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Shigella

Shigella species are a common cause of bacterial diarrhea worldwide. Transmission occurs through direct contact with an infected person or contaminated food or water.

Ingestion of as few as 10 to 100 organisms has been shown to cause disease in volunteers. Shigella are relatively resistant to killing by stomach acid. Thus, ingested bacteria pass into the small intestine where they multiply. In the colonic mucosa, Shigella invades colonic enterocytes, disrupting their normal function and causing inflammation. Symptoms of shigellosis include diarrhea, fever, nausea, vomiting and cramps.

In the United States, where sewage disposal is usually adequate, most cases are probably transmitted by the fecal-oral route from people with symptomatic or recently symptomatic shigellosis. Thus, outbreaks of shigellosis in the United States occur predominantly in institutions such as day care centers or custodial institutions and less commonly by contamination of food or drinking water. In the developing world where sewage disposal is suboptimal or nonexistent, both fecal-oral spread and contamination of common food and water supplies are important mechanisms of transmission.

Shigella are nonmotile, facultatively anaerobic, gram negative rods. There are four species of Shigella:

  • S. dysenteriae (serogroup A)
  • S. flexneri (serogroup B)
  • S. boydii (serogroup C)
  • S. sonnei (serogroup D)

Most cases of shigellosis in the United States are caused by S. sonnei (>75 percent), with S. flexneri the next most frequent isolate. S. dysenteriae was the most common isolate both in Europe and the United States at the beginning of this century but is now rare. In the United States, S. dysenteriae infection is generally limited to imported cases from Mexico and Central America or from laboratory contamination.

The spectrum of disease severity varies according to the serogroup of the infecting organism. S. sonnei commonly causes mild disease, which may be limited to watery diarrhea, while S. dysenteriae 1 or S. flexneri commonly causes bloody diarrhea. In a normal healthy host, the course of disease is generally self-limited, lasting no more than seven days when left untreated. The incubation period ranges from one to seven days, with an average of three days.

Shigellosis is diagnosed by stool culture, which should be performed in all suspected cases prior to therapy. Susceptibility testing is routinely performed on all Shigella isolates. The Health Department recommends treatment with azithromycin or ciprofloxacin due to the frequent resistance of Shigella to ampicillin and trimethoprim/ sulfamethoxazole. Shigellosis is a reportable disease that should be communicated to the local health department within 24 hours of diagnosis.

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