ENTEROTOXIGENIC E. coli (ETEC)
ETEC are the most important cause of traveler’s diarrhea in visitors to developing countries. ETEC also produce diarrhea in infants native to these countries, where they are a leading cause of morbidity and mortality during the first 2 years of life. Repeated bouts of diarrhea caused by ETEC and other infectious agents are an important cause of growth retardation, malnutrition, and developmental delay in the third world countries where ETEC are endemic. ETEC disease is rare in industrialized nations.
Transmission is by consumption of food and water contaminated by human cases or convalescent carriers. Uncooked foods such as salads or marinated meats and vegetables are associated with the greatest risk. Direct person-to-person transmission is unusual, be-cause the infecting dose is high. Animals are not involved in ETEC disease.
ETEC diarrhea is caused by strains of E.coli that produce LT and/or ST enterotoxins in the small intestine. Strains that elaborate both LT and ST cause more severe illness. Adher-ence to surface microvilli mediated by the CFA class of pili is essential for the efficient delivery of toxin to the target enterocytes. The genes encoding the ST, LT, and the CFA pili are borne in plasmids. A single plasmid can carry all three sets of genes. The bacteria remain on the surface, where the adenylate cyclase – stimulating action of the toxin(s) creates the flow of water and electrolytes from the enterocyte into the intestinal lumen. The mucosa becomes hyperemic but is not injured in the process. There is no invasion or inflammation.
Although there can be more than one episode of diarrhea, infections with ETEC can stim-ulate immunity. Travelers from industrialized nations have a much higher attack rate than adults living in the endemic area. This natural immunity is presumably mediated by sIgA specific for LT and CFAs. The small ST peptides are nonimmunogenic. The disease is of very low incidence in breast-fed infants, underscoring the protective effect of maternal antibody and the importance of transmission by contaminated food and water.