Shigellosis

Shigellosis is caused by Shigella species (S. sonnei, S. flexneri, S. dysenteriae, and S. boydii), gram negative, non-motile bacteria with a notably low infectious dose (as few as 10 to 100 organisms), facilitating efficient person-to-person and food/waterborne transmission.

Shigella dysenteriae type 1 produces Shiga toxin and causes the most severe disease, including a recognized risk of hemolytic uremic syndrome.

Classically presents with fever, severe abdominal cramping, and frequent, small-volume, bloody, mucoid stools with tenesmus (bacillary dysentery), reflecting invasion and inflammation of the colonic and rectal mucosa, though a proportion of cases present with watery diarrhea only, particularly early in the illness or with less virulent strains.

Seizures occur disproportionately in young children with shigellosis, even without marked fever, through a mechanism not fully understood.

Reactive arthritis is a recognized post-infectious complication, particularly in HLA-B27 positive individuals.

Stool culture confirms diagnosis and allows susceptibility testing, increasingly important given rising and regionally variable antimicrobial resistance; stool multiplex PCR offers a faster alternative where available.

FBC may show leukocytosis with a striking left shift (bandemia), sometimes disproportionate to the total white cell count.

• Antibiotic treatment is generally recommended for confirmed or strongly suspected shigellosis (unlike most other bacterial gastroenteritides), given evidence of shortened illness duration and reduced transmission, and given the low infectious dose facilitating spread
• Ciprofloxacin 500 mg PO bd for 3 days is a standard first-line regimen where local resistance allows; azithromycin 500 mg PO od for 3 days is an alternative, increasingly preferred in some settings given rising fluoroquinolone resistance
• Ceftriaxone (IV) is used for severe illness or where oral therapy is not tolerated
• Avoid antimotility agents (loperamide), given a theoretical risk of prolonging toxin exposure and precipitating toxic megacolon in this invasive, inflammatory diarrheal illness
• Supportive rehydration as per general gastroenteritis principles
• Strict hand hygiene and, where relevant, exclusion from food handling/childcare settings until symptom-free, given the very low infectious dose and high transmissibility

Referral: infectious disease for severe disease, treatment failure, or suspected drug-resistant infection.