4=checklist for CRRT for outbreak investigation, NICD 2008
The term ‘cluster’ is an aggregation of two or more cases which is not necessarily more than expected.
Magnitude : Measurement of epidemic in terms of morbidity, mortality , loss of productivity, psychological influence on population affected.
Epidemic curve: a graphic plotting of distribution of cases by time of onset.
Limitations: delay in initial report. Improper records. Problems: No evaluation. No follow up. Surveillance system should have been started.
Blood samples from 5 recent and 4 convalescent cases
Late start d/t late info.Investigation not evaluated. No system created for early detection of outbreakRecommendation of sewer and water line distance not practical ???No follow up
outbreak investigation - types of epidemics and investigating them
Presented by: Dr. Timiresh Kumar Das
Moderator: Dr. Neelam Roy
Dept. of Community Medicine
VMMC & Safdarjung Hospital
Determinants of disease outbreaks
Types of epidemics
Objectives of investigation of outbreak/ epidemic
Steps of outbreak investigation
Epidemic: [Greek: epi (upon) demos (the people)]
The occurrence in a community or region of cases of an
illness, specific health related behaviour, or other health
related events clearly in excess of normal expectancy.1
A Dictionary of Epidemiology – 3rd ed; Last JM. 2000.
The “unusual” occurrence in a community or region of
disease, specific health related behaviour, or other health
related events clearly in excess of “expected occurrences”.2
Park’s Textbook of Preventive and Social Medicine – 21st ed; Park JE. 2010
The occurrence in a community or region of a group of
illnesses of similar nature, clearly in excess of normal
expectancy, and derived from a common or from a
Epidemiology – 4th ed; Gordis L. 2004
An epidemic limited to a localised increase in the incidence
of a disease, e.g., in a village, town, or closed institution. (=
A Dictionary of Epidemiology – 3rd ed; Last JM. 2000.
A term used for a small, usually localised epidemic in the
interest of minimising public alarm.2
Park’s Textbook of Preventive and Social Medicine – 21st ed; Park JE. 2010
An outbreak is the occurrence of illness, specific health
related behaviour, or other event clearly in excess of
normal expectancy in a community in a specified time
period. An outbreak is limited or localised to a village,
town, or closed institution.4
Checklist for CRRT for outbreak investigation, NICD 2008
According to the Oxford Textbook of Public Health the criteria for
judging that an outbreak has happened can be one of the
The occurrence of a greater number of cases or events than normally
occur in the same place when compared to the same duration in past
A cluster of cases of the same disease occurs which can be linked to
the same exposure.
E.g. Kaposi's sarcoma, New York - 30 in 1981; only 2-3 previous yrs.
E.g. 3 athletes admitted with acute febrile illness after triathlon in
Springfield, Illinois. Triathlon related to illness. Leptospira.
A single case of disease that has never occurred before or might have a
significant implication for public health policy and practice can be
judged an outbreak which deserves to be investigated.
E.g. - Avian flu (H5N1) Hong Kong in a 3-year boy in May 1997 alerted local
auth. and scientists around the world to start a full-scale investigation.
Endemic: The constant presence of a disease or
infectious agent within a given geographic area or
population group, without importation from outside;
may also refer to the usual or expected frequency of
the disease within such area or population group.
A Dictionary of Epidemiology – 3rd ed; Last JM. 2000.
Pandemic: An epidemic usually affecting a large
proportion of the population, occurring over a wide
geographic area such as a section of a nation, the
entire nation, or a continent or the world.
A Dictionary of Epidemiology – 3rd ed; Last JM. 2000.
When formerly isolated populations are exposed to disease.
19th century – measles in Faroe islands.
When susceptible population is introduced to an endemic
area – streptococcal sore throat outbreaks when new recruits
arrived at Great Lakes Naval Station, USA.
