Causes of diarrhoea


Diarrhoea is commonly the result of an infection or food poisoning. This is mostly resolved after a few days and generally has no serious consequences. It’s important to monitor it closely though, as diarrhoea can have serious complications.

Infectious diarrhoea
Infectious diarrhoea is caused by ingesting microscopic viruses, bacteria or parasites that then live in the intestine. These microbes are usually passed from the stools of others.

Infections can be passed on by infected people who don’t wash their hands after bowel movements and subsequently handle food. These intestinal disorders, characterised by inflammation of the mucous membrane, are often referred to as “dysentery”.

Virus infection is probably the most common cause of short-term diarrhoea. Usually these infections clear up by themselves. Viruses like rotavirus or Norwalk, damages the mucus membrane lining the intestine and disrupts fluid absorption. Rotavirus is often the cause of diarrhoea in children under two years of age. Norwalk virus, on the other hand, is more common in adults and usually results from drinking contaminated water or eating contaminated food.

Bacteria in contaminated food or water produce toxins that trigger intestinal cells to secrete salt and water, resulting in food poisoning of which diarrhoea is a symptom. 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, and 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 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, where it’s 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. Finding the source and testing all contacts is important, as some people spread the infection without having diarrhoea themselves.

In addition to all the above-mentioned infections, HIV-positive people are also at risk of other infections that can cause diarrhoea, e.g. cryptosporidium and isospora belli.

Non-infectious diarrhoea

Certain foods may cause diarrhoea in some people. Many people are intolerant of the milk sugar lactose, due to lactase deficiency. Lactase 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, and fatty or spicy foods can cause diarrhoea in others.

A baby's digestive system may not tolerate large amounts of milk, juice or fruit. Breastfed babies are less likely to develop diarrhoea due to the presence of antibodies in breast milk.

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 microflora and can predispose to infection with a bacteria called Clostridium difficile, resulting in diarrhoea that can range in severity from a mild illness to life-threatening disease necessitating colectomy. Many people become dependent on chemical laxatives like 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.

Chronic (i.e. more than two weeks duration) diarrhoea can be caused by certain intestinal disorders, which include irritable bowel syndrome (IBS), inflammatory bowel diseases (IBD), 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 that causes diarrhoea in elderly people. These are serious diseases requiring medical attention.

IBS is a very common cause of diarrhoea that happens when the intestine doesn’t 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.

Time for the doctor? 
Whatever the cause, it’s important to get help if you experience frequent loose stools, including nocturnal stools, blood and mucus in the stools, associated vomitting and if you’re caring for someone who can’t rehydrate themselves adequately. Symptoms like high fever, severe abdominal pain and diarrhoea that doesn’t improve after 48 hours are serious.


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