Shigella

ORGANISM:

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.