Herd Immunity: The immunity o f a group or
community. The resistance of a group to invasion
and spread of an infectious agent, based on the
resistance to infection of a high proportion of the
individuals members of the group.1
When a large proportion of the population is
immune, the entire population is likely to be
protected, not only those who are immune.
Degree depends on –
Extent of random mixing of the population
Agent and transmission characteristics.
Number of susceptibles and immunes in the population.
Expected number of cases
if effect limited to vaccines
Number of cases observed
Incubation Period: Interval from receipt of infection
to the time of onset of clinical illness.1
Important in case of isolating infected people to prevent
Isolation or quarantine should be greater than maximum
Useful if disease may be introduced into new areas.
Quarantine: The restriction of activities of well
persons or animals who have been exposed to a case
of communicable disease during its period of
communicability (i.e. contacts) to prevent disease
transmission during incubation period if infection
From quarante giorni (40 days).
Plague (Black Death) Europe, 1374 – Venetian Republic
1377, Ragusa detained travellers in an isolated
area, initially for 30 days and, when it did not work, for 40
or point source epidemics
Continuous or multiple exposure epidemics
Slow (modern) epidemics
Point Source: A point source outbreak occurs
when there is one single source that exists for a
very short time and all cases have common
exposure to it in that same particular period.
Ex: food poisoning
Common source single exposure
First case and the last case happen within one
Rapid rise in number of cases followed by rapid decline
due to spoiled food item in a feast.
Bhopal gas tragedy ( 198
Common source multiple exposure: There is only one
source, which provides continuous or intermittent
exposure over a longer period
Prostitute transmitting STD to her clients
Water supply contamination due to leaky pipes.
if leak is constant.
Intermittent if leak occurs during pressure variations.
Propagated epidemic: This
kind of outbreak is
caused by a transmission from one person to
Example: SARS, H1N1 influenza
To define the magnitude
To determine the particular conditions and
factors responsible for the occurrence of the
To identify the cause, source of infection, and
modes of transmission
To formulate prevention and control measures
To control the current outbreak.
Prevention of future outbreaks.
Describe new diseases and learn more about known
Evaluation of the effectiveness of prevention
Evaluation of the effectiveness of the existing
Training health professionals.
Responding to public, political, or legal concern .
The first person who comes across news
of an outbreak / The health worker/ ANM
PHC medical officer / the CHC in charge
The District health officer / District RRT
or DEIT/ State RRT
Specialized agencies like NCDC (NICD).
1. Verification of the diagnosis
2. Confirmation of existence of outbreak
3. Defining population at risk – Map, Count
4. Rapid search for cases and characteristics
5. Data Analysis – Time, Place, Person
6. Formulation of hypothesis
7. Testing of hypothesis
8. Evaluation of ecological factors
9. Further investigations
Technical, Administrative and Logistics
Prepare Outbreak Management Kit according to
Brief members of the investigating team regarding
Roles & responsibilities
Methods of personal protection
Team composed of:
1. Nodal officer (Epi/ PHS)
4. Health Assistant
5. District/ Local administration
6. Other personnel as required
Verification of the diagnosis is usually made on
clinical and laboratory parameters.
Ensure that the problem has been properly
diagnosed -- the outbreak really is what it has been
reported to be
clinical findings and laboratory results for
Visit or talk to several of the people who became ill
For outbreaks involving infectious or toxic chemical
agents, be certain that the increase in diagnosed
cases is not the result of a mistake in the
Incidence rate is calculated by dividing the total cases
by the population at risk.
