Model Answer
0 min readIntroduction
Bacillary dysentery, also known as shigellosis, is an acute inflammatory disease of the intestine caused by bacteria of the genus *Shigella*. It is a significant public health concern, particularly in developing countries, characterized by diarrhea, fever, and abdominal cramps. The disease is highly contagious and spreads easily through the fecal-oral route. Understanding the etiology and employing accurate laboratory diagnosis are crucial for effective management and control of shigellosis outbreaks. Globally, it is estimated that *Shigella* species cause approximately 85 million cases of dysentery each year, leading to over 70,000 deaths, predominantly in children under five years of age (WHO, 2023 - knowledge cutoff).
Etiology of Bacillary Dysentery
Bacillary dysentery is caused by bacteria belonging to the genus *Shigella*. There are four species of *Shigella* that cause disease in humans:
- Shigella dysenteriae: Causes the most severe form of shigellosis, often associated with outbreaks and dysentery with bloody stools. Produces Shiga toxin.
- Shigella flexneri: The most common species globally, causing a milder form of dysentery.
- Shigella boydii: Intermediate in severity between *S. flexneri* and *S. dysenteriae*.
- Shigella sonnei: Common in industrialized countries, typically causing a milder, non-bloody diarrhea.
Transmission and Pathogenesis
The primary mode of transmission is the fecal-oral route. This can occur through:
- Contaminated water and food
- Direct person-to-person contact
- Flies acting as mechanical vectors
The pathogenesis of shigellosis involves several steps:
- Adherence: *Shigella* bacteria adhere to the epithelial cells of the colon.
- Invasion: They invade these cells, multiply intracellularly, and spread to adjacent cells.
- Inflammation: This invasion triggers an inflammatory response, leading to ulceration and the characteristic symptoms of dysentery.
- Toxin Production: Some *Shigella* species, particularly *S. dysenteriae*, produce toxins (like Shiga toxin) that contribute to the severity of the disease. Shiga toxin inhibits protein synthesis and can cause hemolytic uremic syndrome (HUS).
Laboratory Diagnosis of Bacillary Dysentery
1. Specimen Collection
Appropriate specimen collection is crucial for accurate diagnosis. The following specimens are typically collected:
- Stool: The primary specimen for diagnosis. Multiple samples (2-3) collected over 3-5 days increase sensitivity.
- Rectal Swab: Useful, especially in young children or when stool samples are difficult to obtain.
- Sigmoidoscopy/Colonoscopy Biopsies: May be collected in severe or prolonged cases.
2. Microscopy
Microscopic examination of stool samples can provide a preliminary indication of infection:
- Direct Smear: May reveal the presence of polymorphonuclear leukocytes (PMNs) indicating inflammation. *Shigella* bacteria themselves are difficult to visualize directly.
- Staining: Gram staining can show Gram-negative bacilli, but is not specific for *Shigella*.
3. Culture
Culture remains the gold standard for diagnosing shigellosis.
- Selective Media: Stool samples are inoculated onto selective media such as MacConkey agar with crystal violet, Xylose Lysine Deoxycholate (XLD) agar, and Hektoen Enteric (HE) agar. These media inhibit the growth of other intestinal flora while allowing *Shigella* to grow.
- Incubation: Plates are incubated at 37°C for 18-24 hours.
- Colony Morphology: *Shigella* colonies are typically colorless or pale on MacConkey agar and produce hydrogen sulfide (H2S) on XLD agar (black colonies).
4. Biochemical Tests
Once *Shigella* colonies are isolated, biochemical tests are performed for species identification:
| Test | S. dysenteriae | S. flexneri | S. boydii | S. sonnei |
|---|---|---|---|---|
| Lactose Fermentation | - | - | - | ± |
| Sucrose Fermentation | - | - | + | + |
| Urease | + | - | - | - |
| Indole Production | + | + | + | - |
5. Serology
Serological tests (agglutination, ELISA) can detect antibodies against *Shigella* antigens. However, serology is less reliable than culture due to cross-reactivity with other enteric bacteria and delayed antibody response.
6. Molecular Methods
Molecular methods are increasingly used for rapid and accurate diagnosis:
- PCR (Polymerase Chain Reaction): Detects *Shigella*-specific genes (e.g., *ipaH*, *virG*). Highly sensitive and specific.
- Real-time PCR: Allows for quantification of *Shigella* DNA.
- Whole Genome Sequencing (WGS): Provides detailed information about the *Shigella* strain, useful for outbreak investigations and antimicrobial resistance profiling.
Conclusion
Bacillary dysentery remains a significant global health challenge. Accurate diagnosis, relying on a combination of traditional culture methods and modern molecular techniques, is essential for effective treatment and prevention. Rapid diagnostic tests, like PCR, are becoming increasingly important for timely intervention, particularly in outbreak settings. Continued surveillance and improved sanitation practices are crucial for controlling the spread of *Shigella* infections and reducing morbidity and mortality.
Answer Length
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