Epidemiology Case Study

## Questions and Comprehensive Answers --- ## CASE SCENARIO: TYPHOID FEVER OUTBREAK **Background Information:** - Typhoid fever manifests clinically with high fever and headache - A person infected with the typhoid fever agent works in a café but shows no manifestation of disease (asymptomatic carrier) - This person serves 20 people in one day at the café - 5 people got infected with typhoid fever - Only 3 of the 5 infected people developed disease symptoms - Among the 3 who developed typhoid: 2 recovered and 1 died --- ## QUESTION 1 **From the given information, who is likely the mode of transmission?** ### ANSWER: **The Infected Food Handler (Asymptomatic Carrier) is the source of transmission.** **Mode of Transmission: Fecal-Oral Route (Indirect Contact)** **Explanation:** - **Source/Reservoir**: The café worker is an **asymptomatic carrier** (also called a healthy carrier). This person is infected with *Salmonella typhi* but shows no clinical symptoms while still shedding the bacteria. - **Mode of Transmission**:**Primary Mode**: Contaminated food and water (fecal-oral route) - The infected food handler likely contaminated food/beverages served at the café through poor hand hygiene after using the toilet - Customers consumed the contaminated food/drinks, leading to infection - **Chain of Infection**:**Agent**: *Salmonella typhi* bacteria - **Reservoir**: The asymptomatic food handler - **Portal of Exit**: Feces of the infected person - **Mode of Transmission**: Indirect contact via contaminated food/water - **Portal of Entry**: Mouth of the customers (ingestion) - **Susceptible Host**: The 20 customers served **Public Health Significance**: Asymptomatic carriers are particularly dangerous in food service settings because they can unknowingly spread disease to many people. This is similar to the famous case of "Typhoid Mary" in New York. --- ## QUESTION 2 **Each of the affected persons are what type of case, and each one is what? (Clarification: Identify case types and infection outcomes)** ### ANSWER: **Classification of the Affected Individuals:** ### **The Food Handler (Index Case):** - **Type**: **Asymptomatic Carrier** (Subclinical Case/Inapparent Infection) - **Status**: Infected but shows no clinical symptoms - **Role**: Primary source of the outbreak ### **The 5 Infected People:** **Group 1: Two who remained asymptomatic (5 infected - 3 who developed disease = 2 asymptomatic)** - **Type**: **Subclinical Cases/Inapparent Infections** - **Status**: Infected with *Salmonella typhi* but did not develop clinical symptoms - **Outcome**: Remain healthy despite infection **Group 2: Three who developed disease symptoms** - **First Person (Recovered)****Type**: **Clinical Case** (Confirmed/Probable Case) - **Outcome**: Recovery - **Second Person (Recovered)****Type**: **Clinical Case** (Confirmed/Probable Case) - **Outcome**: Recovery - **Third Person (Died)****Type**: **Clinical Case** (Confirmed/Probable Case) - **Outcome**: **Fatal Case** - **Significance**: Death from typhoid fever ### **The 15 People Who Were Not Infected:** - **Type**: **Exposed but Not Infected** (Contacts) - **Status**: Either immune or did not consume contaminated items - **Outcome**: No infection, no disease ### **Summary Table:** ### **Attack Rate Calculation:** - **Attack Rate** = (Number who developed disease / Number exposed) × 100 - Attack Rate = (3/20) × 100 = **15%** ### **Case Fatality Rate:** - **CFR** = (Number of deaths / Number of cases) × 100 - CFR = (1/3) × 100 = **33.3%** --- ## QUESTION 3 **Can you group the 20 people who were served at the café into the 4 stages of natural history of communicable disease?** ### ANSWER: The **Natural History of Disease** has 4 main stages. Here's how the 20 customers fit into each stage: ### **STAGE 1: STAGE OF SUSCEPTIBILITY (Pre-pathogenesis Period)** **Number of People: 15 people** **Characteristics:** - These individuals were exposed to the contaminated food/water but did NOT get infected - They remain susceptible but the agent did not establish infection - Reasons they didn't get infected:May have natural immunity - Did not consume contaminated items - Consumed insufficient infectious dose - Good immune system defense **Intervention at this stage**: Primary prevention (health education, food safety, hand hygiene) --- ### **STAGE 2: STAGE OF SUBCLINICAL DISEASE (Early Pathogenesis)** **Number of People: 2 people + 1 food handler = 3 people total** **Characteristics:** - These individuals are infected with *Salmonella typhi* - The pathogen has entered the body and is multiplying - **NO clinical symptoms** are present (asymptomatic) - They may be carriers capable of transmitting disease - Disease is in the **incubation period** - The person is infected but appears healthy **Intervention at this stage**: Secondary prevention (screening, early detection through laboratory tests) --- ### **STAGE 3: STAGE OF CLINICAL D