Diarrhea is a very frequent complaint; at one point in life everybody will have it. Most of these episodes are due to an acute environmental injury: a bacterial or viral infection, such as salmonella or norovirus, or some irritating food.
There are several ways to categorize diarrhea: acute or chronic, bloody or not bloody, profuse or just loose stools, accompanied by abdominal pain or not. Looking at the underlying mechanism of disease, it can also be divided into secretory diarrhea, when the lining of the gut secretes too much fluid or osmotic diarrhea, when the contents of the gut are too concentrated and pull in water from the gut tissue.
The most frequent cause of diarrhea is infection. These come in many forms: viral, bacterial or parasitic, which can be small, single cell organisms (protozoa) or worms.
Infectious agents can act on the small bowel , causing high volume, mostly non bloody diarrhea, or the colon, which results in frequent, sometimes painful bowel movement, which can be bloody, if there is invasion of the colon wall, but not as high volume as small bowel diarrhea.
Norovirus is the most prevalent agent. It is very infectious and usually causes clusters of cases, such as abord cruise ships. It may present with vomiting and profuse diarrhea and, in an otherwise healthy person, is self limited. As with all, especially high volume diarrheal cases, dehydration is the main cause to worry. Drinking plenty of fluids, especially oral rehydration solutions, is essential, but sometimes intravenous fluids are needed. Rotavirus is the most common cause of diarrhea in children and there is an effective vaccine against it.
The most common bacterial pathogens are Salmonella, Campylobacter, Shigella and E. coli. These can be invasive, causing fever, chills, sometimes bloody diarrhea, weight loss. Treatment is usually symptomatic but, if there are symptoms of invasion, antibiotics might be required.
There are bacteria, which cause diarrhea nit by directly attacking the gut but through toxin production. These range from the relatively minor problems of self limited diarrhea caused by toxins produced by Staphylococcus or B. cereus ( Chinese rice syndrome) in improperly handled food to severe diseases such as cholera (which can be rapidly fatal due to profound dehydration) od C. difficile, which is an increasingly common cause of bloody diarrhea in hospitalized patient or patients after antibiotic treatment. These most be treated quickly to avoid complications.
Toxins can cause diseases, where diarrhea is a minor factor, such as scombroid fish poisoning, where bacteria growing on improperly stored fish produce histamine and histamine-like substances or Ciguatera fish poisoning, where large, predatory fish, like barracuda, eat tiny organisms, which produce a neurotoxin. This is a potentially life threatening illness.
Protozoa, such as Giardia (frequent in non-tropical climates and usually found in contaminated water), which is a non-invasive organism, cause non bloody diarrhea, bloating and weight loss and can be treated fairly easily. Amebae are invasive, cause bloody diarrhea, can live in the liver (and other organs) as well as the gut, and both forms need to be treated in order to get rid of the infection.
Worms come in many forms, depending on the general hygiene in the region. Diarrhea is frequently part of these infestations but usually not the leading symptom. Most worms, especially when diagnosed promptly, can be treated effectively.
A special category of infectious diarrhea is Traveler’s Diarrhea. This is an acute, most likely self limited diarrhea, occurring in tourists coming into a tropical or subtropical country from more moderate climates, where the usually food or water borne pathogens are less frequent. Diarrhea can be quite debilitating but is usually over in 2-3 days. It can be treated with various antibiotics and prevented by taking PeptoBismol or Xifaxan
