Shigella

Kiyoshi Shiga first isolated Shigella dysenteriae type 1 during a severe dysentery epidemic in Japan in 1896, when more than 90000 cases were described with a mortality rate approaching 30%.18 Shigellae are Gram-negative, non-lactose fermenting, non-motile bacilli, with S. sonnei the main type found in industrialized countries, and S. flexneri and S. dysenteriae predominating in underdeveloped countries. Humans are the only natural hosts and transmission occurs by fecal-oral contact. The low infectious inoculum (as few as ten organisms)19 renders shigellae highly contagious. Shigella causes 250 million cases of diarrhea and 650 000 deaths worldwide.20 In the USA, S. dysenteriae infection is seen almost exclusively among travelers. After an incubation of 1-4 days, shigellosis begins with fever, headache, malaise, anorexia and occasional vomiting and watery diarrhea with progression to dysentery within hours to days. Unusual extraintestinal manifestations may occur, including hemolytic uremic syndrome (HUS) in children and throm-botic thrombocytopenic purpura in adults. Most episodes of shigellosis in otherwise healthy individuals resolve within 7 days. Shigellae are extremely fastidious and are best isolated from fresh stool rapidly inoculated onto selective culture plates incubated immediately at 37 oC. Appropriate antibiotics (ampicillin or TMP-SMX) given for 5 days significantly decrease the duration of fever, diarrhea, intestinal protein loss and pathogen excretion. However, Shigella strains that are resistant to one or both drugs have been identified. Several promising Shigella mutants with deletions in virulence genes have entered clinical trials as oral vaccine candidates.

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