zoonoses and communicable diseases common to ... - PAHO/WHO

SHIGELLOSIS 247SHIGELLOSISICD-10 A03.0 shigellosis due to Shigella dysenteriae;A03.1 shigellosis due to Shigella flexneri;A03.2 shigellosis due to Shigella boydii;A03.3 shigellosis due to Shigella sonnei; A03.8 other shigellosisSynonyms: Bacillary dysentery.Etiology: The genus Shigella belongs to the family Enterobacteriaceae. Shigellaeare small gram-negative, nonmotile, unencapsulated bacilli; they are anaerogenic(with a few exceptions) and non-lactose fermenting (or slow fermenters).The genus Shigella may be considered genetically as a single species, closelyrelated in DNA analyses to E. coli. However, it is divided into four species based onphenotype traits. Each species is a distinct serogroup: Shigella dysenteriae(serogroup A), S. flexneri (serogroup B), S. boydii (serogroup C), and S. sonnei(serogroup D). The four serogroups contain a total of 38 serotypes. Serotyping isimportant in epidemiological investigation. Diagnostic laboratories are generallylimited to identifying the serogroup and sending cultures to a reference laboratoryfor identification of the serotype.The primary virulence factor of a Shigella strain is its ability to invade cells of theintestinal mucosa. The invasive capacity depends on factors controlled by both chromosomicand plasmidic genes (Keusch and Bennish, 1991).The invasive capacity of a strain can be demonstrated with the Sereny test, whichconsists of inoculating a culture in a guinea pig’s conjunctival sac. Invasive strainsproduce keratoconjunctivitis in 24 to 48 hours. Cultures obtained from clinical casesalways yield a positive Sereny test result. Shigellae also produce cytotoxins, particularlyin the case of S. dysenteriae serotype 1 (Shiga toxin).Geographic Distribution: Worldwide.Occurrence in Man: Shigellosis can be either epidemic or endemic. Epidemic orpandemic shigellosis is usually caused by S. dysenteriae serotype 1 (Shiga bacillus),the most virulent and toxigenic strain. In 1969–1970, a widespread epidemic causedby S. dysenteriae serotype 1 occurred in Central America and Mexico, with highmorbidity and mortality rates, particularly in children. More than 13,000 patientsdied as a result. The infection was introduced in the US, where 140 cases occurredfrom 1970 to 1972. The epidemic spread to Central Africa and Asia (India,Bangladesh, and Sri Lanka). Plasmid analysis demonstrated that the pandemic wasnot produced by a single bacterial clone. Thus, it is difficult to explain the appearanceof the disease in such distant areas. The strains isolated in all areas proved tobe multiresistant to antibiotics (WHO, 1987). In Guatemala, there were 112,000cases with 10,000 deaths from 1969 to 1972. A new outbreak appeared in Guatemalain 1991 caused by S. dysenteriae serotype 1; it affected 540 people in the course ofone month, both in Guatemala City and in Verapaz, a city of 10,000 inhabitants(CDC, 1991).Endemic shigellosis is usually caused by S. flexneri and S. sonnei. The first occursprimarily in developing countries, the second, in economically advanced countries.Morbidity and mortality rates are high in developing countries, particularly in chil-