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 flex-neri (B), Shigella boydii (C), and Shigella sonnei are less dangerous. More than 40 serotypes are recognized and are useful in tracing the spread of outbreaks.

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 hy gienic 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.

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