Epidemic, Endemic, Pandemic & Sporadic - Definitions and Epidemic Investigation

PART I - DEFINITIONS

1. EPIDEMIC

• The number of cases indicating an epidemic varies according to the agent, size and type of population exposed, previous experience or lack of exposure to the disease, and time and place of occurrence.
• Epidemicity is relative - it depends on the usual frequency of the disease in the same area, among the specified population, at the same season of the year.
• A single case of a communicable disease long absent from a population or first invasion by a disease not previously recognized in that area requires immediate reporting and full field investigation; two such cases associated in time and place may be sufficient to be considered an epidemic.

2. ENDEMIC

• Example: Common cold is endemic because somebody always has one.
• Hyperendemic: Disease is constantly present at a high incidence and/or prevalence rate and affects all age groups equally.
• Holoendemic: A high level of infection beginning early in life and affecting most of the child population, leading to a state of equilibrium such that adults show evidence of disease much less commonly than children (e.g., malaria).
• An endemic disease, when conditions are favourable, may burst into an epidemic (e.g., hepatitis A, typhoid fever).

3. PANDEMIC

• Only some pandemics cause severe disease in individuals or at a population level.
• Characteristics of an infectious agent that can cause a pandemic: the agent must be able to (a) infect humans, (b) cause disease in humans, and (c) spread easily from human to human.
• Examples: Influenza pandemics, cholera pandemics, COVID-19 pandemic.

4. SPORADIC

• Cases are so few and separated widely in space and time that they show little or no connection with each other, nor a recognizable common source of infection.
• Examples: Tetanus, herpes zoster, meningococcal meningitis.
• A sporadic disease may become the starting point of an epidemic when conditions favour its spread.
• Many zoonotic diseases are characterised by sporadic transmission to man.

PART II - STEPS IN INVESTIGATION OF AN EPIDEMIC

• (a) Define the magnitude in terms of time, place and person
• (b) Determine the conditions and factors responsible
• (c) Identify cause, source(s) of infection and modes of transmission
• (d) Make recommendations to prevent recurrence

Note: No single step-by-step "cookbook" approach applies in all situations, and some steps can be done concurrently.

Step 1 - Verification of Diagnosis

• This is the first and most important step.
• Confirm that the reported cases are genuine - reports may sometimes be spurious due to misinterpretation of signs and symptoms by the lay public.
• A clinical examination of a sample of cases may suffice; laboratory investigations help confirm diagnosis but epidemiological investigation should not be delayed pending laboratory results.

Step 2 - Confirmation of the Existence of an Epidemic

• Compare disease frequencies during the same period of previous years.
• An epidemic exists when the number of observed cases is in excess of the expected frequency for that population based on past experience.
• An arbitrary limit of two standard errors from endemic occurrence is used to define the epidemic threshold for common diseases like influenza.
• For common-source epidemics (cholera, food poisoning, hepatitis A) - existence is often obvious. For modern epidemics (cancer, cardiovascular diseases) - comparison with previous experience is necessary.

Step 3 - Defining the Population at Risk

Step 4 - Rapid Search for All Cases and Their Characteristics

Step 5 - Data Analysis

• Time: Prepare a chronological distribution of dates of onset and construct an "epidemic curve". This suggests: (a) time relationship with exposure to a suspected source, (b) whether it is a common-source or propagated epidemic, (c) seasonal or cyclic pattern.
• Place: Prepare a "spot map" (geographic distribution) of cases and their relation to possible infection sources. Clustering suggests a common source (as demonstrated by John Snow in the London cholera outbreak).
• Person: Analyze by age, sex, occupation and other possible risk factors. Determine attack rates/case fatality rates for those exposed and not exposed. In food-borne outbreaks, calculate food-specific attack rates for each food eaten.

Step 6 - Formulation of Hypotheses

• (a) Possible source
• (b) Causative agent
• (c) Possible modes of spread
• (d) Environmental factors which enabled it to occur

Step 7 - Testing of Hypotheses

• Compare attack rates in groups exposed vs. not exposed to each suspected factor.
• Determine which hypothesis is consistent with all known facts.
• When divergent theories exist, it is instructive to turn to arguments tested by subsequent events.

