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MI4.1-9 | Gastrointestinal & Hepatobiliary Infections — PBL Case

CLINICAL SETTING

It is a Monday morning in a medical college campus in Puducherry. The college health centre is overwhelmed: 47 first-year MBBS students have reported with acute gastrointestinal illness since Sunday evening. The hostel mess served Sunday lunch (vegetable biryani, raita, and fruit custard) and Sunday dinner (dal, rice, and chicken curry) to about 200 students. Most affected students report profuse watery diarrhoea, some with vomiting; five have bloody mucoid stools and one has a temperature of 38.8°C. Three students are severely dehydrated and have been sent to the emergency ward. The college dean has asked you — the public health medicine faculty and a microbiologist — to jointly investigate the outbreak and identify the causative agent.

Trigger 1: Initial Presentation

The health centre records show: 42 students have watery diarrhoea (some profuse, rice-water-like), 5 students have bloody mucoid stools with tenesmus and fever. All cases began 6–18 hours after Sunday lunch. The cafeteria manager reports the raita (yoghurt-based) was made from milk that was kept unrefrigerated for 3 hours before mixing, and the custard was prepared the previous evening and stored at room temperature. The chicken curry was fully cooked but held warm for 5 hours before serving. You collect stool specimens from 10 affected students and arrange to test leftover food samples.

DISCUSSION POINTS

  • Using the clinical presentations (watery diarrhoea in 42 vs. bloody mucoid stools in 5), categorise the type of diarrhoeal illness for each group. What is the likely aetiology for each group and how does the incubation period help differentiate them?
  • Which food items are most likely implicated, and what is the microbiological basis for contamination at room temperature? Apply principles of food poisoning type (intoxication vs. toxico-infection vs. infection) to each suspect food.
  • What stool samples would you collect from each group (volume, container, transport conditions) and which bedside/laboratory tests would you perform first to distinguish watery diarrhoea from dysentery?
Click to reveal Trigger 2: Laboratory Results (discuss previous trigger first!)

Trigger 2: Laboratory Results

Results come in 24–36 hours later. Stool microscopy from the 42 watery-diarrhoea students: no pus cells, no parasites, no trophozoites. Stool culture on TCBS: no growth. MacConkey plates from 5 dysentery students: pale, non-lactose-fermenting colonies; further testing shows non-motile, H₂S-negative organisms that agglutinate with Shigella group B antiserum. Food culture of the custard: Staphylococcus aureus 10⁷ CFU/g; coagulase-positive, phage type 80/81. The raita culture: also S. aureus-positive. The chicken curry culture: negative. However, re-heating the custard and the raita to 80°C before re-culturing shows no bacterial growth. Yet the extracted filtrate of the re-heated custard is inoculated into a cat (in a controlled research model) and produces vomiting within 30 minutes.

DISCUSSION POINTS

  • The MacConkey result and Shigella agglutination confirm bacillary dysentery in 5 students. Explain the pathogenesis of Shigella dysentery from ingestion to clinical presentation, including the role of the Type III secretion system and Shiga toxin in S. dysenteriae type 1.
  • Despite re-heating eliminating all bacteria, the custard filtrate still causes vomiting in the cat model. Explain this finding in terms of toxin biochemistry, and discuss its significance for food outbreak investigation: why 'bacteria-negative food' does NOT mean 'safe food'.
  • What additional tests would you apply to the 42 watery-diarrhoea students to look for a viral aetiology? What specific antigen detection, electron microscopy finding, or molecular test would confirm your hypothesis?
Click to reveal Trigger 3: Diagnosis and Management (discuss previous trigger first!)

Trigger 3: Diagnosis and Management

You consolidate the findings: the custard and raita were the vehicles for S. aureus intoxication (explaining the rapid-onset watery diarrhoea and vomiting in 42 students). ELISA on stool concentrates of the 42 students detects Rotavirus antigen in 18; the remaining 24 are attributed to S. aureus enterotoxin. The 5 students with bloody mucoid stools are confirmed Shigella flexneri infections. Two of the three severely dehydrated patients are stabilised with IV Ringer's lactate; one requires ICU observation. The dean now asks for a report covering: case management, notification to the health authorities, and a specific set of preventive recommendations to prevent future outbreaks.

