Abstract
During the seven-year period 1974–1980, 93,516 Shigella isolates from humans were reported to the Centers for Disease Control, Atlanta, through a nationwide surveillance system. Over the past 30 years, the reported incidence of shigellosis has been declining in contrast to that reported for salmonellosis. Shigella sonnei (group D) now accounts for ∼70% of the Shigella isolates reported, since most of the decline has been due to the increasingly less frequent infections caused by Shigella flexneri (group B); Shigella dysenteriae infections are now uncommon. The highest reported rates of shigellosis are from the western states and among children from one to five years of age. Among the total population of the United States, the ratio of S. flexneri to S. sonnei isolates was 0.28 among persons from one to 19 years of age and 0.45 among persons ⩾20 years of age; among American Indians the proportion of S. flexneri was much greater, the corresponding ratios being 2.1 and 2.9, respectively; in both groups the ratios for children younger than one year of age resembled those obtaining for adults. Age-related acquisition of immunity to S. sonnei may explain these observations.