Step 8 - Evaluation of Ecological Factors

• Sanitary status of eating establishments
• Water and milk supply; breakdown in water supply systems
• Movements of human population
• Atmospheric changes (temperature, humidity, air pollution)
• Population dynamics of insects and animal reservoirs

Step 9 - Further Investigation of Population at Risk

• Medical examination, screening tests
• Examination of suspected food, faeces or blood samples
• Biochemical studies, assessment of immunity status
• Serological study may reveal clinically inapparent cases
• Healthy individuals from the same universe may be studied in a case-control fashion to classify all members as to: (a) exposure to specific potential vehicles; (b) whether ill or not

Step 10 - Writing the Report

• Description of the epidemic
• Conclusions drawn
• Recommendations for control and prevention of recurrence

Definitions & Investigation of an Epidemic

PART A - DEFINITIONS

1. EPIDEMIC

• The number of cases indicating an epidemic varies according to: the agent, size and type of population exposed, previous experience or lack of exposure, and time and place of occurrence.
• Epidemicity is relative to the usual frequency of the disease in the same area, among the same population, at the same season of the year.
• A single case of a disease long absent from a population, or first invasion of a previously unrecognized disease, requires immediate reporting and full field investigation.
• Two cases of such disease associated in time and place may be sufficient evidence to be considered an epidemic.

2. ENDEMIC

• Example: Common cold is endemic because somebody always has one.
• Hyperendemic: Disease is constantly present at a high incidence/prevalence and affects all age groups equally.
• Holoendemic: High level of infection beginning early in life, affecting most children, with adults showing evidence of disease far less commonly - e.g., malaria.
• An endemic disease may burst into an epidemic when conditions become favourable (e.g., hepatitis A, typhoid fever).

3. SPORADIC

• Cases are so few and separated widely in space and time that they show little or no connection with each other, nor a recognizable common source of infection.
• Examples: Tetanus, herpes zoster, meningococcal meningitis.
• A sporadic disease may become the starting point of an epidemic when conditions favour its spread.
• Many zoonotic diseases are characterised by sporadic transmission to man.

4. PANDEMIC

• Only some pandemics cause severe disease in individuals or at population level.
• Characteristics of an infectious agent that can cause a pandemic - the agent must be able to:
  1. Infect humans
  2. Cause disease in humans
  3. Spread easily from human to human
• Examples: Influenza pandemics, cholera pandemics, COVID-19 pandemic.

Quick Memory Aid:

Term	Scope	Frequency
Sporadic	Isolated cases	Irregular, infrequent
Endemic	One area/region	Constant, expected
Epidemic	Community/region	Clearly in excess of normal
Pandemic	International/worldwide	Widespread, large numbers

PART B - STEPS IN INVESTIGATION OF AN EPIDEMIC

• (a) Define magnitude in terms of time, place and person
• (b) Determine conditions and factors responsible for the epidemic
• (c) Identify cause, source(s) of infection and modes of transmission
• (d) Make recommendations to prevent recurrence

Note: No single step-by-step "cookbook" applies in all situations. Some steps can be done concurrently.

Step 1 - Verification of Diagnosis

• The first and foremost step.
• Reports may sometimes be spurious, arising from misinterpretation of signs and symptoms by the lay public.
• Clinical examination of a sample of cases may suffice - not necessary to examine all cases.
• Laboratory investigations are useful to confirm the diagnosis.
• Epidemiological investigation must NOT be delayed pending laboratory results.

Step 2 - Confirmation of the Existence of an Epidemic

• Compare disease frequencies during the same period of previous years.
• An epidemic exists when observed cases are in excess of expected frequency based on past experience.
• An arbitrary limit of two standard errors from endemic occurrence is used to define the epidemic threshold (e.g., for influenza).
• Common-source epidemics (cholera, food poisoning, hepatitis A) are easily recognized.
• Modern epidemics (cancer, cardiovascular disease) are not easily recognized without comparison to previous experience.

