Updated 25 July 2018

Causes of gastroenteritis

The most common cause of gastroenteritis is a virus.

The microorganisms that cause gastroenteritis spread easily from one person to another.  

Contamination can occur in the following cases:

  • If a person with diarrhoea doesn’t wash their hands thoroughly after using the toilet.
  • If a person touches their mouth after touching an object that may be contaminated.
  • If a person eats food or drinks water that’s been contaminated.
  • If a person eats food that isn’t cooked thoroughly or pasteurised.
  • If a person swims in a pond or pool that’s been contaminated by another person or animal.
  • If a person comes into contact with an animal that’s carrying the microorganism. 

The most common causes of gastroenteritis are:

  • Viruses (most common)
  • Bacteria
  • Parasites
  • Chemical toxins
  • Drugs

Viruses are the most common cause of gastroenteritis. The four viruses that account for the majority of cases are norovirus, rotavirus, astrovirus and adenovirus.  

  • Norovirus generally infects older children and adults. It’s the most common cause of gastroenteritis, causing 50-70% of cases and is highly contagious. Infections occur anytime during the year and most infections occur due to swallowing contaminated food or water.
  • Rotavirus is the most severe, dehydrating gastroenteritis among young children, particularly between the ages of 3 and 15 months. This infection is generally spread by contact with contaminated faeces. Adults can get infected but the illness is generally mild. The most severe infections generally occur in winter.
  • Astrovirus usually infects infants and young children, but can affect people of all ages. The infection is spread by contact with contaminated faeces and occurs most commonly in winter.
  • Adenovirus generally affects children under the age of two. The infection is also spread by contact with contaminated faeces and occurs most commonly in summer, but can occur all year round.

Other viruses (such as cytomegalovirus and enterovirus) can cause gastroenteritis in people who have impaired immunity.

Bacterial gastroenteritis is less common than viral gastroenteritis. Common bacterial causes include Campylobacter, Salmonella, Shigella and certain strains of Escherichia coli (E. coli).

Salmonella and Campylobacter are the most common bacterial causes of diarrhoea. They’re most often acquired from eating undercooked chicken or eggs, or using unpasteurised milk. 

Species of Shigella are the third most common bacterial cause of diarrhoea and are usually transmitted from person to person.

Some bacterial infections (specifically Clostridium difficile) occur after treatment with antibiotics, as the antibiotics kill the healthy bacteria that are normally present in the gut. This allows the bacteria to grow in their place, producing a toxin that causes the diarrhoea.

Several different subtypes of E. coli cause diarrhoea. They’re generally spread through undercooked meat, unpasteurised milk and contaminated water. Person-to-person transmission can also occur in crowded living areas. A common subtype of E. coli infection is traveller’s diarrhoea. 

Some bacteria (such as Staphylococcus aureus, Bacillus cereus, and Clostridium perfringens) produce toxins that cause food poisoning. These toxins generally cause severe nausea, vomiting and diarrhoea. Symptoms generally begin within 12 hours of ingesting the contaminated food and subside within 36 hours.

Certain intestinal parasites (Giardia being the most common) enter the lining of the intestine and cause nausea, vomiting, diarrhoea and a general sick feeling. These infections are spread most commonly through contaminated drinking water.

If the disease continues long term (i.e. becomes chronic), the intestines won’t be able to absorb nutrients, leading to a condition known as malabsorption syndrome.    

Chemical toxicity
Gastroenteritis may result from ingesting chemical toxins. These may toxins come from plants, such as poisonous mushrooms, or from certain kinds of exotic seafood.  

Gastroenteritis due to chemical toxicity can also occur by drinking water or eating food that’s contaminated by chemicals such as arsenic, lead, mercury or cadmium. Heavy-metal poisoning often causes nausea, vomiting, abdominal pain and diarrhoea. 

As we get older, we develop immunity to the more common viruses and bacteria in our environment. This means that, despite regular exposure, we’re no longer vulnerable to many of them.

These micro-organisms are usually acquired directly or indirectly from another infected person. However, some are present in soil, water and even air. Animals (both livestock and pets) may be a source of infection with some of the micro-organisms that cause gastroenteritis.

If you have a child with ongoing gastrointestinal symptoms, it may be worth considering that he or she is lactose intolerant (an intolerance to milk sugar).

Reviewed by Kim Hofmann, registered dietitian, BSc Medical (Honours) Nutrition and Dietetics, BSc (Honours) Psychology. December 2017.