An outbreak or an epidemic is the occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time. 

In an epidemic or outbreak, it is usually presumed that the cases are related to one another or that they have a common cause.

Outbreak - Increase in cases is more of a localised situation e.g. village, institution, housing society etc.

Epidemic - involves large numbers of people over a wide geographic area. Epidemic implies a crisis situation.

Cluster - aggregation of cases in a given area over a particular period without regard to whether the number of cases is more than expected. 

1. Health dept alerted by a Clinician (most common way) who finds an increased number of cases  or unusual clinical presentation.

2. Analysis of surveillance data reveals increased number of cases or unusual clustering of cases.

3. Members of affected community report to the health dept when they observe several similar cases in their community.

Investigation helps to

1. define the magnitude of the epidemic.

2. determine the conditions and factors that led to the occurrence of the epidemic.

3. institute prevention and control measures.

4. recommend measures for preventing future epidemics of same or similar nature.

5. identify the causative agent, sources of infection, and modes of transmission. 

6. understand natural history of disease, risk factors and population at risk.

7. assess the effectiveness of programme strategies that are already in place. e.g.  measles outbreak in a country where measles vaccination is an ongoing activity.

8. demonstrate that the government cares for the health of its citizens and thereby maintain good  public relations.

9. rule out any biological warfare for addressing political concerns.

1. Establish the existence of an epidemic

2. Verify the diagnosis

3. Construct a working case definition

4. Search for cases systematically and record information

5. Perform descriptive epidemiology

6. Develop hypotheses

7. Evaluate hypotheses epidemiologically

8. If needed, reconsider, refine and re-evaluate hypotheses 

9. Implement control and prevention measures

10. Communicate findings

• To confirm existence of an epidemic compare observed figures with expected figures. 

• Observed figures should be more than expected.

• In some cases, comparison not required (e.g. common source epidemics like cholera, food poisoning). 

• A single case of a disease that is not known to exist in country is also considered as epidemic (e.g. polio in a country that has eliminated polio from last 10 years).

Expected number is the number that you would normally expect to see (can compare from the previous few weeks or months, or from a comparable period during the previous few years).

Expected number could be 

1.    Obtained from health department surveillance records.

2.    Retrieved from existing local data (hospital discharge records, mortality statistics, cancer or birth defect registries).

3.    Calculated by applying  disease rates from neighbouring states or national data if local data is not available.

4.    Obtained by conducting a telephone survey of physicians to determine whether they have seen more cases of the disease than usual.

5.    Obtained by conducting survey of the community to establish the background or historical level of disease.

Mostly yes. But, Not every time the increased numbers are due to outbreak. It could be due to 

1.    changes in local reporting procedures.

2.    changes in the case definition.

3.    increased interest because of local or national awareness.

4.    improvements in diagnostic procedures.

5.    consistent reporting by health care facility.

6.    changes in size of population.

1.    To ensure disease has been accurately diagnosed.

2.    To rule out laboratory error.

1. By reviewing clinical findings and laboratory findings

Correlate clinical and laboratory findings. If laboratory findings are inconsistent with clinical and epidemiological findings, ask an expert to review technique or send to reference lab for confirmation

2. By talking to patients

Talking to patients - Helps in better understanding of disease. Conversation with the patients will help in

a.    Forming a visual image of patient affected by disease. 

b.    Identification of the exposures before the person became ill. 

c.    Finding out if the case patients know anyone with similar illness or they have anything in common with those who have the disease.

d.    Generating hypothesis about disease aetiology and spread.

3. By summarising the clinical finding with frequency distribution, the epidemiologist can 

a.    Assess if clinical findings are consistent with the diagnosis.

b.    Characterise the spectrum of illness.

c.    Frame case definitions.

A case definition is a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest.

Case definition should be able to detect most of the cases and should have very few false positive cases.

Initial case definition can be broad, subsequent ones more specific.

If uncertainty formulate 3 definitions - possible/suspect, probable and confirmed cases.

Once decided, apply consistently to all persons being investigated.

A case definition should include

• • Clinical criteria –simple and objective (e.g. Fever 38 deg C)

  • Restricted by time (e.g. onset of symptoms in past 14 days)

  • Restricted by place (e.g. residents of village X)

  • Restricted by person (e.g. persons aged below 5 years))

Case definition should not include exposure or risk factor that you want to test (e.g. if your hypothesis is that curry A is associated with food poisoning, do not mention that in case definition).

Possible case – has few of typical clinical features

Example: A child aged 5-10 years (person characteristic) studying in 'PVD' school (Place restriction) with symptoms of abdominal pain and diarrhoea (clinical criteria) with onset of symptoms between 11-16th Jul (time restriction).

Probable case – has typical clinical features without laboratory confirmation

Example: A child aged 5-10 years (person characteristic) studying in 'PVD' school who had attended school picnic (Place restriction) and presenting with symptoms of abdominal pain, diarrhoea and blood in stools (clinical criteria) with onset of symptoms between 11-16th Jul (time restriction).

Confirmed / Definite case – clinical features and lab confirmation

A child aged 5-10 years (person characteristic) studying in 'PVD' school who had attended school picnic (Place restriction) and presenting with symptoms of abdominal pain, diarrhoea and blood in stools (clinical criteria) with onset of symptoms between 11-16th Jul (time restriction) and a positive stool culture report for E.coli (laboratory confirmaiton).

Having Possible, Probable & Confirmed Case Definitions can be useful

• In keeping a track of cases even when diagnosis is not established.

