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Amaltas Institute - Community Medicine 2nd Sessional Exam - ANSWERS

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SECTION A - LONG ANSWER QUESTIONS

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Q.1(a) Epidemiological Determinants of Typhoid Fever (5 marks)

Agent Factors

• Agent: Salmonella typhi - the major cause of enteric fever. Has three main antigens: O (somatic), H (flagellar), and Vi (virulence). At least 80 phage types known - phage typing is an important epidemiological tool to trace source of epidemics.
• Reservoir of infection: Man is the ONLY known reservoir - both cases and carriers.
  • Cases: May be mild, missed or severe. Infectious as long as bacilli appear in stool/urine.
  • Carriers: (i) Temporary carriers - incubatory or convalescent (excrete for 6-8 weeks); (ii) Chronic carriers - excrete bacilli for >1 year, often housed in the gall bladder/biliary tract. Carrier rate ~3% at 1 year. Famous example: "Typhoid Mary" caused >1300 cases.
• Source of infection: Primary sources = faeces and urine of cases/carriers. Secondary sources = contaminated water, food, fingers, flies.

Host Factors

• Age: Highest incidence in 5-19 years. Incidence falls after age 20 due to acquired immunity.
• Sex: More cases in males (greater exposure); but carrier rate is higher in females.
• Immunity: All ages are susceptible. Cell-mediated immunity (CMI) plays a major role as S. typhi is intracellular. O-antibody is higher in active disease; H-antibody is higher after immunization.
• Gastric acidity and local intestinal immunity are important host defences.

Environmental & Social Factors

• Peak incidence: July-September (rainy season + increased fly population)
• S. typhi survives in water for ~7 days, in ice for >1 month, in soil for up to 70 days.
• Bacteria multiply in milk rapidly without altering its taste or appearance.
• Risk factors: open-air defecation, contaminated water supplies, poor food hygiene, low socioeconomic status.
• Incubation period: usually 10-14 days (range 3 days to 3 weeks, dose-dependent).
• Mode of transmission: faecal-oral or urine-oral route - through contaminated water, food, milk, or flies.

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Q.1(b) Laboratory Diagnosis and Control of Yellow Fever (10 marks)

Yellow Fever - Overview

Laboratory Diagnosis

1. Serology: Detection of IgM antibody in serum - most common diagnostic method (ELISA).
2. Molecular diagnosis: Viral RNA detection by RT-PCR in blood (best in early infection, first 3-5 days).
3. Culture/Isolation: Virus isolation in cell culture or intracerebral inoculation in mice - rarely done clinically.
4. Histopathology (post-mortem): Characteristic Councilman bodies (eosinophilic inclusions) and midzonal hepatic necrosis in liver biopsy.
5. Plaque neutralization test (PRNT): Gold standard for serological confirmation but complex.

• Note: Aspirin and NSAIDs should be avoided (increase bleeding risk).

Control of Yellow Fever

1. Immunization (most effective measure):
   • 17-D live attenuated vaccine - highly effective, single dose gives lifelong immunity.
   • Mandatory for travellers to endemic areas (WHO requirement).
   • Required for international travel under International Health Regulations.
2. Vector control:
   • Elimination of Aedes mosquito breeding sites (stagnant water, containers).
   • Larviciding (temephos), adulticiding (malathion sprays).
   • Use of insecticide-treated bed nets and repellents.
3. Surveillance and notification:
   • Yellow fever is a notifiable disease under International Health Regulations.
   • Rapid epidemiological investigation of cases.
   • Emergency mass vaccination during outbreaks.
4. Prevention in travellers:
   • Vaccination certificate (valid 10 days after primary vaccination, lifetime validity).
   • Personal protection: protective clothing, DEET repellents.
5. Treatment: Supportive only - fluid management, management of bleeding, organ support.

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Q.2 Study on Pesticide Exposure and Parkinson's Disease

(a) Ideal Study Design (1 mark)

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(b) Steps in Conducting a Cohort Study (6 marks)

• Identify a group exposed to pesticides (e.g., agricultural workers, pesticide factory workers) - the "exposed cohort."
• Identify a comparison group not exposed to pesticides but similar in all other respects (age, sex, socioeconomic status) - the "unexposed cohort."
• Confirm that both groups are free from Parkinson's disease at the start (baseline examination - exclude prevalent cases).

