Shigellosis
Shigellosis diarrhoeal illness caused by the enteric bacterial pathogen Shigella of which there are four species, all of which are pathogenic to man. These are: Shigella dysenteriae, S. flexneri, S. sonnei and S. boydii. The bacteria enter the gut through the oral route with contaminated food and water. Contamination is believed to be brought about mainly by person to person contact since the practice of hand-washing prior to handling food has been found to be effective in lowering disease incidence. In the intestine, they multiply, produce toxins and invade the intestinal epithelial cells and spread to adjacent cells causing thereby a focus of infection.
The consequence of these is onset of diarrhoea with stool mixed with blood and mucus and triggering fever and severe abdominal pain. The disease usually lasts for a week but should preferably be treated with antibiotics since the illness can lead to other complications such as failure of kidney and if not treated promptly, may be fatal. The disease is one of the major causes of morbidity and mortality in developing countries, the most vulnerable group being children under the age of five.
In Bangladesh, the disease also known as bacillary dysentery or blood dysentery, is prevalent throughout the year at a certain level of incidence, called the endemic level, but every year sporadic epidemics break out in parts of the country some of which are quite explosive causing considerable morbidity and mortality. The winter months show relatively low incidence of shigellosis, February being the month with generally the lowest incidence. The incidence then begins to rise and reaches a peak during early monsoon months - June and July. A post-monsoon period of high incidence has also occasionally been recorded in some years. In addition to the year round occurrence and sporadic epidemics for which no precipitating factors can be readily identified, the disease like other enteric infections is also common following natural disasters such as cyclone and monsoon flooding. Poor personal health and sanitation are the main factors aiding the spread of the disease.
The total estimated number of cases of shigellosis in Bangladesh in a year varies from year to year that parallels with the magnitude of the epidemic in a particular year. Reliable data on the number are not available because of lack of adequate disease surveillance system. It is estimated that approximately 2,00,000 to 300,000 cases of shigellosis occur in Bangladesh in a typical year, barring those years when explosive epidemics occur in which case the number will be higher.
Unlike cholera and other types of diarrhoeal diseases such as those caused by some toxin-producing E. coli known as enterotoxigenic E. coli (ETEC), shigellosis is not regarded as a water-borne diarrhoeal disease. Also, unlike cholera the disease is not associated with discharge of profuse quantity of watery stool with the consequence that severe dehydration is not an usual accompanying symptom of the disease. Thus, oral rehydration therapy (ORT) such as use of oral rehydration saline is not generally critical in the treatment of shigellosis. Although the disease is self-limiting, use of antibiotics is often recommended for treatment after proper microbiological investigations and antibacterial sensitivity tests have been carried out. Several antibiotics are useful in treatment of shigellosis including ampicillin, trimethoprim and quinolone antibiotics such as ciprofloxacin. The bacteria are able to develop resistance to antibiotics rather rapidly so that drug resistance is a major problem in the treatment of shigellosis.
Shigellosis is an immunising infection. That is, an episode of illness results in the development of immunity against subsequent infections caused by the pathogen. Thus, an effective vaccine is believed to be a good option for the prevention of shigellosis. However, there are several serological types of Shigella which thus requires that the vaccine used should be able to protect against all these different types. Killed whole-cell vaccines that have been used in the past either orally or given through injection have not been effective in developing protective immunity. Recently, live vaccines produced by genetically modifying the bacteria to reduce its virulence while maintaining the immunity- triggering properties, are being developed and tested. Several live candidate vaccines against shigellosis are now in various stages of trial in humans, but to-date no practical and effective vaccine is available for mass use. In the development of such vaccines, the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b) located in Dhaka plays an important role. With its modern clinical, laboratory and field study facilities, the Centre often carries out extensive human trials of candidate vaccines developed by laboratories in the industrialised countries.
There are several field-study stations operated by ICDDR,B in different parts of Bangladesh where studies on the epidemiology of the disease and trials of drugs and vaccines are conducted. One such field station is located in Matlab, about 40 kilometres south east of Dhaka where 200,000 people are under year round surveillance for various diseases, population parameters, migration etc., and is widely acclaimed as the best longitudinal demographic surveillance facility of its kind in the developing world.
National institutions engaged in biomedical research often carry out collaborative research on different aspects of shigellosis with ICDDR,B. Because the disease is particularly widespread in children due to their lack of prior exposure to the pathogen and thus of immunity, the Dhaka Children Hospital also works particularly closely with ICDDR,B in shigellosis and other diarrhoeal diseases in children. [Zia Uddin Ahmed]
