Diarrhea is defined by the World Health Organization as having 3 or more loose or liquid stools per day, or as having more stools than is normal for that person.

Secretory

Secretory diarrhea means that there is an increase in the active secretion, or there is an inhibition of absorption. There is little to no structural damage. The most common cause of this type of diarrhea is a cholera toxin that stimulates the secretion of anions, especially chloride ions. Therefore, to maintain a charge balance in the lumen , sodium is carried with it, along with water. In this type of diarrhea intestinal fluid secretion is isotonic with plasma even during fasting. It continues even when there is no oral food intake.

Osmotic

Osmotic diarrhea occurs when too much water is drawn into the bowels.

This can be the result of maldigestion (e.g., pancreatic disease or celiac disease ), in which the nutrients are left in the lumen to pull in water. Osmotic diarrhea can also be caused by osmotic laxatives (which work to alleviate constipation by drawing water into the bowels). In healthy individuals, too much magnesium or vitamin C or undigested lactose can produce osmotic diarrhea and distention of the bowel. A person who has lactose intolerance can have difficulty absorbing lactose after an extraordinarily high intake of dairy products. In persons who have fructose malabsorption, excess fructose intake can also cause diarrhea. High-fructose foods that also have a high glucose content are more absorbable and less likely to cause diarrhea. Sugar alcohols such as sorbitol (often found in sugar-free foods) are difficult for the body to absorb and, in large amounts, may lead to osmotic diarrhea. Diarrhea stops when offending agent (e.g. milk, sorbitol) is stopped.

Exudative

Exudative diarrhea occurs with the presence of blood and pus in the stool. This occurs with inflammatory bowel diseases, such as Crohn's disease or ulcerative colitis, and other severe infections such as E. coli or other forms of food poisoning.

Motility-related

Motility-related diarrhea is caused by the rapid movement of food through the intestines (hypermotility). If the food moves too quickly through the gastrointestinal tract, there is not enough time for sufficient nutrients and water to be absorbed. This can be due to a vagotomy or diabetic neuropathy, or a complication of menstruation. Hyperthyroidism can produce hypermotility and lead to pseudodiarrhea and occasionally real diarrhea. Diarrhea can be treated with antimotility agents (such as loperamide). Hypermotility can be observed in people who have had portions of their bowel removed, allowing less total time for absorption of nutrients.

Inflammatory

Inflammatory diarrhea occurs when there is damage to the mucosal lining or brush border, which leads to a passive loss of protein-rich fluids, and a decreased ability to absorb these lost fluids. Features of all three of the other types of diarrhea can be found in this type of diarrhea. It can be caused by bacterial infections, viral infections, parasitic infections, or autoimmune problems such as inflammatory bowel diseases. It can also be caused by tuberculosis, colon cancer, and enteritis.

Dysentery

Generally, if there is blood visible in the stools, it is not diarrhea, but dysentery. The blood is trace of an invasion of bowel tissue. Dysentery is a symptom of, among others, Shigella, Entamoeba histolytica, and Salmonella.

Diarrhea is most commonly due to viral gastroenteritis with rotavirus accounting for 40% of cases in children under five.(p. 17) In travelers however bacterial infections predominate.

It can also be the part of the presentations of a number of medical conditions such as: Crohn's disease or mushroom poisoning .

Infections

There are many causes of infectious diarrhea, which include viruses, bacteria and parasites.

The bacterium campylobacter is a common cause of bacterial diarrhea, but infections by salmonellae, shigellae and some strains of Escherichia coli (E.coli) are frequent.

Parasites do not often cause diarrhea except for the protozoan Giardia, which can cause chronic infections if these are not diagnosed and treated with drugs such as metronidazole,

Other infectious agents such as parasites and bacteria l toxins also occur.

Malabsorption

Malabsorption is the inability to absorb food, mostly in the small bowel but also due to the pancreas.

Causes include:

• enzyme deficiencies or mucosal abnormality, as in food allergy and food intolerance, (e.g. celiac disease (gluten intolerance), lactose intolerance (intolerance to milk sugar, common in non-Europeans), fructose malabsorption)

• pernicious anemia (impaired bowel function due to the inability to absorb vitamin B₁₂ ),

• loss of pancreatic secretions (may be due to cystic fibrosis or pancreatitis),

• structural defects, like short bowel syndrome (surgically removed bowel) and radiation fibrosis (usually following cancer treatment and other drugs, including agents used in chemotherapy),

• certain drugs (like orlistat, which inhibits the absorption of fat).