• Collect detailed information on pesticide exposure through:
  • Personal interviews or questionnaires (type, duration, intensity of exposure)
  • Occupational records, employment history
  • Environmental surveys or biological monitoring (urinary pesticide metabolites)
  • Classify subjects by degree of exposure (low, medium, high)

• The comparison group may be:
  • General population (using national incidence rates)
  • An internal unexposed cohort drawn from the same population
• Both groups must be comparable in age, sex, and other confounders.

• Both groups are followed prospectively over a period of time (Parkinson's disease requires long follow-up - many years).
• Regular medical examinations to detect onset of Parkinson's disease.
• Minimize loss-to-follow-up (it introduces bias).
• Periodic re-assessment of exposure status.

• Calculate Incidence rate of Parkinson's disease in exposed vs. unexposed groups.
• Calculate Relative Risk (RR) = Incidence in exposed / Incidence in unexposed.
• Calculate Attributable Risk to measure the excess risk due to pesticide exposure.
• Control for confounding variables by stratification or multivariate analysis.

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(c) Advantages and Disadvantages of Cohort Study (4 marks)

1. Establishes the temporal sequence - exposure precedes disease (causality can be inferred).
2. Gives direct measurement of incidence rates and relative risk.
3. Can study multiple outcomes of a single exposure.
4. Less prone to recall bias (exposure is recorded prospectively).
5. Allows study of rare exposures.
6. Considered the most reliable observational design (approximates experimental model).

1. Time-consuming and expensive - especially for diseases with long latency (like Parkinson's).
2. Large sample size required.
3. Loss to follow-up is a major problem - can introduce bias.
4. Not suitable for rare diseases (need very large cohorts).
5. Exposure status may change during follow-up.
6. Not suitable as a first study - requires prior evidence of association.

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(d) Isolation vs. Quarantine (4 marks)

• A typhoid fever patient isolated in hospital until 3 negative stool/urine cultures.
• A COVID-19 positive patient kept in isolation facility.
• A leprosy patient with infectious lesions kept away from family.

• Contacts of cholera patients kept under surveillance for 5 days (incubation period).
• Passengers arriving from plague-endemic countries quarantined for 6 days.
• Household contacts of COVID-19 case quarantined for 14 days.
• A ship detained in harbour because of suspected communicable disease on board (maritime quarantine).

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SECTION B - SHORT ANSWER QUESTIONS (5 marks each)

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B.1 Investigation of a Case of Food Poisoning

• Foods eaten in the previous 24-48 hours
• Place and time of eating
• Time of onset of symptoms
• Nature of symptoms (nausea, vomiting, diarrhoea, abdominal pain, fever)

• Stool samples from affected persons
• Vomitus samples
• Remnants of incriminated food
• Stool samples from food handlers/kitchen staff

• Stool culture on appropriate media (MacConkey, TCBS, etc.) - aerobically and anaerobically.
• Identify causative organism (Staphylococcus aureus, Salmonella, Clostridium, E. coli, etc.)
• Determine total bacterial count and relative numbers of each organism.
• Phage typing for complete characterization.
• Examine food handlers' stools.
• For botulism: animal protection tests (inject saline filtrate of food into mice).

• Calculate attack rates for each food item consumed.
• Plot an epidemic curve (time of onset vs. number of cases) to determine type of outbreak.
• Calculate the incubation period to suspect the probable causative agent:
  • 1-6 hours: Staphylococcal or B. cereus toxin
  • 8-16 hours: Clostridium perfringens
  • 16-48 hours: Salmonella, Shigella

• Notify health authorities.
• Treat cases (fluid replacement, antibiotics if indicated).
• Condemn and destroy incriminated food.
• Identify and exclude infected food handlers.
• Improve food hygiene and storage.

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B.2 Standardized Mortality Ratio (SMR)

SMR = (Observed Deaths / Expected Deaths) × 100

• SMR = 100: mortality in study group equals that of standard population.
• SMR > 100: the study group has excess mortality compared to the standard population (unfavourable).
• SMR < 100: study group has lower mortality than standard population (favourable).

