Question
Shigellosis.
Answer
Shigellosis: an acute bacterial dysenteric illness caused by Shigella species — Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei (of increasing prevalence order in different regions; S. dysenteriae type 1 causes the most severe disease, producing Shiga toxin).
Transmission: faeco-oral route, notably via a very low infective dose (as few as 10–100 organisms), facilitating easy person-to-person and food/water-borne spread.
Pathogenesis: Shigella invades the colonic mucosal epithelium (via M cells and subsequent lateral cell-to-cell spread), causing epithelial cell death, mucosal ulceration, and an intense inflammatory response — accounting for the characteristic dysenteric stool (blood, mucus, pus). S. dysenteriae type 1 additionally produces Shiga toxin, which inhibits host protein synthesis and can cause severe complications including Haemolytic Uraemic Syndrome (HUS).
Clinical features: abrupt onset of fever, abdominal cramps, and frequent, small-volume, bloody, mucoid stools (dysentery); can be complicated by toxic megacolon, seizures (particularly in children), reactive arthritis, and HUS (with S. dysenteriae type 1).
Laboratory diagnosis:
- Stool microscopy — numerous pus cells (neutrophils) and red blood cells.
- Stool culture — on selective media (e.g., XLD, MacConkey, Hektoen enteric agar), showing non-lactose-fermenting colonies; biochemical and serological confirmation identifies the specific Shigella species/serotype.
- Antibiotic susceptibility testing — important given increasing antimicrobial resistance in Shigella.
Treatment: antibiotics (e.g., Ciprofloxacin, Azithromycin, guided by local resistance patterns) are used for moderate-to-severe disease, along with oral/IV rehydration; antimotility agents are avoided given the risk of prolonging toxin exposure/complications.