Diarrhoeal Diseases (Updated February 2009)
Diarrhoeal diseases represent a major health problem in developing countries and also a high risk to travellers who visit these countries. Conservative estimates place the global death toll from diarrhoeal diseases at about two million deaths per year (1.7 - 2.5 million deaths), ranking third among all causes of infectious disease deaths worldwide. Most of these deaths occur in children under five years of age   . An average morbidity attack rate of 3.2 episodes of diarrhoea per year per child has been reported, but in some settings in developing countries, this number can be as high as 12 episodes per year per child. Evidence has been accumulating for long-term consequences of such heavy disease burden in early childhood on physical and mental development of children that may eventually translate into costly impairment of human fitness and productivity at an adult age . Moreover, outbreaks of cholera, shigellosis and typhoid fever most often occur in resource-poor countries, adding to the burden of disease among the most vulnerable such as refugees, internally displaced populations and groups living in shanty towns.
The wide diversity of bacterial and viral infections that may cause diarrhoea  complicates accurate surveillance and diagnosis, especially in developing countries with little or no access to modern laboratory procedures. The specific disease burden attributable to a particular infectious agent is particularly complex, given the multiplicity of these agents and their serotypes, and its accurate documentation depends largely on laboratory facilities. While, in the long term, access to clean water, better hygiene, adequate nutrition, and improvement of sanitary measures would certainly have the greatest impact on diarrhoeal diseases, immunization against specific diseases is the best hope for the short- and mid-term. The development of vaccines against enteric pathogens however represents a serious challenge because of the large number of pathogens and the requirement to induce mucosal immunity in the gut  .
Among the principal bacterial agents of diarrhoeal diseases are Vibrio cholerae (cholera), a variety of Salmonella spp, including S typhi (typhoid fever), and of Shigella spp, the agents of shigellosis (bacterial dysentery), Campylobacter spp (especially C jejuni) and a variety of enteropathogenic Escherichia coli strains, including the enterotoxigenic ETEC strains that are the main agents of travellers’ diarrhoea.
Diarrhoeas can also be caused by a variety of bacterial pathogens such as Staphylococcus aureus, Clostridium perfringens, Clostridium difficile or Klebsiella, as well as by various protozoa including Giardia, Cyclospora and Cryptosporidium spp (e.g. Giardia lambia, Cryptosporidium parvum) and Entamoeba histolytica.
Among the enteric viruses, rotaviruses remain the leading cause of diarrhoeal disease in young children in the world. Every year, before the advent of rotavirus vaccines, an estimated 527 000 young children died from severe diarrhoea caused by rotavirus infection, most of them occurring in developing countries in South Asia and sub-Saharan Africa . The progressive implementation of rotavirus vaccines in the field will hopefully soon change this bleak picture. Other viruses causing diarrhoeas include enteric adenoviruses, astroviruses and caliciviruses, the latter (noroviruses) being responsible for most gastroenteritis winter outbreaks in industrialized countries .
Diarrhoea also is the most common health problem among travellers from industrialized countries who visit developing areas, especially in the tropics  . Up to 80% of diarrhoeal episodes in travellers are bacterial in nature, caused principally by enterotoxigenic Escherichia coli (ETEC) strains, as well as, quite commonly, by Shigella, Campylobacter and Salmonella spp. In addition, mild cases of cholera, caused by Vibrio cholerae, are often indistinguishable from other causes of acute diarrhoeal disease. The increased frequency of antibacterial drug-resistance among these pathogens is a source of major concern   .
The CDC's Foodborne Diseases Active Surveillance Network (FoodNet) recently estimated that the incidence of illnesses caused by foodborne pathogens in the USA had not decreased since 2004 as far as campylobacter, listeria, E coli O157, salmonella, shigella, vibrio and yersinia infections were concerned, whereas cryptosporidium infections increased by 44%  . Salmonella remained the most common cause of foodborne illness as of 2007 in the USA. Foodborne outbreaks attributable to leafy greens such as lettuce, cabbage and spinach increased substantially during the past 35 years and continue to cause public health problems throughout the USA, in part due to increased consumption of ready-to-eat products and in part due to the modified atmosphere packaging that is used to keep the produce looking fresh.
All these pathogens are transmitted by the faecal-oral route.