1. Comparing mortality between occupational groups.
2. Basis for allocation of health funding in countries like England.
3. Used when age-specific rates in subgroups are not available (indirect standardization).
4. Useful in occupational epidemiology to assess mortality risk of specific jobs.
5. Adjusts for confounding by age and other factors using a standard population.

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B.3 Depot Formulations of Contraceptives

• 150 mg IM injection every 3 months (13 weeks)
• Very effective (>99%), no daily compliance needed
• Mechanism: inhibits ovulation, thickens cervical mucus, atrophies endometrium

• 200 mg IM injection every 2 months (8 weeks)
• Used in many developing countries

• Monthly injection combining progestogen + estrogen
• Regular menstrual cycles maintained

• Implanon/Nexplanon: single rod, 3-year duration
• Jadelle: two rods, 5-year duration

• High efficacy, no daily compliance issue
• Useful in women who forget pills
• Reversible (fertility returns after discontinuation)
• Suitable for postpartum and lactating women (progestogen-only types)

• Menstrual irregularities (amenorrhoea, irregular bleeding)
• Delayed return to fertility (3-18 months for DMPA)
• Cannot be immediately reversed once injected
• Bone mineral density decrease with long-term DMPA use

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B.4 Nosocomial Infections

1. Urinary tract infections (UTI) - most common (associated with urinary catheters)
2. Surgical site infections (SSI) - wound infections post-operatively
3. Pneumonia - especially ventilator-associated pneumonia (VAP)
4. Bloodstream infections - catheter-related bloodstream infections (CRBSI)
5. Hepatitis B - from needlestick injuries or blood products

• MRSA (Staphylococcus aureus)
• Pseudomonas aeruginosa
• Klebsiella pneumoniae
• E. coli
• Candida species

• Invasive procedures (urinary catheters, IV lines, ventilators)
• Prolonged hospitalization
• Immunocompromised state
• Overuse of broad-spectrum antibiotics (leads to resistant organisms)
• Poor hand hygiene among healthcare workers

1. Strict hand hygiene (WHO's 5 moments)
2. Standard and transmission-based precautions
3. Aseptic technique for all invasive procedures
4. Early removal of invasive devices
5. Antibiotic stewardship programs
6. Surveillance and reporting systems

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B.5 Syndromic Management of Urethral Discharge in Male

• Neisseria gonorrhoeae (Gonorrhoea)
• Chlamydia trachomatis (Non-gonococcal urethritis)

• Ceftriaxone 250 mg IM single dose OR
• Cefixime 400 mg orally single dose (if IM not available)

• Azithromycin 1 g orally single dose OR
• Doxycycline 100 mg orally twice daily for 7 days

• Counsel regarding safe sex practices and condom use
• Test for HIV, syphilis, Hepatitis B
• Partner notification and treatment (treat all sexual partners within 60 days)
• Follow-up in 7 days if symptoms persist

• Avoid unprotected sexual intercourse until patient and partner are treated
• Avoid alcohol
• Explain about drug compliance

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B.6 Indices of Thermal Comfort

• Ordinary air temperature in the shade
• Does not account for humidity or air movement

• Temperature recorded by a thermometer with its bulb wrapped in wet cloth
• Accounts for cooling by evaporation (humidity effect)

• WBGT = 0.7 × WBT + 0.2 × Globe temperature + 0.1 × DBT
• Most widely used index for occupational heat stress
• Accounts for radiation, humidity, and air movement

• Combines DBT, WBT, and air movement into a single index
• Represents the temperature of still, saturated air producing the same sensation as the actual conditions

• Modification of ET that also accounts for radiant heat (using globe thermometer instead of DBT)

• Combines air temperature and relative humidity
• Indicates "how hot it feels"

• Combines temperature and wind speed
• Indicates cold stress

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SECTION C - VERY SHORT ANSWER QUESTIONS (2 marks each)

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C.1 Secondary Attack Rate (SAR)

SAR = (Number of cases among contacts / Total susceptible exposed contacts) × 100

• Measures the transmissibility of an infectious disease.
• Helps identify the source and spread of infection in a household/close contact setting.
• Useful in evaluating effectiveness of control measures.

