Etiology and Epidemiology

Four species or subgroups of Shigella cause human disease. Shigella dysenteriae (subgroup A), the first to be discovered, is the most virulent. Shigella flex­neri (B), Shigella boydii (C), and Shigella sonnei are less dangerous.

Shigella organisms are passed in the feces and spread from person to person by the fecal-oral route. Bacteria are excreted during the illness and for about 4 weeks after recovery, but some asympto­matic individuals may act as carriers for a year or more. Contaminated food and water are the most common modes of transmission. Direct fecal contami­nation or mechanical carriage by flies can introduce bacteria into food, milk, or water. Sick, convalescent, or even healthy food handlers who have poor hy­gienic practices are especially dangerous; proper handwashing after defecation is a simple but effec­tive preventive measure. Crowding and poor sanita­tion favor transmission, and outbreaks are common in jails and institutions for the retarded and men­tally ill.

Epizootics have been reported in colonies of cap­tive primates, and two species have been isolated from dogs, but animal reservoirs have no known epidemiological significance.