenteritis (Stomach Flu)

Medical Author : Charles Patrick Davis, MD, PhD

Medical Editor : John P. Cunha, DO, FACOEP

Definition of gastroenteritis

Gastroenteritis (often referred to as the "stomach flu," however, it is not related to the influenza virus. Gastroenteritis is a nonspecific term for various inflammatory problems in the gastrointestinal tract with the most common symptoms and signs being diarrhea, nausea, vomiting, and abdominal pains.

As previously mentioned, although it is not caused by influenza viruses, gastroenteritis  is commonly referred to as the "stomach flu" because most people have acute symptoms that last a day or so, and then begin to resolve, like the symptoms of more benign flu strains. In the U.S., less than 2% of the estimated 100 million persons with gastroenteritis symptoms per year ever require hospitalization, but in developing countries it is a leading cause of death, mainly due to dehydration. Severe gastroenteritis can cause dehydration. Also, people with symptoms of diarrhea, bloody diarrhea, fever greater than 101 F (38.33 C) for longer than 5 days, or have severe infection (sepsis), and other problems will be considered to have another disease (for example, shigellosis). Not all doctors agree on the nonspecific term of gastroenteritis so for this article, the parameters are presented that comprise the causes and symptoms that many researchers consider to occur with gastroenteritis.

What causes gastroenteritis?

Infectious agents such as bacteria and viruses are the most frequent causes of gastroenteritis in the US and worldwide. Infections cause diarrhea and other symptoms by causing inflammation of the gastrointestinal (GI) tissue. The infections increase the fluid content in the intestines and colon by changing the gastrointestinal tract's ability to absorb water and by increasing the speed of transit (motility) for things you ingest. This, in turn, causes diarrhea. Infectious agents may physically damage intestinal cells directly or indirectly with secreted toxins.

What are the most common causes of gastroenteritis?

The most prevalent cause of gastroenteritis in the U.S. and the world is Norovirus. It causes about 50% to 70% of viral gastroenteritis while Rotavirus, Astrovirus, Adenovirus, and Sapovirus strains cause most of the other viral gastroenteritis infections. Norovirus was also listed as the leading cause of gastroenteritis in children under 5 years old according to the Centers for Disease Control (CDC).

Bacterial causes of gastroenteritis that occur worldwide are Salmonella, Shigella, Campylobacter Aeromonas, and Escherichia coli (E. coli) strains of bacteria. Other bacteria like Clostridium, Vibrio, Campylobacter, and Yersinia spp can cause outbreaks occasionally.

Parasites such as Giardia, Cryptosporidium, and Entamoeba infections can cause gastroenteritis and occasionally, other parasites have outbreaks such as the Cyclospora outbreak that occurred in 2012 to 2013 in the U.S.

There are many other less frequent causes of gastroenteritis such as food allergies, antibiotics, and toxins. Gastroenteritis symptoms are frequently listed as possible side effects of many medicines.

What are the symptoms of gastroenteritis?

The primary symptom of gastroenteritis is diarrhea (non-bloody). Nausea, vomiting, and some abdominal cramping may accompany the diarrhea; mild fever (about 100 F or 37.77 C), chills, headache, and muscle aches along with feeling tired may occur in some individuals. Vomiting is occasional and the symptoms usually last about 2 to 5 days and begin to resolve. Severe gastroenteritis means the person has signs of dehydration; this is a medical emergency.

Children with gastroenteritis usually have diarrhea, but may have other symptoms, sometimes conflicting, of refusing to eat or drink or are very thirsty, either increased or low or no urine output. Weight loss, lethargy, and pinched skin that does not rapidly go back to normal are signs of dehydration, along with decreased fluid intake. 

Is gastroenteritis contagious?

The large majority of causes (viral and bacterial) of gastroenteritis are contagious, usually through food or water contamination; in addition, they can be transferred person-to-person. A few causes are not contagious such as food allergies or the side effects of medications. Exposures to body fluids (for example, feces or droplets containing infectious agents) are common sources that transmit the disease to others (See transmission section below).

Who is at risk for gastroenteritis?

Gastroenteritis is a common, worldwide disease and almost everyone suffers from it a few times in their life because it is almost impossible to avoid contact with some of the viral and bacterial causes.

  • People living in crowded conditions (military, cruise ships, dorms) are at higher risk, as are people living in developing countries who often have a diet that contains contaminated food or water.
  • Infants, children and some adults (elderly, immunosuppressed) are at higher risk because of immature or depressed immune systems and also because they can become dehydrated faster than older children and adults.
  • Some patients taking antibiotics are at higher risk because the antibiotics depress the normal GI microbes and allow bacteria or viruses like Clostridium diffficile to predominate and cause disease.
  • People who do not practice good hygiene and hand washing techniques are at higher risk, as are those who eat under cooked and/or unwashed foods or drink from potentially contaminated fluid sources (rivers, streams, unpasteurized milk, for example).

How is gastroenteritis transmitted?

The majority of all patients can transfer most viral and bacterial causative agents to other people by direct and indirect contact, usually by the fecal – oral route. Direct contact could involve an infant's hand touching feces-contaminated surfaces and then touching a sibling or relative; indirect contact would be like touching a door knob or railing on a cruise ship or in a dorm that is contaminated and the person touches the contaminated surface and transfers the agent by touching their mouth. Another common way to get gastroenteritis is drinking or eating contaminated foods and liquids. 

How does food become contaminated with gastroenteritis-causing bacteria or viruses?

In most instances, food and drinks come into contact with feces contaminated with the infecting agent. This can happen in the fields, or in transport, storage, and processing of food and drinks. In processed foods and drinks, this contamination is relatively rare, but when it occurs, an outbreak of the disease is often traced back to faulty equipment, human errors in the processing and/or a breakdown of quality-control procedures.