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C.2 Levels of Prevention and Their Interventions

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C.3 Human Development Index (HDI)

1. Health - Life expectancy at birth
2. Education - Mean years of schooling + Expected years of schooling
3. Standard of Living - Gross National Income (GNI) per capita (PPP $)

• Very High HDI: ≥0.800
• High HDI: 0.700-0.799
• Medium HDI: 0.550-0.699
• Low HDI: <0.550

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C.4 Measures of Dispersion

1. Range = Maximum value - Minimum value (simplest, but affected by extreme values)
2. Interquartile Range (IQR) = Q3 - Q1 (middle 50% spread; robust to outliers)
3. Mean Deviation = Average of absolute deviations from mean
4. Variance = Mean of squared deviations from mean (σ²)
5. Standard Deviation (SD) = Square root of variance (σ) - most commonly used
6. Coefficient of Variation (CV) = (SD/Mean) × 100 - used to compare variability between datasets with different units

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C.5 Confounding Factors

1. Is associated with the exposure under study, AND
2. Is independently associated with the outcome/disease, AND
3. Is NOT an intermediate step in the causal pathway between exposure and outcome.

• At design stage: Randomization, restriction, matching
• At analysis stage: Stratification (Mantel-Haenszel method), multivariate analysis (logistic regression)

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C.6 Criteria for Assessment of Obesity

• BMI = Weight (kg) / Height² (m²)
• WHO Classification:
  • Normal: 18.5-24.9
  • Overweight: 25.0-29.9
  • Obese Class I: 30-34.9
  • Obese Class II: 35-39.9
  • Obese Class III (morbid): ≥40
• Asian cut-offs (India): Overweight ≥23, Obese ≥25 kg/m²

• Males: >102 cm (WHO); >90 cm (Asian cut-off)
• Females: >88 cm (WHO); >80 cm (Asian cut-off)

• Males: >0.90 = abdominal obesity
• Females: >0.85 = abdominal obesity

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C.7 Eight Fertility-Related Indicators

1. Crude Birth Rate (CBR) - live births per 1000 mid-year population
2. General Fertility Rate (GFR) - live births per 1000 women aged 15-44 years
3. Age-Specific Fertility Rate (ASFR) - live births per 1000 women in a specific age group
4. Total Fertility Rate (TFR) - average number of children a woman would bear during her lifetime
5. Gross Reproduction Rate (GRR) - average number of female children born per woman
6. Net Reproduction Rate (NRR) - GRR adjusted for mortality (NRR=1 means replacement level)
7. Child-Woman Ratio (CWR) - children 0-4 years per 1000 women aged 15-44 years
8. Infant Mortality Rate (IMR) - deaths in infants <1 year per 1000 live births

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C.8 Classification of Water-Related Diseases

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C.9 CBNAAT (Cartridge-Based Nucleic Acid Amplification Test)

• A patient sample (sputum, CSF, lymph node aspirate) is loaded into a single-use cartridge.
• The cartridge contains all reagents for DNA extraction, amplification, and detection.
• Results available in ~2 hours (versus 6-8 weeks for culture).

1. Presence of Mycobacterium tuberculosis (MTB) DNA
2. Rifampicin resistance (as a proxy for MDR-TB) - detects mutations in the rpoB gene

• Rapid results (same day)
• High sensitivity and specificity
• Detects drug resistance simultaneously
• Minimal biosafety requirements
• Automated, minimal technical skill needed

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C.10 Sanitary Well

1. Lining/Casing: Cemented masonry or concrete lining to a depth of at least 3 meters below ground level to prevent seepage of surface water.
2. Platform: Impervious cement/concrete platform extending at least 1 meter on all sides, with a raised parapet wall (30 cm high) to prevent run-off from entering.
3. Drainage: Platform slopes away from the well with a proper drain channel to carry wastewater away.
4. Cover: A tight-fitting cover to prevent entry of dust, insects, and animals.
5. Pump: Mechanical pump preferred over a rope and bucket (which can contaminate the water).
6. Location: Sited at least 15 meters away from any source of pollution (latrines, garbage dumps, drains).
7. Depth: Should tap deep groundwater (>3 meters).

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