Shigella | Medical Science Optional Notes for UPSC PDF Download

Epidemiology of Shigella

Shigella, a gram-negative bacterium that does not form spores and lacks motility, is also non-lactose fermenting.

Classification

• Shigella dysenteriae (Serogroup A) - Shiga's bacillus
• Shigella flexneri (Serogroup B) - Flexner's Bacillus / Strong's Bacillus
• Shigella boydii (Serogroup C) - Neucastle-Manchester Bacillus
• Shigella sonnei (Serogroup D) - Duval's bacillus

Among the major disease-causing species, S. flexneri is the most commonly isolated worldwide, constituting 60% of cases in the developing world. S. sonnei causes 77% of cases in the developed world and only 15% in the developing world. S. dysenteriae is typically responsible for dysentery epidemics, especially in confined populations like refugee camps. S. dysenteriae type 1, marked by the presence of Shiga toxin-encoding genes, is the most severe and often manifests in epidemic form.

Shigella primarily resides in the human intestinal tract. Transmission occurs through the fecal-oral route, flies, and in some cases, sexually (as seen in gay bowel syndrome). Shigellosis, particularly S. sonnei infection, may manifest as food poisoning. Epidemics are associated with poor hygiene. Shigella is a leading pathogen linked to moderate to severe diarrhea, ranking first among children aged 12-59 months.

In India, shigella dysenteriae type I is the most common cause of hemolytic uremic syndrome, while in the West, E. coli O157: H7 takes precedence. Breast milk, containing the Bifidus factor, offers protection against Salmonella, Shigella, and E. coli. Shigella colonies on MacConkey agar remain colorless due to the absence of lactose fermentation. Shigella sonnei is an exception, fermenting lactose late and forming pale pink colonies.

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Pathogenesis of shigella

Lab Diagnosis of Shigella

• Exclusive to humans, this pathogen relies solely on them as its natural hosts.
• The primary mode of transmission is oral, often occurring through the consumption of contaminated food or water.
• The highest incidence of this condition is observed in regions of South Central and Southeast Asia.
• Urban areas are more prone to its occurrence compared to rural settings.
• Significant risk factors include poor sanitation and a lack of access to clean drinking water.
• Other risk factors encompass:
  (a) Reduced stomach acidity, particularly in individuals under one year of age or those with conditions leading to decreased acidity such as antacid ingestion or achlorhydria.
  (b) Compromised intestinal integrity, evident in cases of inflammatory bowel disease (IBD), prior gastrointestinal surgery, or previous antibiotic therapy.
• Up to 10% of cases may transition to becoming carriers, often referred to as "Typhoid Mary," with the gall bladder serving as the storage site.

Pathogenicity of Salmonella

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Lab Diagnosis of Salmonella

Widal Test

• Employing a tube agglutination approach, the Widal test detects H and O antibodies.
• The O agglutination manifests as dense, granular, chalky clumps.
• Conversely, H agglutination presents as fluffy, cotton-woolly clumps.
• The test is considered significant when the H titre exceeds 200, and the O titre exceeds 100.
• Initially, two types of tubes were utilized in the Widal Test: (1) Dreyer's tube, characterized by a narrow tube with a conical bottom for H agglutination, and (2) Felix tube, featuring a short, round-bottomed tube for O agglutination.
• False positives may arise due to an anamnestic response or prior immunization.
• False negatives can be observed in the early or late stages of illness or in patients receiving antibiotic treatment.

Typhoid Fever - Repeats

• Identify bacterial diseases transmitted through contaminated water. Explain the laboratory methods for diagnosing typhoid fever and outline precautions necessary for interpreting laboratory results (1994).
• List the complications associated with typhoid fever and provide details on the dosage and duration of antimicrobial treatments used for typhoid (2004).
• Explore the pathogenesis of enteric fever. Examine the various mechanisms by which Salmonella develops antimicrobial resistance (2017).

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Shigella - Repeats

• Identify the SHIGELLA species responsible for causing diseases in humans. Describe the pathogenesis of bacillary dysentery. Explain the laboratory procedures used in the diagnosis of bacillary dysentery. Highlight the distinctions in laboratory findings between bacillary dysentery and Amoebic dysentery (2010).
• Discuss the etiology of bacillary dysentery and elaborate on its laboratory diagnosis (2014).
• Examine the epidemiology, pathogenesis, and laboratory diagnosis of a patient presenting with bacillary dysentery (2018).