Shigella spp dysentery

Last edited 02/2020 and last reviewed 02/2020

Bacillary dysentery is caused by Shigella, with Shigella sonnei responsible for more than 90% of cases in UK, but causing the mildest form of the condition, S. flexneri, S. boydii and S. dysenteriae causing progressively more severe dysentery.

The inoculating dose is small - about 200 organisms.

The organisms invade the superficial colonic mucosa causing inflammation. Local multiplication and invasion is possible via a virulence factor, together with the production of a toxin. A cholera-like presentation may occur if the disease is due to S. dysenteriae, which produces a cytotoxin, Shiga toxin. The shiga-like toxins are related, but less potent, and are produced by Shigella sp. and also by enterohaemorrhagic E. Coli.

Outbreaks occur in nurseries, mental and geriatric institutions - the latter two are on the decline - underlining the fact that the faecal-oral route is needed. Some 50% of cases occur in children less than 10 years old.



  • 4 species of shigella: Shigella sonnei, Shigella flexneri, Shigella boydii, Shigella dysenteriae


  • Humans


  • infections peak in late summer in the UK
  • highest rates of infection occur in children aged < 5 years, followed by 5-14 year age group
  • S. sonnei is the most common species in Western Europe and both S. sonnei and S. flexneri are endemic in UK
  • most cases of S. boydii and S. dysenteriae are imported but all strains may be travel-associated


  • faeco-oral transmission directly amongst households, nursery and infant schools is most common. Foodborne infections occur but are rare
  • direct transmission between men-who-have-sex-with-men (MSM) is also an important transmission route
  • environmental contamination during episodes of acute diarrhoea can occur, where bacilli may be aerosolised during toilet flushing and settle on surrounding surfaces and survive for weeks in cool and humid locations

Incubation period:

  • 12 hours – 4 days (usually 1-3 days) but up to 1 week for S. dysenteriae

Common clinical features:

  • clinical features vary depending on Shigella species
    • S. sonnei causes mild illness in most cases with symptoms of diarrhoea (may be bloody in 10-50%) and abdominal pain with/without nausea, vomiting, headache and malaise lasting average of 4-5 days (range 1 day – 2 weeks)
    • S. flexneri causes similar symptoms to S. sonnei but illness may be more severe with dysentery more prominent, longer duration of illness and hospitalisation rates higher
    • complications include reactive arthritis and Reiter’s syndrome
    • S. boydii causes diarrhoeal illness like that of S. flexneri
    • S. dysenteriae type 1 infection causes more severe illness, with dysentery in most cases and complications including haemolytic uraemic syndrome (HUS)


  • cases are most infectious when diarrhoea is present but considered infectious as long as organisms are excreted in stool (average of 2- 4 weeks but prolonged carriage of several months has been reported)


  • PHE (2019). Recommendations for the Public Health Management of Gastrointestinal Infections