Week 15 - Outbreak Investigation

Baseline

Amount of a disease that is usually present in a community; The observed but not necessarily the desired level; regarded as the expected level of disease

Sporadic

A disease that occurs infrequently and irregularly.

Endemic

The constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area.

Hyperendemic

The persistently high levels of disease occurrence.

Epidemic

An increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area.

Outbreak

Similar to an epidemic but is often used for a more limited geographic area.

Cluster

An aggregation of cases grouped in a place and time that are suspected to be greater than the number expected, even though the expected number maybe unknown.

Pandemic

An epidemic that has spread over several countries or continents, usually affecting a large number of people.

Epidemic

Occurrence of disease cases of deaths; Limited to specified group of individuals during a short period of time; Excess of normal expectancy.

👾 Agents and hosts are present in adequate number. 👾 Agent can be effectively conveyed from source to host.

How does epidemic occur?

🦭 Recent increase in virulence of agent 🦭 Recent introduction of agent into a settling 🦭 Enhanced mode of transmission 🦭 Change in susceptibility 🦭 Increase host exposure 🦭 Introduction of agent through new portals entry

Factors which may trigger an epidemic:

Prevention and Control

Primary Public Health reason for investigation; Principal considerations

Prevent occurrence of additional cases

Main goal if cases are continuing to occur:

To assess the extent of the outbreak, the size, and characteristics of the population at risk.

Main objective if cases are continuing to occur:

Prevent similar outbreaks in the future.

Main goal if an outbreak appears to be almost over:

To identify factors which contributed to the outbreak.

Main objective if an outbreak appears to be almost over:

Gain additional knowledge/research

An opportunity to study the natural history of the disease in question; Field investigation provides an opportunity to define the natural history; Are able to characterize the populations at greater risk; Assessing impact of control measures.

Training

Requires combination of diplomacy, logical thinking, problem-solving ability, quantitative skills, epidemiologic know-how and judgment; Improve with practice and experience; Seasoned epidemiologist with trainee/s.

Public, Political, and Legal Concerns

Override scientific concerns in the decision to conduct investigation; Public has taken an interest in ‘disease clusters’ and potential environmental exposures.

Program consideration

Targeted by a public health program; May reveal a weakness in that program and provide an opportunity to modify/strengthen the program’s efforts; May identify populations which may have been overlooked.

🦈 Investigation 🦈 Administration 🦈 Consultation

Preparation can be grouped into:

Investigation

Appropriate scientific knowledge, supplies, and equipment; consult regarding epidemic, laboratory specimen collection, storage, and transport.

Administration

Travel arrangements and travel orders.

Consultation

Specific roles and responsibilities.

Verify the Diagnosis and Confirm Outbreak

Confirm laboratory testing; Rule out misdiagnoses or laboratory error.

🐔 Regular analysis of surveillance data-detect unusual patterns 🐔 Calls from a health care provider who knows of several cases 🐔 Media reports

Knowledge/Awareness of an existence of an epidemic through:

🫁 True outbreaks with a common cause 🫁 Sporadic and unrelated cases of the same disease 🫁 Unrelated cases of similar but unrelated disease

Initial investigation may turn out to be:

🥖 Number of cases from the previous few weeks 🥖 Number of cases from a comparable period 🥖 Surveillance data if disease is a notifiable disease 🥖 Hospitals discharge record 🥖 Apply rates from neighboring areas 🥖 Conduct survey of the community

Comparison of the number of observed number of cases with the expected number which can be:

🥼 Existence of a true outbreak 🥼 Changes in local reporting procedures 🥼 Changes in the case definition 🥼 Increased interest 🥼 Improvements in diagnostic procedures 🥼 Sudden increases in size of population

Factors which could explain excess cases:

💨 Severity of the illness 💨 The potential for spread 💨 Political consideration 💨 Public relations 💨 Availability of resources

Factors influencing decision regarding investigation of the outbreak:

