Causes of diarrhoea

There are many causes of diarrhoea, including food poisoning, infection, malabsorption and inflammatory disease of the gut. 

Diarrhoea is often caused by viral stomach flu or food poisoning. For most people, diarrhoea is due to a self limiting infection or food poisoning which is resolved after a few days, and has no serious consequences.

Diabetes Mellitus can also be a cause of chronic diarrhoea as can other diseases like thyroid disease. 

Infectious Diarrhoea:

Infectious diarrhoea is caused by ingesting microscopic viruses, bacteria or parasites which then live in the intestine. These microbes are usually passed from the diarrhoea of others. Infections can be passed on by infected people who don’t wash their hands after bowel movements and having casual contact, such as preparing food. These intestinal disorders, characterised by inflammation of the mucous membrane, are often referred to as dysentery. 

Virus infection 

This is probably the most common cause of short-term diarrhoea. Usually these infections clear up by themselves. 
A virus such as rotavirus damages the mucous membrane lining the intestine, and disrupts fluid absorption. Rotavirus is often the cause of diarrhoea in children under two years old. Norwalk virus is more common in adults, and usually results from drinking contaminated water or food. 

Bacteria 

Bacteria in contaminated food or water produce toxins that trigger intestinal cells to secrete salt and water, resulting in diarrhoea. This is a type of food poisoning. Salmonella and Campylobacter are the most common types of bacterial infection. 
These serious conditions require medical evaluation. Traveller's diarrhoea is caused primarily by E. coli. 
It most often occurs in visitors to developing countries where sanitation is poor. Cholera, which has acute diarrhoea as one of its symptoms, is contracted mainly by drinking contaminated water. 

Parasites 

Parasites such as amoeba and giardia can attack the intestine. Giardia may occur in wild animals and contaminated water. 
Giardiasis usually infects young children, often in day care. In these environments, it is transmitted by direct contact, or by adults changing different children’s nappies without hand washing in between. Family members of affected children are also at risk. 
Giardiasis can last months without treatment and spread rapidly. It’s important to find the source and test all contacts, as some people spread the infection without having diarrhoea themselves.
In addition to all the above-mentioned infections, HIV positive individuals are also at risk of other opportunistic infections that can cause diarrhoea, e.g. cryptosporidium and isospora belli. 

Non-infectious Diarrhoea:

Food

Certain foods may cause diarrhoea in some people. Many people are intolerant of the milk sugar lactose, due to lactase deficiency. This is an enzyme that breaks down lactose to glucose, which is then readily absorbed by the intestinal tract. 

Even small amounts of milk or dairy products can cause diarrhoea in individuals with lactase deficiency. Large amounts of alcohol, caffeine, artificial sweetener, fatty or spicy foods cause diarrhoea in others.
A baby's digestive system may not tolerate large amounts of milk, juice or fruit. Breast-fed babies are less likely to develop diarrhoea, due to the presence of antibodies in breast milk. 

Medications

If diarrhoea occurs after taking a new medicine, contact your doctor. Diarrhoea can also develop up to a month after taking antibiotics. Antibiotics change the gut micro flora and can predispose to infection with bacteria called Clostridium difficile. This results in diarrhoea that can range in severity from a mild illness, to life-threatening disease necessitating colectomy.
Many people become dependent on chemical laxatives such as Milk of Magnesia, magnesium sulphate (Epsom salts), cascara and phenolphthalein. Magnesium can be inadvertently ingested in certain over-the-counter products. Sorbitol, an artificial sweetener used in some products such as jams, is also a laxative. 

Diseases 

Chronic diarrhoea (i.e. > 2 weeks duration) can be caused by certain intestinal disorders. These include Irritable Bowel Syndrome, Inflammatory Bowel Diseases, including ulcerative colitis and Crohn's disease, chronic pancreatitis, coeliac disease, colon cancers and certain tumours of the small intestine.

Microscopic colitis is an unusual condition which causes diarrhoea in elderly people. These are serious diseases requiring medical attention.

Irritable Bowel Syndrome (IBS) is a very common cause of diarrhoea and occurs when the intestine does not contract smoothly and rhythmically. The contractions can be too strong, causing diarrhoea, or too weak, causing constipation. There may be alternating constipation and diarrhoea. IBS is associated with abdominal pain and bloating, but no blood in the stool or loss of weight. Emotional stress may aggravate these symptoms.

Read more: 

Risk factors of diarrhoea

Symptoms of diarrhoea  

Causes of diarrhoea  

^{Revised and reviewed by Dr Saville Furman, MBChB MFGP (SA). Family Physician, Part time lecturer in family medicine and primary care at UCT, Red Cross Children’s Hospital and Groote Schuur hospital, Cape Town. February 2015.}