Step 3 - Defining the Population at Risk

Step 4 - Rapid Search for All Cases and Their Characteristics

• Name, age, sex, occupation, social class
• Travel history, history of previous exposure
• Time of onset, signs and symptoms
• Personal contacts (home, work, school)
• Foods eaten, exposure to common vehicles (water, food, milk)
• History of injections/blood products, attendance at large gatherings

Step 5 - Data Analysis

• Time: Construct an "epidemic curve" (chronological distribution of dates of onset). Suggests:
  • Time relationship with exposure to a suspected source
  • Whether it is a common-source or propagated epidemic
  • Seasonal or cyclic pattern
• Place: Prepare a "spot map" (geographic distribution of cases) and their relation to possible infection sources (water supply, food, occupation). Clustering indicates a common source (demonstrated by John Snow in the London cholera outbreak).
• Person: Analyze by age, sex, occupation and other risk factors. Determine attack rates and case fatality rates for those exposed vs. not exposed. In food-borne outbreaks, calculate food-specific attack rates for each food eaten.

Step 6 - Formulation of Hypotheses

• (a) Possible source
• (b) Causative agent
• (c) Possible modes of spread
• (d) Environmental factors enabling the epidemic

Step 7 - Testing of Hypotheses

• Compare attack rates in groups exposed vs. not exposed to each suspected factor.
• Ascertain which hypothesis is consistent with all known facts.
• When divergent theories exist, turn to arguments that have been tested by the subsequent course of events.

Step 8 - Evaluation of Ecological Factors

• Sanitary status of eating establishments
• Water and milk supply; breakdown in water supply systems
• Movements of human population
• Atmospheric changes (temperature, humidity, air pollution)
• Population dynamics of insects and animal reservoirs
• The outbreak can be studied in a case-control fashion to relate disease to environmental factors.

Step 9 - Further Investigation of Population at Risk

• Medical examination, screening tests
• Examination of suspected food, faeces or blood samples
• Biochemical studies, assessment of immunity status
• Serological study may reveal clinically inapparent cases and throw light on pathogenesis
• Healthy individuals from the same universe studied in a case-control fashion to classify all members as to: (a) exposure to specific potential vehicles; (b) whether ill or not

Step 10 - Writing the Report

Important note from Park: "It may be necessary to implement temporary control measures at the commencement of an epidemic on the basis of known facts of the disease. These measures may be modified or replaced in the light of new knowledge acquired by the epidemic investigation."

Epidemic, Endemic, Pandemic & Sporadic - Definitions + Investigation of an Epidemic

PART A - DEFINITIONS

1. EPIDEMIC

• The number of cases indicating an epidemic varies with: the agent, size and type of population exposed, previous experience or lack of exposure, and time and place of occurrence.
• Epidemicity is relative - it relates to the usual frequency of the disease in the same area, among the same population, at the same season of the year.
• A single case of a disease long absent from a population, or first invasion of a disease not previously recognized in an area, requires immediate reporting and full field investigation.
• Two such cases associated in time and place may be sufficient evidence to be considered an epidemic.

2. ENDEMIC

• Example: Common cold is endemic because somebody always has one.
• Hyperendemic: Disease is constantly present at a high incidence/prevalence rate and affects all age groups equally.
• Holoendemic: A high level of infection beginning early in life, affecting most children; adults show evidence of disease far less commonly than children (e.g., malaria).
• An endemic disease may burst into an epidemic when conditions become favourable (e.g., hepatitis A, typhoid fever). As new control/preventive measures are applied, the endemic status of a disease may change.

3. SPORADIC

• Cases are so few and separated widely in space and time that they show little or no connection with each other, nor a recognizable common source of infection.
• Examples: Tetanus, herpes zoster, meningococcal meningitis.
• A sporadic disease may be the starting point of an epidemic when conditions favour its spread.
• Many zoonotic diseases are characterised by sporadic transmission to man.