🎃 Symptoms/Laboratory Results 🎃 Time period 🎃 Location

Develop a specific case definition using:

🥶 Existing surveillance 🥶 Active surveillance

Conduct surveillance using case definition:

Case Definition

Allows a simple, uniform way to identify cases; “Standardizes” the investigation; Unique to outbreak; Objective Criteria

Confirmed Probable Suspected

Categories of Cases

Confirmed

Symptoms characteristics of the agent; Lab test; Epidemiologic link

Probable

Symptoms confirmed; No lab or epidemiologic link

Suspected

Symptoms are reported but not confirmed; No lab or epidemiologic link

Tabulate and Orient Data

Create a line listing of identified cases.

Epidemic Curve

Graphical depiction of the number of cases of illness by the date of illness onset; Can provide information on the following characteristics of an outbreak.

Pattern of spread Magnitude Outliers Time trend Exposure/Incubation Period

Characteristics of an outbreak:

Common Source Propagated Mixed

Types of Outbreak

Common Space

People are exposed continuously or intermittently; Exposure maybe brief or long; Exposure often results in an epi curve with irregular peaks

Common Source - Common Point Source

Cases were all exposed to the same source during a brief period; Sharp upward slope and a gradual downward slope; All cases occur within one incubation period.

Common Source - Continuous Exposure

Continuous exposure will often cause cases to rise gradually.

Common Source - Intermittent

Brief, sporadic exposure period; Irregular peaks reflect timing and extent of exposure.

Propagated Epidemic

Spread is from person-to-person; Can last longer than source outbreaks; Agent may replicate in a susceptible host; May have multiple waves

👾 Acute food poisoning; Staphylococci toxins, C. perfringens 👾 Heavy metal exposure: cadmium, copper, zinc 👾 Other poisonings: MSG, mushroom, shellfish poisoning

Time Frame (Incubation Period): Hours

👾 Some food poisonings: Salmonella (1-2 days), V. cholerae, C. jejuni 👾 Bacterial infections: Legionnaire’s disease, Mycoplasma pneumoniae 👾 Viral infections: influenza (1-3 days), adenovirus (1-5 days), enteroviral infections (5-6 days)

Time Frame (Incubation Period): Days

🎃 Common childhood disease: measles, mumps, rubella (2-3 weeks) 🎃 Hepatitis A (2-6 weeks)

Time Frame (Incubation Period): Weeks

🌷 Hepatitis B (2-6 weeks) 🌷 Rabies (0.5-12 months)

Time Frame (Incubation Period): Months

🦭 Radiation induced leukemia 🦭 Kuru (1-27 years)

Time Frame (Incubation Period): Years

Mixed Pandemic

Both a single, common exposure to an infectious agent and secondary propagative spread to other individuals; Either same disease introduced separately or different diseases with different incubation periods.

Number of susceptible persons The period of time The minimum and maximum incubation periods

Factors Affecting Duration of an Epidemic

Attack Rate

Special cumulative incidence that measures the proportion of persons in an population who experience an acute health event during a limited time.

Attack Rate

Number of new cases of a health problem during an outbreak divided by the size of a population at the beginning of the period who are at risk of the event.

Secondary Attack Rate

Frequency of new cases of a disease among the contacts of known patients; The ratio of the number cases among contacts occurring within the acceptable incubation period.

Outbreak Magnitude Outbreak Time Trend Outbreak Outliers

Epidemic Curve:

Outbreak Outliers

Outliers are cases at the very beginning and end that may not appear to be related to the rest of the cases.

Outbreak Magnitude

Can provide a sense of the magnitude of an outbreak Additional information can be obtained by stratifying the epi curve

Communicate Findings

8th Step in Outbreak Investigation

Implement and Evaluate Control Measures

7th Step in Outbreak Investigation

Institute Immediate Control Measures

4th Step in Outbreak Investigation

Formulate and Test Hypothesis

5th Step in Outbreak Investigation

Plan and Execute Additional Studies

6th Step in Outbreak Investigation