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
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Sporadic
A disease that occurs infrequently and irregularly.
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Endemic
The constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area.
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Hyperendemic
The persistently high levels of disease occurrence.
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Epidemic
An increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area.
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Outbreak
Similar to an epidemic but is often used for a more limited geographic area.
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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.
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Pandemic
An epidemic that has spread over several countries or continents, usually affecting a large number of people.
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Epidemic
Occurrence of disease cases of deaths; Limited to specified group of individuals during a short period of time; Excess of normal expectancy.
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👾 Agents and hosts are present in adequate number. 👾 Agent can be effectively conveyed from source to host.
How does epidemic occur?
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🦭 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:
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Prevention and Control
Primary Public Health reason for investigation; Principal considerations
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Prevent occurrence of additional cases
Main goal if cases are continuing to occur:
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To assess the extent of the outbreak, the size, and characteristics of the population at risk.
Main objective if cases are continuing to occur:
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Prevent similar outbreaks in the future.
Main goal if an outbreak appears to be almost over:
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To identify factors which contributed to the outbreak.
Main objective if an outbreak appears to be almost over:
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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.
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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.
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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.
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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.
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🦈 Investigation 🦈 Administration 🦈 Consultation
Preparation can be grouped into:
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Investigation
Appropriate scientific knowledge, supplies, and equipment; consult regarding epidemic, laboratory specimen collection, storage, and transport.
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Administration
Travel arrangements and travel orders.
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Consultation
Specific roles and responsibilities.
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Verify the Diagnosis and Confirm Outbreak
Confirm laboratory testing; Rule out misdiagnoses or laboratory error.
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🐔 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:
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🫁 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:
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🥖 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:
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🥼 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:
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💨 Severity of the illness 💨 The potential for spread 💨 Political consideration 💨 Public relations 💨 Availability of resources
Factors influencing decision regarding investigation of the outbreak:
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🎃 Symptoms/Laboratory Results 🎃 Time period 🎃 Location
Develop a specific case definition using:
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🥶 Existing surveillance 🥶 Active surveillance
Conduct surveillance using case definition:
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Case Definition
Allows a simple, uniform way to identify cases; “Standardizes” the investigation; Unique to outbreak; Objective Criteria
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Confirmed Probable Suspected
Categories of Cases
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Confirmed
Symptoms characteristics of the agent; Lab test; Epidemiologic link
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Probable
Symptoms confirmed; No lab or epidemiologic link
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Suspected
Symptoms are reported but not confirmed; No lab or epidemiologic link
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Tabulate and Orient Data
Create a line listing of identified cases.
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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.
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Pattern of spread Magnitude Outliers Time trend Exposure/Incubation Period
Characteristics of an outbreak:
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Common Source Propagated Mixed
Types of Outbreak
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Common Space
People are exposed continuously or intermittently; Exposure maybe brief or long; Exposure often results in an epi curve with irregular peaks
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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.
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Common Source - Continuous Exposure
Continuous exposure will often cause cases to rise gradually.
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Common Source - Intermittent
Brief, sporadic exposure period; Irregular peaks reflect timing and extent of exposure.
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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
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👾 Acute food poisoning; Staphylococci toxins, C. perfringens 👾 Heavy metal exposure: cadmium, copper, zinc 👾 Other poisonings: MSG, mushroom, shellfish poisoning
🎃 Common childhood disease: measles, mumps, rubella (2-3 weeks) 🎃 Hepatitis A (2-6 weeks)
Time Frame (Incubation Period): Weeks
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🌷 Hepatitis B (2-6 weeks) 🌷 Rabies (0.5-12 months)
Time Frame (Incubation Period): Months
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🦭 Radiation induced leukemia 🦭 Kuru (1-27 years)
Time Frame (Incubation Period): Years
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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.
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Number of susceptible persons The period of time The minimum and maximum incubation periods
Factors Affecting Duration of an Epidemic
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Attack Rate
Special cumulative incidence that measures the proportion of persons in an population who experience an acute health event during a limited time.
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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.
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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.
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Outbreak Magnitude Outbreak Time Trend Outbreak Outliers
Epidemic Curve:
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Outbreak Outliers
Outliers are cases at the very beginning and end that may not appear to be related to the rest of the cases.
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Outbreak Magnitude
Can provide a sense of the magnitude of an outbreak Additional information can be obtained by stratifying the epi curve
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