4. PANDEMIC

• Only some pandemics cause severe disease in individuals or at population level.
• An infectious agent capable of causing a pandemic must be able to:
  1. Infect humans
  2. Cause disease in humans
  3. Spread easily from human to human
• Examples: Influenza pandemics, cholera pandemics, COVID-19 pandemic.

Comparison at a glance:

Term	Geographic scope	Frequency
Sporadic	Isolated, scattered	Irregular, infrequent
Endemic	Fixed area/region	Constant, expected
Epidemic	Community or region	Clearly in excess of normal
Pandemic	International / worldwide	Very widespread, large numbers

PART B - STEPS IN INVESTIGATION OF AN EPIDEMIC

• (a) Define magnitude in terms of time, place and person
• (b) Determine conditions and factors responsible
• (c) Identify cause, source(s) of infection and modes of transmission
• (d) Make recommendations to prevent recurrence

Note: No single "cook-book" approach applies to all situations. Some steps can be done concurrently.

Step 1 - Verification of Diagnosis

• The first step in any epidemic investigation.
• Reports may be spurious, arising from misinterpretation of signs and symptoms by the lay public.
• Clinical examination of a sample of cases suffices - it is not necessary to examine all cases.
• Laboratory investigations are useful to confirm diagnosis, but epidemiological investigation must NOT be delayed pending lab results.

Step 2 - Confirmation of the Existence of an Epidemic

• Compare disease frequencies during the same period of previous years.
• An epidemic exists when the observed frequency exceeds the expected frequency based on past experience.
• An arbitrary limit of two standard errors from endemic occurrence defines the epidemic threshold for common diseases (e.g., influenza).
• Common-source epidemics (cholera, food poisoning, hepatitis A) are easily and obviously recognized.
• Modern epidemics (cancer, cardiovascular disease) are not easily recognized without historical comparison.

Step 3 - Defining the Population at Risk

Step 4 - Rapid Search for All Cases and Their Characteristics

• Name, age, sex, occupation, social class
• Travel history, history of previous exposure
• Time of onset of disease, signs and symptoms
• Personal contacts at home, work, school
• Special events attended, foods eaten, exposure to common vehicles (water, food, milk)
• History of injections or blood products, attendance at large gatherings

Step 5 - Data Analysis

• Time: Construct an "epidemic curve" (chronological distribution of dates of onset). This suggests:
  • Time relationship with exposure to a suspected source
  • Whether it is a common-source or propagated epidemic
  • Seasonal or cyclic pattern
• Place: Prepare a "spot map" (geographic distribution of cases) and their relation to possible infection sources (water supply, food, occupation, etc.). Clustering indicates a common source (classically demonstrated by John Snow in the London cholera outbreak).
• Person: Analyze by age, sex, occupation and other risk factors. Determine attack rates and case fatality rates for those exposed vs. not exposed. In food-borne outbreaks, calculate food-specific attack rates for each food eaten.

Step 6 - Formulation of Hypotheses

• (a) Possible source
• (b) Causative agent
• (c) Possible modes of spread
• (d) Environmental factors which enabled it to occur

Step 7 - Testing of Hypotheses

• Compare attack rates in groups exposed vs. not exposed to each suspected factor.
• Ascertain which hypothesis is consistent with all known facts.
• When divergent theories exist, turn to arguments that have been tested by the subsequent course of events.

Step 8 - Evaluation of Ecological Factors

• Sanitary status of eating establishments
• Water and milk supply; breakdown in the water supply system
• Movements of human population
• Atmospheric changes - temperature, humidity, air pollution
• Population dynamics of insects and animal reservoirs

Step 9 - Further Investigation of Population at Risk

• Medical examination, screening tests
• Examination of suspected food, faeces or blood samples
• Biochemical studies, assessment of immunity status
• Serological study to reveal clinically inapparent cases and illuminate pathogenesis
• Healthy individuals studied in a case-control fashion to classify all members as to: (a) exposure to specific potential vehicles; (b) whether ill or not

Step 10 - Writing the Report

Important: Temporary control measures may need to be implemented at the commencement of an epidemic based on known facts. These may be modified or replaced in the light of new knowledge acquired during the investigation.