Inflammatory bowel disease

The two overlapping types here are of unknown origin:

• Ulcerative colitis is marked by chronic bloody diarrhea and inflammation mostly affects the distal colon near the rectum.

• Crohn's disease typically affects fairly well demarcated segments of bowel in the colon and often affects the end of the small bowel.

Irritable bowel syndrome

Another possible cause of diarrhea is irritable bowel syndrome (IBS) which usually presents with abdominal discomfort relieved by defecation and unusual stool (diarrhea or constipation) for at least 3 days a week over the previous 3 months. There is no direct treatment for IBS, however symptoms can be managed through a combination of dietary changes, soluble fiber supplements, and/or medications.

Other causes

• Diarrhea can be caused by chronic ethanol ingestion.

• Ischemic bowel disease. This usually affects older people and can be due to blocked arteries.

• Hormone-secreting tumors: some hormones (e.g., serotonin) can cause diarrhea if excreted in excess (usually from a tumor).

• Chronic mild diarrhea in infants and toddlers may occur with no obvious cause and with no other ill effects; this condition is called toddler's diarrhea.

Evolution

According to two researchers, Nesse and Williams , diarrhea may function as an evolved expulsion defense mechanism. As a result, if it is stopped, there might be a delay in recovery. They cite in support of this argument research published in 1973 which found that treating Shigella with the anti-diarrhea drug (Co-phenotrope, Lomotil) caused people to stay feverish twice as long as those not so treated. The researchers indeed themselves observed that: "Lomotil may be contraindicated in shigellosis. Diarrhea may represent a defense mechanism".

The following types of diarrhea may indicate further investigation is needed:

• In infants

• Moderate or severe diarrhea in young children

• Associated with blood

• Continues for more than two days

• Associated non-cramping abdominal pain, fever, weight loss, etc.

• In travelers

• In food handlers, because of the potential to infect others;

• In institutions such as hospitals, child care centers, or geriatric and convalescent homes.

A severity score is used to aid diagnosis in children.

A rotavirus vaccine has the potential to decrease rates of diarrhea. There are currently two licensed vaccines against rotavirus. New vaccines against rotavirus, Shigella , ETEC, and cholera are under development, as well as other causes of infectious diarrhea.

In many cases of diarrhea, replacing lost fluid and salts is the only treatment needed. This is usually by mouth – oral rehydration therapy – or, in severe cases, intravenously. Diet restrictions such as the BRAT diet are no longer recommended. Research does not support the limiting of milk to children as doing so has no effect on duration of diarrhea.

Medications such as loperamide (Imodium), bismuth subsalicylate may be beneficial, however they may be contraindicated in certain situations.

Medications

Antibiotics

While antibiotics are beneficial in certain type of acute diarrhea they are usually not used except in specific situations. However, some bacteria are developing antibiotic resistance, particularly Shigella .

Anti motility agents

Anti motility agents like loperamide are effective at reducing the duration of diarrhea.

Bismuth compounds

While bismuth compounds ( Pepto-Bismol) decreased the number of bowel movements in those with travelers' diarrhea it does not decrease the length of illness. These agents should only be used if bloody diarrhea is not present.

Codeine Phosphate

Codeine Phosphate is used in the treatment of diarrhea to slow down Peristalsis and the passage of fecal material through the bowels - this means that more time is given for water to reabsorb back into the body, which gives a firmer stool, and also means that feces is passed less frequently.

Alternative therapies

The probiotic lactobacillus can help prevent antibiotic associated diarrhea in adults but possibly not children. For those who suffer from lactose intolerance, taking digestive enzymes containing lactase when consuming dairy products is recommended.

World wide in 2004 approximately 2.5 billion cases of diarrhea occurred which results in 1.5 million deaths among children under the age of five.

• Rotavirus

• Shigella

• Enterotoxigenic Escherichia coli

• Cholera

• Salmonella typhimurium

• Brainerd diarrhea

• Travelers' Diarrhea