•  When lab results are awaited.

•  When test is costly and difficult to obtain (you may choose to rely on clinical features).

•  When an outbreak occurs in confined settings like ship, hostel etc, diagnosis is confirmed from a few affected persons and then all others having similar illness are identified as probable case.

Only few cases get reported, so you have to search for more cases.

To determine geographical extent of problem & population affected.

1. Direct case finding at facilities where diagnosis is likely to happen – hospitals, clinics, nursing home.

2. Alert the public through media - e.g outbreaks due to food product.

3. Survey the entire population if an outbreak occurs in a restricted population. e.g. school, ship, workplace.

4. Ask case patients if they know someone with similar illness.

Search for a period equal to two incubation period, since the occurrence of the last case.

1. Identifying information

Name for checking duplicate records.

Address  for the purpose of mapping. 

Address and Phone number to contact them for further questions or notifying them results and outcome of investigations.

2. Demographic information

Record age, sex, race, occupation etc. for characterising population at risk (person characteristics of descriptive epidemiology).

3. Clinical information

Record symptoms / signs, Date of onset, Other clinical information, duration of illness,hospitalisation,death etc. to verify if case definition is met. Date of onset will help you to chart epidemic curve. Spectrum of illness can be described based on this information.

4. Risk Factor information

Tells you about the incriminating factors. E.g. in case of diarrhoeal disease outbreak, enquire about exposure to food and water sources.

5. Reporter information

Record name, address, phone number of the physician,clinic, hospital, lab etc. from where report is obtained. This information can be useful at later stages if you need to seek additional clinical information or to report on  the results of your investigations.

Collect the data in a 

• Standard case report format or 

• Epidemiological case sheet

When data collected in epidemiological sheet is abstracted with selected critical items into a tabular form, it is called as a line list.

click here to learn more about time distribution of cases 

click here to learn more about maps in outbreak investigation 

Person characteristic to analyse will depend on the disease that is being investigated.

Example of Host characteristics that need to be described

• Age

• Sex

• Race

• Marital status

• Medical status

Example of Exposure factors that need to be described

• Occupation

• Leisure activities

• Use of medications

• Use of tobacco / drugs / alcohol

A hypothesis is a statement based on the data collected, as to what is the likely source of infection, the causative agent, the mode of spread of diseases and the exposure or environmental factors that led to the occurrence of the disease.

1. Information about the disease  - Reservoir of infection, routes of transmission, vehicles commonly implicated, known risk factors.

2. Epidemiological data of cases (Time, place and person distribution) - Is the epidemic curve showing narrow period of exposure, why only people in a particular area affected, why only some groups of people are affected etc. Are there any outliers in the data. 

3. Information gathered through talks with patients (reveals about risk factors) and local authorities (reveals local practices).

4. Information obtained through observations in field.

There are two ways of testing / evaluating hypothesis i.e.

1.    Compare with established facts

When clinical, laboratory, environmental or epidemiological evidence very obviously supports the evidence

e.g. Hypervitaminosis D outbreak in Massachusetts 1991

2.    Undertake analytical epidemiological studies

Cohort study – e.g. outbreak of gastro-enteritis in New York 1940

Case control study –e.g. Legionnaires disease, in Louisiana

Hypothesis may need to be reconsidered, refined or re-evaluated if

1. Analytical studies do not reveal anything.

Example an outbreak of food poisoning where a food curry was  postulated to be the reason for the outbreak. However the laboratory testing of the food samples do not reveal the causative micro-organism. Further studies will need to be undertaken to find out the source of exposure.

2 An association is proved, and you want to further narrow down an identify the responsible source.

Example an outbreak investigation identifies a source of exposure say a restaurant X. further studies can identify which food item was responsible.

Depending on the disease, the prevention and control measures will vary. Broady these measures can be classifed as measures against

1. source of infection

2. interrupt transmission

3. protect susceptible host

Examples of prevention and control measures

• Eliminate source of Infection or reservoir of infection 

  • Destroy mosquito breeding sites

  • Stop the infectious food handler from working in the kitchen

  • Sterilise contaminated water

  • Treat the individual and render the person non-infectious

• Interrupt transmission

  • Use personal protective equipment like N-95 mask

  • Isolate the infectious case

  • Quarantine the healthy contact of an infectious case

  • Use insecticide to kill vectors spreading diseases like malaria, dengue

• Protect susceptible host 

  • Vaccinate with COVID-!9 vaccine

  • Chemoprophylaxis against Malaria

• Text book of Park, 26th Edition

• Principles of EPIDEMIOLOGY  in Public Health Practice Third Edition An Introduction to Applied Epidemiology and Biostatistics - Self Studycourse SS-1978. U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention (CDC) Office of Workforce and Career Development Atlanta, GA 30333

https://www.cdc.gov/csels/dsepd/ss1978/index.html

• Principles of EPIDEMIOLOGY, An Introduction to Applied Epidemiology and Biostatistics. Second Edition. SELF-STUDY Course 3030-G. U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES, Public Health Service, Centers for Disease Control and Prevention (CDC), Epidemiology Program Office, Public Health Practice Program Office, Atlanta, Georgia 30333

• Epidemic curve

https://www.cdc.gov/training/quicklearns/createepi/

• Focus Epidemiology

https://sph.unc.edu/epid/focus/

• Outbreak Investigation | National Health Portal Of India (nhp.gov.in)