What is the pathophysiology of Shigella infection?

Updated: Apr 03, 2018
  • Author: Jaya Sureshbabu, MBBS, MRCPCH(UK), MRCPI(Paeds), MRCPS(Glasg), DCH(Glasg); Chief Editor: Russell W Steele, MD  more...
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Answer

Shigella infection is a major public health problem in developing countries where sanitation is poor. Humans are the natural reservoir, although other primates may be infected. No natural food products harbor endogenous Shigella species, but a wide variety of foods may be contaminated.

Shigellosis is spread by means of fecal-oral transmission. Other modes of transmission include ingestion of contaminated food or water (untreated wading pools, interactive water fountain), contact with a contaminated inanimate object, and certain mode of sexual contact. Vectors like the housefly can spread the disease by physically transporting infected feces.

The infectivity dose (ID) is extremely low. As few as 10 S dysenteriae bacilli can cause clinical disease, whereas 100-200 bacilli are needed for S sonnei or S flexneri infection. The reasons for this low-dose response are not completely clear. One possible explanation is that virulent Shigellae can withstand the low pH of gastric juice. Most isolates of Shigella survive acidic treatment at pH 2.5 for at least 2 h. [2]

The incubation period varies from 12 hours to 7 days but is typically 2-4 days; the incubation period is inversely proportional to the load of ingested bacteria. The disease is communicable as long as an infected person excretes the organism in the stool, which can extend as long as 4 weeks from the onset of illness. Bacterial shedding usually ceases within 4 weeks of the onset of illness; rarely, it can persist for months. Appropriate antimicrobial treatment can reduce the duration of carriage to a few days.

DNA-DNA hybridization studies demonstrate that E.Coli  share more than 75% nucleotide similarity with Shigella species and Shigella species appears to be metabolically inactive biogroups of E.coli. [3]


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