Shigellosis and Bacillary Dysentery
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Direct answer
Shigellosis is bacillary dysentery caused by Shigella species, which invade the colonic mucosa after an infective dose of as few as 10 to 100 organisms. Shiga toxin, produced mainly by Shigella dysenteriae type 1, can cause haemolytic uraemic syndrome, while bloody, mucoid diarrhoea with tenesmus is the typical picture. Antibiotics shorten the illness, but resistance is widespread, so the choice follows local sensitivity data.
What you must remember
- Low inoculum: as few as 10 to 100 organisms cause infection, which explains rapid spread in crowded settings with poor sanitation.
- Invasion: Shigella invades colonic epithelium, causing ulceration, bloody and mucoid stools, tenesmus and abdominal cramps.
- Shiga toxin: produced by Shigella dysenteriae type 1, it damages endothelium and can cause haemolytic uraemic syndrome in children.
- Species pattern: S. flexneri is the usual cause in India and other developing settings, while S. sonnei is more common in higher-income countries.
- Diagnosis: stool culture on selective media such as MacConkey or XLD, with non-lactose-fermenting colonies, and microscopy showing many polymorphs.
- Antibiotics: shorten illness and reduce shedding, with azithromycin often preferred where fluoroquinolone resistance is high.
- Supportive care: oral rehydration and zinc in children, with antimotility drugs avoided in dysentery.
A bedside route for bloody diarrhoea
Shigellosis is managed by matching the severity of the illness to the stool findings and the risk of complications. The steps below set out the order in which decisions are normally made.
- Assess severity: dehydration, high fever, toxicity and signs such as lethargy or seizures in a child decide whether admission is needed.
- Test the stool: send stool for culture on selective media and microscopy, where many polymorphs and visible blood point to invasive disease.
- Start rehydration: give oral rehydration solution, or intravenous fluids when needed, with zinc supplementation in children.
- Choose the antibiotic: pick azithromycin or a fluoroquinolone according to local sensitivity, since resistance to older agents is widespread.
- Avoid antimotility drugs: loperamide and similar agents can prolong illness in bloody or febrile diarrhoea, so they are withheld.
- Watch for HUS: falling urine output, haemolytic anaemia and low platelets signal haemolytic uraemic syndrome, which is most important after Shigella dysenteriae type 1 infection in children.
Where the exam tries to catch you
Examiners test the low infective dose, the toxin complication and the reasons for withholding antimotility drugs. The traps below are common errors in this area.
- Trap: shigellosis needs a large inoculum to cause disease Correct: Shigella causes infection with very few organisms, which is why it spreads easily in crowded and poorly sanitised settings.
- Trap: loperamide relieves symptoms in dysentery Correct: antimotility drugs are avoided in bloody diarrhoea with fever, since they can trap organisms and toxins in the colon.
- Trap: S. sonnei is the commonest cause in India Correct: S. flexneri is the usual cause in India and other developing settings, while S. sonnei is more common in higher-income countries.
Frequently asked questions
Which Shigella species causes haemolytic uraemic syndrome?
Shigella dysenteriae type 1 causes haemolytic uraemic syndrome through Shiga toxin, which damages the endothelium in the kidneys. It is most important in children with bloody diarrhoea and falling urine output.
Why does shigellosis spread so easily between people?
Shigellosis spreads easily because Shigella causes infection with a very low inoculum, as few as 10 to 100 organisms. Faecal contamination of hands, food and water in crowded settings then allows rapid transmission.
Which antibiotic is preferred for shigellosis when resistance is high?
Azithromycin is often preferred for shigellosis where fluoroquinolone resistance is high, and the final choice follows local sensitivity data. Antibiotics shorten the illness and reduce faecal shedding, while oral rehydration remains essential.
Why are antimotility drugs avoided in shigellosis?
Antimotility drugs such as loperamide are avoided in shigellosis with bloody or febrile diarrhoea, because they slow the clearance of organisms and toxins from the colon. Oral rehydration and antibiotics are the safer approach in dysentery.
What is the typical stool finding in bacillary dysentery?
Bacillary dysentery typically produces small volumes of blood and mucus in the stool, with tenesmus and abdominal cramps. Microscopy shows many polymorphonuclear cells, which supports an invasive colonic infection.