Infectious Disease Online
Pathology of Shigellosis
wikimedia-commons: This photomicrograph revealed stool exudates in a patient with shigellosis (Centers for Disease Control and Prevention Publich Health Image Library)
Members of the genus Shigella cause a readily communicable infectious colitis, bacillary dysentery.
Bacillary dysentery has historically affected troops in the field, prisoners of war, victims of natural disaster and those living in unsanitary overcrowded conditions.
The disease was distinguished from amebic dysentery in 1896 by Shiga who recognized the first representative of the genus now known as Shigella dysenteriae.
The four species of Shigella ( S. dysenteriae, S. flexneri, S. boydii and S. sonnei ) are gram negative bacilli. These are among the most virulent enteropathogens.
Disease is produced by ingestion of as few as 10 to 100 organisms, and there are few asymptomatic carriers.
The shigellae are worldwide and are most conspicuous in tropical and developing countries, where they are a major cause of morbidity and mortality. In the developed countries, S. flexneri and S. sonnei are more common, and infection is sporadic.
The lesions, limited to the colon, are destructive as evidenced by the bloody mucoid stools characteristic of shigellosis.
Unlike the salmonella infection which also invade and colonize the intestinal mucosa, shigellae have no significant animal reservoir.
Shiga toxin contributes to the profuse diarrhea and precedes dysentery in some patients.
This enterotoxin activates membrane-associated adenyl cyclase. Thus shiga toxin like cholera toxina and and E. coli enterotoxin induces hypersecretion of fluid and electrolytes from mucosa of the terminal ileum.
Water and electrolyte balance must be maintained to prevent dehydration, prostration and impaired mental status.
Mode of transmission: Transmission occurs by the fecal-oral route.
Clinical presentation: Symptoms appear 2 to 5 days after the ingestion of bacteria. The dose of organisms and the status of host defenses influence the incubation period and severity.
Shigella infection ranges from mild diarrhea to incapacitating and life-threatening dysentery, the latter caused primarily by S. dysenteriae.
Diffuse involvement of the colon is associated with high fever, shaking chills, toxemia and shock.
Appropriate antibiotic therapy is critical in shortening the illness, preventing relapse and reducing transmission.
The key to the pathogenecity of Shigella is its ability to invade and multiply in the epithelium and lamina propria of the terminal ileum and colon.
The mucosa becomes edematous and hyperemic and is covered by pus and mucus.
The mucosa becomes soft and friable and irregular superficial ulcerations appears.
The ulcerated mucosa becomes covered with a granular, dirty-yellow pseudomembrane.
Sloughed pseudomembrane, together with blood tinged mucus comprise the characteristic dysenteric stool of shigellosis.
There is a predominantly mononuclear leukocytic infiltrate within the lamina propria.
The surfaces of the ulcers are covered with an acute, suppurative, neutrophilic reaction accompanied by congestion, marked edema, fibrin deposition, and thromboses of small vessels.
The epithelium persists only in the depths of the crypts and goblet cells contain no mucus in the acute stage.
As the disease progresses, the ulcer margins are transformed into active granulation tissue.
When the disease remits, the granulation tissue fills the defect and the ulcers heal by regeneration of the mucosal epithelium.
Epithelial regeneration is rapid and healing is complete in 2 weeks.
Investigations: Detection methods for Shigella include conventional culture methods, immunological methods, and molecular microbiological methods.
Conventional culture of Shigella in foods is often problematic due to the lack of appropriate selective media.
Immunological methods for Shigella have been researched, yet there is only one commercially available test kit.
Molecular microbiological methods such as PCR, oligonucleotide microarrays, and rep-PCR have also been developed for the detection and identification of Shigella.
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