How is gastroenteritis diagnosed?

Gastroenteritis is most often presumptively diagnosed by the symptom(s) it produces (mainly diarrhea). Because gastroenteritis is usually a self-limited disease, the large majority of people are never seen or diagnosed by a doctor. There are no specific tests for gastroenteritis. However, during outbreaks like those seen on cruise ships, viral and bacterial cultures or PCR and other immunologically-based tests can eventually identify the causative pathogen. By the time this identification occurs, most of the patients with gastroenteritis have begun to recover. When gastroenteritis symptoms become severe, most public health officials and health care professionals run such tests to identify the causative agent of a specific disease, based on all of the patient's history, physical exam, and symptoms. In addition, patients with similar histories of recent food or drink they had in common with others often helps to discover the source of the disease (for example, people who got diarrhea had salads from the same food provider).

How is gastroenteritis treated?

Most people with gastroenteritis require no formal treatment. The key to a rapid and safe recovery at home (home remedy) is proper hydration. Home treatment consists of adequate fluid intake so dehydration is prevented. Clear fluids are recommended (Pedialyte especially for young children, Gatorade, PowerAde and other sports drinks), but not fruit juices or milk as they may prolong the symptoms. If dehydration occurs, the patient should be evaluated by a doctor. Many health care professionals choose to begin IV fluids, the treatment of choice for rapid rehydration.

Other medications may be prescribed to reduce the symptoms of gastroenteritis. To reduce vomiting, promethazine (Phenergan), prochlorperazine (Compazine), or ondansetron (Zofran) are often used. Some physicians suggest using these agents only as a suppository or rapidly disintegrating tablet on the tongue since patients may vomit the pills up. Others may prescribe diphenoxylate and atropineomotil (Lomotil) or lopermadine (Imodium) to slow diarrhea while others do not as the drugs may prolong the disease in some individuals. Many doctors recommend no medical treatment for gastroenteritis symptoms as all of the drugs have side effects and if the patient stays well hydrated, the symptoms usually stop soon anyway.

As the gastroenteritis symptoms abate, especially vomiting, doctors may recommend a BRAT diet (bananas, rice, apples and toast) for a day or two before returning to the patient's regular diet. Potatoes, lean meat like chicken and whole grains can help replace nutrients and electrolytes lost with diarrhea.

Patients who have more serious symptoms or other symptoms in addition to gastroenteritis need to be evaluated, diagnosed, and treated by a physician because the patient will likely have a specific disease that will need treatment. The treatment will depend on the cause of the illness (for example, salmonellosis or Clostridium difficile toxin). Antibiotics and other treatments may not be recommended for some of these diseases so an accurate diagnosis of the disease is important. For Clostridium difficile infected patients, antibiotic sensitivity testing may need to be done to determine the most effective antibiotics to use.

When should I call my doctor for gastroenteritis?

If gastroenteritis symptoms last more than about 5 days, increase in severity (fever of 101 F or 38.33 C or higher), or bloody diarrhea develops, dehydration, constant abdominal pain or other symptoms, a patient should see a physician. The patient may have some gastroenteritis symptoms but may have a disease more serious than self-limiting gastroenteritis. Signs and symptoms of dehydration may include decreased or no urine production, dry mucus membranes, dry mouth or skin, no tears, weakness, lightheadedness and low blood pressure, while children may show little or no urination, become lethargic, have skin that "tents up" when pinched. Signs of dehydration in anyone are good reasons to see a doctor immediately.

What are complications of gastroenteritis?

Most patients who get gastroenteritis have no complications and will completely recover. The major complication for some patients is dehydration; infants, children, the elderly and immunosuppressed are at higher risk for this complication. In many third world countries, hydration of infants is difficult at best so there are many infant deaths worldwide due to dehydration caused by gastroenteritis.

Can gastroenteritis be prevented?

There is no diet that will prevent gastroenteritis but food preparation plays a strong role in preventing gastroenteritis. In general, there are some actions people can do to prevent or reduce the chance of getting gastroenteritis, including:

  • Hand washing, especially before eating and after any close association with an infected person or items (clothing, bedding, toys) they have touched
  • Launder items daily that infected persons wear
  • Avoid direct contact with infected individuals when possible
  • Do not eat undercooked foods, especially meats
  • Do not eat or drink raw foods or untreated water
  • Do not drink untreated or unpasteurized fluids, especially milk
  • Thoroughly wash any produce, especially in third world countries, before eating
  • While traveling, avoid all raw foods and ice; drink only from sealed bottled products and use bottled water for tooth brushing

In addition, there is a vaccine available against rotavirus that has reduced this infection in children. Also, there is a vaccine available against cholera-causing bacteria (Vibrio), but it is not widely available. Research is ongoing; a clinical trial of a Norovirus vaccine was done with some success. It is likely in the near future commercial vaccines against some causes will be available.

What is the prognosis for gastroenteritis?

The prognosis (outlook) for complete recovery is excellent in most people infected with viral and bacterial caused gastroenteritis, as long as the person keeps well hydrated. Because infants, children, pregnant women, and adults that are elderly or immunosuppressed, usually dehydrate faster than healthy adults and sometimes are more difficult to rehydrate orally, their prognosis can range from excellent to poor. Their prognosis depends on how dehydrated they become and how effective are the attempts to rehydrate the patient.

The prognosis for those patients that develop gastroenteritis symptoms as part of a specific disease process (for example, shigellosis) vary from good to poor, depending on the severity of the specific disease process.

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