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Shigella


ORGANISM:

  • Genus: Shigella
  • Species: dysenteriae


GENERAL CONCEPTS:
  • Shigella dysenteriae is responsible for bacillary dysentery, a disease most often associated with crowded, unsanitary conditions.
  • Other species of Shigella may produce milder forms of diarrheal disease.


DISTINCTIVE PROPERTIES:
  • Shigellae are facultative, non-motile, Gram-negative bacilli. They possess the heat stable endotoxin (LPS) characteristic of Gram-negative bacteria.
  • Shigellae are pathogenic primarily due to their ability to invade intestinal epithelial cells.
  • S. dysenteriae also produces a heat labile exotoxin that is a neurotoxin acting upon the gray matter of the central nervous system.


PATHOGENESIS:
  • Dysentery is an oral infection transmitted via fecal contamination of water or food. During the 1-4 day incubation period, penetration of bacteria into the mucosal epithelial cells of the intestine causes an intense irritation of the intestinal wall, producing cramps and a watery, bloody diarrhea.


HOST DEFENSES:
  • Unlike the salmonellae, shigellae are acid tolerant. As a consequence, gastric acidity provides little protection against infection.
  • Protective defenses include the normal flora, secretory IgA and phagocytosis.


EPIDEMIOLOGY:
  • Dysentery and other shigelloses occur worldwide but the hosts are limited to humans and primates.
  • Because of their acid tolerance, relatively few organisms are required to produce disease. Indeed, as few as 10 cells can cause disease in 10% of healthy persons; 200 cells may cause disease in 40% of persons.
  • Contaminated food and water are the primary sources for contracting dysentery, but person to person transmission may occur because of the low dose required to produce disease.
  • About 60% of cases occur in children aged 1-10 years.


DIAGNOSIS:

  • Clinical: As with other diarrheal diseases, clinical diagnosis alone is equivocal. Diarrhea, fever and a watery bright red blood tinged stool are classical symptoms, but isolation of the organisms is required for confirmation.
  • Laboratory: Shigella can be readily isolated and characterized using standard bacteriologic media or rapid identification systems. Shigellae are non-motile, incapable of fermenting lactose and do not produce H2S. Serological techniques may be used for epidemiological characterization.


CONTROL:

  • Sanitary: As with other fecal-oral diseases, dysentery is best prevented by assuring a safe water supply and proper disposal of feces.
  • Immunological: No vaccines are currently available.
  • Chemotherapeutic: The use of antibiotics is debatable because the disease is self-limiting. If required, ampicillin or a trimethoprim-sulphamethoxizole combination may be employed. Replacing lost fluids to prevent dehydration is most important for treating the disease.

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