Week 15 - Outbreak Investigation

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

A disease that occurs infrequently and irregularly.

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

The persistently high levels of disease occurrence.

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

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

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.

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

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

How does epidemic occur?

Factors which may trigger an epidemic:

Primary Public Health reason for investigation; Principal considerations

Main goal if cases are continuing to occur:

Main objective if cases are continuing to occur:

Main goal if an outbreak appears to be almost over:

Main objective if an outbreak appears to be almost over:

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.

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.

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

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.

Preparation can be grouped into:

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

Travel arrangements and travel orders.

Specific roles and responsibilities.

Confirm laboratory testing; Rule out misdiagnoses or laboratory error.

Knowledge/Awareness of an existence of an epidemic through:

Initial investigation may turn out to be:

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

Factors which could explain excess cases:

Factors influencing decision regarding investigation of the outbreak:

Develop a specific case definition using:

Conduct surveillance using case definition:

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

Categories of Cases

Symptoms characteristics of the agent; Lab test; Epidemiologic link

Symptoms confirmed; No lab or epidemiologic link

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

Create a line listing of identified cases.

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.

Characteristics of an outbreak:

Types of Outbreak

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

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.

Continuous exposure will often cause cases to rise gradually.

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

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

Time Frame (Incubation Period): Hours

Time Frame (Incubation Period): Days

Time Frame (Incubation Period): Weeks

Time Frame (Incubation Period): Months

Time Frame (Incubation Period): Years

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.

Factors Affecting Duration of an Epidemic

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

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.

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.

Epidemic Curve:

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

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

8th Step in Outbreak Investigation

7th Step in Outbreak Investigation

4th Step in Outbreak Investigation

5th Step in Outbreak Investigation

6th Step in Outbreak Investigation