Diarrhoeal Diseases conditions showing frequent and excessive discharge of watery material from the bowel. Diarrhoeal diseases mostly result from ingestion of harmful germs with food and water, although in some cases short episodes of diarrhoea may result from eating poisonous substances, and difficult to digest food, or through physiological intolerance of certain foods. Diarrhoea caused by virus, bacteria or parasites possesses two characteristics- firstly, the offending organisms colonise the intestine and as a consequence cause inflammation of the intestine or enteritis; and secondly, they upset the balance of intestinal fluid absorption and secretion mechanism, often enhancing the latter very considerably, which is then manifested as watery stool discharged frequently in large volumes. Most diarrhoeal episodes are acute infections of the intestine and are self-limiting illnesses lasting for a few days but requiring treatment either to correct the lost fluid or to contain the tissue damage or other complications of the infection.
Viral Diarrhoea Diarrhoea caused by viruses is generally mild. The two common viral agents commonly associated with diarrhoea are the 'Norwalk' virus and rotavirus. Viral diarrhoea is often associated with watery stool, nausea, vomiting, low-grade fever, headache, abdominal cramp and general malaise. The International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b) conducts research on rotavirus diarrhoea that is aimed at developing a live oral vaccine consisting of an attenuated virus to be administered as oral drops similar to oral polio vaccines. No diagnostic tests are performed to detect Norwalk agents in Bangladesh.
Bacterial Diarrhoea The most prevalent and frequently the most severe types of life threatening diarrhoea. Among bacterial diarrhoea, the two most common types are watery diarrhoea, the prototype of which is cholera caused by Vibrio cholerae and some toxin-producing strains of the colon bacteria Eschericia coli, and bacillary or bloody dysentery caused by the enteric bacteria Shigella. The toxin-producing E. coli is also called enterotoxigenic E. coli or ETEC, and the diarrhoea referred to as ETEC diarrhoea. In addition, there is a type of enteritis caused by Salmonella typhi, a close relative of Shigella, whose primary manifestation is not, however, diarrhoea. The route of entry of the pathogen is the intestine from where it travels to the blood stream, thereby causing typhoid fever.
Cholera is caused by Vibrio cholerae, a comma-shaped motile Gram-negative bacteria which when ingested with contaminated food or water, grows and colonises the small intestine of susceptible individuals. During the process it produces a toxin, the cholera toxin (CT) which is adsorbed onto the gangliosides of the intestinal epithelial cells. The toxin stimulates hypersecretion of water and chloride ions from all parts of the small intestine into the lumen of the gut while simultaneously inhibiting absorption of sodium by the gut wall from the luminal fluid. The result is profuse watery diarrhoea with an outpouring of fluids and electrolytes lost with the stool. So potent is the secretogenic effect of cholera toxin that a patient may loose as much as 30 liters of fluid in one day or twice his body weight during the course of the disease. Severe dehydration thus results which is often accompanied by acidosis of the blood. Unless these are corrected, death results rapidly. In oral rehydration therapy (ORT), fluid replacement can be brought about in the patient by oral administration of a solution containing salt and glucose, instead of injection of such fluid through the intra-venous route. Because of this methods, cholera mortality in Bangladesh has been greatly reduced in recent years. A somewhat milder form of watery diarrhoea is caused by some E. coli which is generally referred to as ETEC diarrhoea and is common in children.
Cholera and ETEC diarrhoea are non-invasive diseases since the bacteria never reache the bloodstream so that the illness is usually self-limiting and can be managed by adequate fluid replacement. Cholera and ETEC diarrhoea together may account for nearly three-quarter of the million cases of diarrhoea annually occurring in Bangladesh.
In contrast to cholera and ETEC infections that are non-invasive, enteritis caused by Shigella and Salmonella is invasive, that is, the bacteria enter cells of the intestine and spread within the intestinal tissue. Shigella causes bloody dysentery or bacillary dysentery by invading the epithelial cells of the large intestinal mucosa, causing inflammation, tissue damage, and bleeding. Infective dose of Shigella is remarkably small; as few as 100 bacteria can potentially trigger an episode of the disease. Onset of the disease may be associated with mild watery diarrhoea, but bloody stool with abdominal pain and fever are more pronounced symptoms than diarrhoea related dehydration in episodes of bacillary dysentery. Cases of bacillary dysentery may run to about a quarter of a million a year in Bangladesh. Enteritis caused by Salmonella is also initiated as an intestinal infection, similar to Shigella infection. The infective dose of Salmonella is about 100,000 bacteria. When ingested with contaminated food, they invade the small intestine and reach the bloodstream where they multiply. At early stages, constipation may result, but as the disease progresses, bloody diarrhoea with associated high fever can cause severe incapacitation. Duration of the disease is several weeks. Antibiotics are the preferred course of treatment.
Parasitic Diarrhoea Intestinal parasitic diarrhoea include amoebic dysentery or amoebiasis, caused by the non-flagellated protozoan parasite Entamoeba histolytica, and giardiasis, or diarrhoea caused by the flagellate protozoa Giardia lamblia. E. histolytica enters human intestine through the oral route and invades the mucosa. Mucosal invasion is aided by the production of proteolytic enzymes by the parasite causing tissue lysis and production of discrete ulcers. Disease symptoms include abdominal tenderness, pain and fulminating dysentery - 30 or more movements of mucoid stool in a day - nausea and frequent urgent desire to defecate. As opposed to E. histolytica, G. lamblia is a relatively weak pathogen. Infection by G. lamblia triggers diarrhoea with loose semi-solid stool, abdominal distension, flatulence, malaise and weight loss. The organism occasionally invades the gall bladder and bile ducts. Children are more susceptible to clinical giardiasis than adults. Both amoebiasis and giardiasis are common parasitic infections in children in Bangladesh. Three-quarters of children under the age of 15 years have exhibited immunological evidence of E. histolytica infection at an earlier stage of their life. Thus, amoebic dysentery is a disease of high prevalence in Bangladesh. Estimates on the prevalence of giardiasis are not available, but in ICDDR,B surveillance, about 5% of diarrhoeal patients have shown presence of G. lamblia in their stool. [Zia Uddin Ahmed]