Common organisms causing diarrhea are:

Viruses      :       Rotavirus
                          Enterovirus

Bacteria     :       E. coli
                          Shigella
                          Camylobacter jejuni
                          V. Cholerae
                          Salmonella (non typhoidal)

Protozoal   :      Giardia duodenalis
                          Entamoeba Histolytica
                          Cryptosporidium

Most of these organisms affect the small bowel except shigella, entero invasive E. coli (affect colon), C.jejuni and entamoeba histolytica which affect the large bowel. Small bowel diarrhea are frequent, large quantity watery diarrhea (greenish to white) whereas large bowel diarrhea are semi loose, small quantity diarrhea with/without blood in stools, small bowel diarrhea usually can lead to dehydration and fluid replacement is essential. Antimicrobials are not needed for small bowel diarrhea as it is self limiting except for cholera. Large bowel diarrhea do not cause dehydration unless caused by E. coli or Shigella and need treatment with antimicrobials.

Rotavirus: - It is one of commonest cause of diarrhea in children between 6-24 months of age. It affects the small intestines and spreads through faeco-oral route mainly in winter seasons. It causes watery diarrhea with vomiting.

E. coli: - It is one of the commonest cause of diarrhea in developing countries. It spreads through contaminated food and water. Five types of E. coli have been identified:

a)	Enterotoxigenic E. coli (ETEC): It produces enterotoxins that cause secretion of fluid and electrolytes and is commonest cause of traveler's diarrhea. This diarrhea is self limited.
b)	Localized adherent E. coli (LA-EC): Enteroadherance and production of a potent cytotoxin are important mechanisms for causing diarrhea. This disease is usually self limited.
c)	Diffuse adherent E. coli (DA-EC): Similar to LA-EC.
d)	Enteroinvasive E. coli (EIEC): They are uncommon and occur in sporadic food borne outbreaks and resemble shigellosis. It affects colon and treatment with antimicrobials is essential.
e)	Enterohemorrhagic E. coli (EHEC): It is seen in Europe and parts of North & South America where outbreaks can be caused by undercooked meat. EHEC produces Shiga like toxin that affects colon. It leads to acute onset of cramps, bloody diarrhea. Type O157:47 can lead to hemolytic uremic syndrome.

Shigella:- Shigella is the commonest cause of dysentery in children. Spread occurs from person to person contact and commonly seen in hot, humid climates. Infectious dose is low (10 to 100 organisms). It affects the colon by invasion and also releases Shiga toxin which is cytotoxic and neurotoxic that causes watery diarrhea. S. flexneri is commonest in developing countries whereas S. sonnei is commonest in developed countries. S. boydii is less common whereas S. dysenteriae causes epidemics. Patients present with dysentery. Treatment with antimicrobials is essential.

Campylobacter Jejuni:- It produces diarrhea by invasion of the ileum and large intestine. Diarrhea is watery but in few cases of dysentery may occur.

Vibrio cholerae:- It is seen in Africa, Asia and Latin America and spreads through contaminated food and water. It adheres to small intestine and produces an enterotoxin similar to E. coli (ETEC). Fluid replacement and treatment with antimicrobials is essential.

Salmonella:- It causes 1-5% of gastroenteritis and occurs from ingestion of contaminated animal products. It affects the ileum and releases enterotoxin that causes watery diarrhea. Antimicrobials are not needed and can even prolong shedding of the pathogen in the stool.

Giardia duodenalis:- Children between 1-5 years are most commonly affected and spread through faeco-oral route. It affects the small bowel and leads to acute or persistent diarrhea with malabsorption and bloating. Treatment with metronidazole (5 mg/kg/dose tds for 5 days) or tinidazole (50 mg/kg single dose; max 2 doses) should be given.

Entamoeba histolytica:- It invades the large intestines and causes bloody diarrhea. Presence of trophozoites in stools is suggestive of invasive disease and treatment with metronidazole (10 mg/kg/dose tds for 5 days) is required. 90% of infections are asymptomatic and are caused by non-pathogenic strains of E. histolytica that need no treatment.

Cryptosporidium:- It accounts for acute watery for persistent diarrhea in 5-15% of children in developing countries. It spreads through faecal-oral route and affects small intestines causing mucosal damage and malabsorption. Treatment with Nitazoxanide is effective.

Last updated : 1^st July 2009


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