DISCUSSION POINTS

  • Outline the immediate management of: (a) S. aureus food poisoning (supportive only; no antibiotics — why?), (b) confirmed Shigella dysentery in a student (antibiotic of choice in 2024, given rising fluoroquinolone resistance in India), and (c) severe dehydration from watery diarrhoea (ORS composition and IV fluid of choice).
  • This is a notifiable disease outbreak. Describe the steps of outbreak investigation (describe the epidemic curve, calculate attack rate, identify the vehicle, and notify which authority in Tamil Nadu/Puducherry). What is the role of the microbiologist in an outbreak investigation team?
  • Draft five specific preventive recommendations for the college mess covering: food storage temperature, refrigeration of dairy products, food handlers' hygiene (including nasal swab screening), surface cleaning of kitchen equipment, and early case reporting from the hostel to the health centre.

Group Task Assignments

Group 1: Diarrhoea and Dysentery — Aetiology, Pathogenesis and Laboratory Diagnosis

  • Prepare a comparative table of all bacterial, viral, and parasitic agents of acute diarrhoea encountered in this case and in the SDL, covering: incubation period, mechanism (secretory/invasive/cytotoxic), stool characteristics, key diagnostic test, and antibiotic relevance.
  • Demonstrate stool wet mount preparation using the SDL microscopy technique, identifying PMNs, RBCs, and any trophozoites/cysts present.

Competencies: MI4.1, MI4.2, MI4.3, MI4.4

Group 2: Food Poisoning — Toxin Biochemistry and Outbreak Investigation

  • Compare S. aureus enterotoxin, B. cereus cereulide, and C. perfringens CPE in terms of heat stability, target receptor, incubation period, and the principle 'bacteria-negative does not mean toxin-negative'. Present as a clinical decision chart for the outbreak investigation officer.
  • Outline the steps of a formal outbreak investigation, including case definition, attack rate calculation, Epidemic curve construction, and notification pathway under India's IDSP.

Competencies: MI4.1, MI4.5

Group 3: H. pylori and Viral Hepatitis — Integration with Cluster Theme

  • One month after the outbreak, a follow-up survey finds 8 students have dyspepsia. HPYLORI stool antigen testing is positive in 3. Prepare a brief diagnostic algorithm for H. pylori, comparing invasive (CLO, culture, histology) vs. non-invasive (UBT, stool antigen, serology) methods with their best clinical indications.
  • Three of the 47 outbreak students are now found to have elevated LFTs at follow-up. Construct a serology interpretation table for all five hepatitis viruses, detailing: transmission route, acuity/chronicity, key diagnostic markers, and prevention strategy — as a reference card for the health centre.

Competencies: MI4.6, MI4.7, MI4.8, MI4.9

Learning Issues

Research these questions and bring your findings to the discussion.

  1. [MI4.1] What are the precise definitions of diarrhoea, dysentery, and food poisoning, and which organisms span more than one category?
  2. [MI4.2] What makes Vibrio cholerae a purely secretory pathogen without mucosal invasion, and how does this differ from E. coli ETEC vs. EIEC vs. EHEC?
  3. [MI4.3] Why does stool microscopy in bacillary dysentery show sheets of PMNs, while amoebic dysentery shows fewer PMNs despite similar bloody mucoid stools?
  4. [MI4.4] What is the correct procedure for a stool wet mount to identify trophozoites vs. cysts, and what preparation-to-examination time limit applies?
  5. [MI4.5] How does the heat stability of S. aureus enterotoxin explain why re-heating contaminated food does not prevent intoxication, and what are the practical food-safety implications?
  6. [MI4.7] Why does HEV genotype 1 cause disproportionately high mortality in third-trimester pregnancy compared to all other viral hepatitides?
  7. [MI4.8] What is the serological window period of HBV, which marker bridges it, and how does this affect blood bank screening practices?
  8. [MI4.9] How would you interpret: HBsAg positive, HBeAg negative, anti-HBe positive, HBV DNA 500,000 IU/mL — and how does this differ from the inactive carrier state?