Principles of Disease Investigation and Control

1. Concept of an Outbreak

• Outbreak is a situation where the number of cases of a particular disease or deaths from the disease exceeds the average number over many years in a community (CĐ).

• Outbreak investigation is the process of collecting information about the severity and distribution of a disease to identify relevant factors:

• Distribution (who, when, where): Who got sick? When did they get sick? Where did they get sick?

• Source, causative agent: What is the source of the disease? What is the causative agent?

• Mode of transmission: How is the disease transmitted?

• Population at risk: Who is at high risk of getting the disease?

2. Factors Influencing Outbreak Evolution

There are three main factors influencing the course of an outbreak:

a. Agent (Causative agent):

• Increased dosage, change in virulence: The causative agent may become stronger, more toxic.

• New entry routes: The causative agent may find new entry routes into the body.

• Variation: The causative agent may change in structure, leading to drug resistance or easier transmission.

• New emergence: A new causative agent emerges, for which there is no treatment or vaccine.

• Combined attack by multiple pathogens: Multiple causative agents attack the body simultaneously, making the disease worse.

b. Environment:

• New environment: For example, Lyme disease emerges due to changes in living environment.

• New mode of transmission: The causative agent may find a new mode of transmission, for example, transmission through blood.

• Healthcare services: The quality of healthcare services can affect the ability to prevent and treat the disease.

• New socio-economic factors: For example, the HIV/AIDS epidemic emerged due to changes in lifestyle and social culture.

c. Host (Human):

• Change in susceptibility: The human immune system may become weaker, making it easier to get sick.

• Change in infection rate: The way humans interact and their behavior may create conditions for the causative agent to enter the body.

• Culture, behavior: Lifestyle and living habits may facilitate the spread of disease.

• High risk: High-risk groups (young children, elderly, immunocompromised individuals) are more susceptible to disease.

3. R0 – Index Reflecting the Level of Disease Transmission

• R0 (Basic Reproduction Number) is an index that reflects the ability of a disease to spread in a population.

• R0 indicates how many people, on average, one infected person can infect in a population that is not yet immune.

• Example:

• Measles: R0 = 12-18

• Diphtheria: R0 = 6-7

• Smallpox, Polio, Rubella: R0 = 5-7

• Mumps: R0 = 4-7

• HIV/AIDS, SARS: R0 = 2-5

4. Epidemic Curve

• The epidemic curve depicts the change in the number of cases over time. It helps to determine:

• Time of initial exposure: When did the first person come into contact with the causative agent?

• Incubation period: How long after exposure to the causative agent do patients exhibit symptoms?

• Peak of the epidemic: When is the highest number of cases?

• Tail of the epidemic: When does the number of cases begin to decrease?

5. Types of Epidemic Curves

There are three types of epidemic curves:

a. Single-peak curve:

• Suggests simultaneous exposure to a common source of infection in a group of people.

• Commonly encountered in cases of food poisoning.

• The shape of the curve depends on the incubation period and host response.

• Single-peak curves can be curved, pointed, with short duration, and no secondary peaks.

b. Continuous/Intermittent curve:

• Suggests exposure to multiple sources of infection over the same period of time.

• Commonly encountered in cholera outbreaks.

c. Propagated curve:

• Suggests transmission of the disease from infected people to others, with or without an intermediary vector.

6. Why Conduct Outbreak Investigations?

• Community demand: Protect public health.

• Opportunity for research and training: Provide information about diseases, improve epidemiological knowledge.

• Implementation of prevention programs: Based on outbreak investigation results, effective prevention measures can be implemented.

• Legal responsibility and political reasons: Reporting disease outbreaks is a legal responsibility of health authorities, and it is also a crucial political issue.

7. When to Conduct Outbreak Investigations?

• When a suspected outbreak is reported.

• When periodic analysis reveals an increase in disease incidence or mortality.

• When scientists warn of unusual disease patterns.

• When deaths of unknown cause occur.

8. Steps in Outbreak Investigation

There are 10 main steps in outbreak investigation:

1. Investigation preparation: Define objectives, scope of investigation, develop a plan, mobilize personnel, finance, equipment,…

2. Community verification: Identify the community with the outbreak, its scale, and characteristics.

3. Confirming the existence of the disease: Identify the disease currently occurring with certainty.

4. Defining the case: Define criteria for classifying suspected and confirmed cases.

5. Describing the disease: Collect information on the time, place, and people who are sick.

6. Developing hypotheses: Propose hypotheses to explain the cause and mode of transmission of the outbreak.

7. Evaluating and testing hypotheses: Gather evidence to verify hypotheses.

8. Refining hypotheses and conducting additional research: Conduct further research to clarify the cause and transmission mechanism.

9. Implementing control measures: Based on outbreak investigation results, implement appropriate measures to control the outbreak.

10. Reporting outbreak investigation results: Provide information to relevant authorities, the public, and stakeholders.

9. Important Issues in Outbreak Investigation Preparation

4M:

• Manpower: Ensure sufficient number of staff with relevant expertise.

• Money: Ensure funding for investigation activities.

• Materials: Prepare complete equipment and materials for the investigation.

• Management: Effectively manage resources and investigation progress.

10. Case Definition

• Two levels:

• Suspected case: Extract information from records, public health archives, community information.

• Confirmed case: Suspected case + laboratory confirmation of the causative agent +…

• Note: Case definition criteria must be clearly standardized to ensure accurate and objective investigation results.

11. Testing Hypotheses

• Testing hypotheses can be conducted in two ways:

• Finding evidence from history, laboratory tests, epidemiology: Collect information about exposure, laboratory confirmation of the causative agent, and identify factors related to the disease.

• Measuring the relationship: Apply statistical methods to determine the relationship between risk factors and the disease.

12. Additional Research

• Commonly used additional research:

• Case-control study:

• Apply when: The outbreak occurs in a population that is not clearly defined.

• Select control group: Select healthy individuals during the outbreak.

• Calculate: Odds Ratio (OR).

• Cohort study:

• When: The population is small and well defined.

• Collect: Information about exposure and suspected medical factors.

• Calculate: Attack Rate (AR) in exposed and unexposed groups, Relative Risk (RR) for each medical factor.

13. Prevention

a. For the source of transmission:

• Early detection: Quickly identify the source of the disease.

• Reporting: Report to health authorities promptly.

• Isolation: Isolate the sick person, animals, water source, food,…

• Disinfection: Eliminate pathogens from the environment.

• Care and monitoring: Monitor the health of the sick person and provide timely medical services.

b. For the mode of transmission:

• Water, feces, and garbage treatment:

• Eliminate intermediate animals: For example, mosquitoes, rats,…

• Personal hygiene: Wash hands frequently, don’t spit indiscriminately, sleep under mosquito nets,…

• Avoid contact with sick people: Maintain safe distance, limit direct contact.

c. Prevention of respiratory diseases:

• Impact on the susceptible group: Vaccination.

d. Prevention of diseases spread through the digestive tract and skin mucous membranes:

• Impact on the mode of transmission:

• Food and water hygiene.

• Wash hands frequently.

• Avoid eating raw food.

• Do not share food or eating utensils.

Note:

• Outbreak investigation requires close coordination between health authorities, the community, and stakeholders.

• Active participation of the public is necessary to provide accurate information and fully implement disease prevention measures.

• Outbreak response must be evidence-based, ensuring fairness, transparency, and effectiveness.

Besides the information above, you can learn more about specific types of outbreaks, effective prevention and treatment measures to protect the health of yourself and the community.