7 ye ar ave rage incide nce vs . 2003 incide nce of D e ngue in
D e lhi
n o . o f c as e s
This rate is
compared with the
rate occurring in
period of the
months of reporting
Clustering of cases or deaths
Increases in cases or deaths
Single case of disease of epidemic potential
Acute febrile illness of an unknown etiology
Two or more linked cases of disease with outbreak
potential(e.g., Measles, Cholera, Dengue, Japanese
encephalitis or plague)
Unusual isolate (Cholera O 139)
Environmental factors e.g. rainfall, climate
Shift in age distribution of cases
High vector density
To be kept in standardized format in each
Private and public sector
Important source of information, not to neglect
Review of routine data – surveillance data
Triggers (There are triggers for each condition under
surveillance, Various trigger levels may lead to local or
Threshold for diseases under surveillance that trigger pre-
determined actions at various levels
Based upon the number of cases in weekly report
Trigger levels depend on:
Type of disease
Case fatality (Death / case ratio)
Number of evolving cases
Usual trend in the region
Levels of response
Suspected /limited outbreak
• Local response by health
worker and medical officer
• Local and district response
by district surveillance
officer and rapid response
• Local, district and state
Wide spread epidemic
• State level response
• Local, district, state and
Trigger levels for Dengue
• Clustering of 2 similar case of probable Dengue fever in a village
• Single case of Dengue hemorrhagic fever
• More than 4 cases of Dengue fever in a village 1000 population.
Triggers for syndromic surveillance
• More than 2 similar case in the village (1000 Population)
• More than two cases of jaundice in different houses irrespective
of age in a village or 1000 population
responded to by
2nd week (ending
• Severity of illness
• Number of cases
• Source / mode of transmission
• Availability of preventive & control measures
• Availability of staff & resources
• Public, political and legal concerns
• Public health program considerations
•Potential to affect others if the control
measures are not taken
• Research opportunity
in the numerator:
Reporting of prevalent cases as incident cases (e.g.;
hepatitis C, sleeping sickness)
Variation of the denominator:
Rapidly changing population denominators
Migrants or refugees
Obtain a map of the area
Counting of the population
Helps to calculate the denominator for further
calculation of attack rates.
Provides us with the possible number of people at
Mapping helps us to know area: ecological and
Detailed, Current map of the area.
If not available – prepare
Information: Natural landmarks, Roads, All dwelling
units, Sources of water, Other important features
Census by age and sex
Lay health workers
House to house visits
Includes: Framing a case definition, searching for
cases and doing a epidemiological survey.
A case definition is a standard set of criteria for
deciding whether an individual should be classified as
having the health condition of interest.
Clinical and/or biological criteria,
Case definition should be
balanced, practical, reliable and applied without bias.
Example - Measles: 3 definitions
Fever and runny nose
Too many other illnesses produce same symptoms
Call many illnesses “measles”
Fever and rash and Koplik’s spots and conjunctivitis
Many cases of measles do not have all these signs
Miss many real cases of measles
IDSP case definition: Fever of 3-7 days duration, with
generalized maculopapular rashes; with history of cough, coryza,
conjunctivitis or Koplik’s spots.
Suspect -Fewer of the typical clinical features
Probable- Typical clinical features of the disease
without laboratory confirmation.
Confirmed- Typical clinical features with laboratory
Example: E. coli O157 outbreak at
Restaurant X on 31/3/2010
Possible: diarrhea (3 loose stools per day) and
ate food purchased at restaurant X on 31/3
Probable: bloody diarrhea and ate food
purchased at restaurant X during on 31/3
Confirmed: culture positive with “outbreak”
PFGE pattern and ate food purchased from
restaurant X on 31/3
Case Definition may need to be updated within
Broad to specific
with E. coli O157 vs. infection with the outbreak
strain (defined by PFGE pattern)
Location of exposure
SARS outbreak (travel
within 10 d of onset):
• In February: China/HK/Hanoi/Singapore
• In April: Toronto, Canada added
• In May: Taiwan added
Dates of exposure can change
outbreak: to meet the case definition-dates of
exposure dependant on location of exposure
The first cases to be recognized are usually only a small
proportion of the total number
To identify other cases, use as many sources possible
Passive Surveillance - Relies on routine notifications by
Active Surveillance - Involves regular outreach to potential
reporters to stimulate reporting of specific conditions;
investigators are sent to the afflicted area to collect more
Contact physician offices, hospitals, schools to find
persons with similar symptoms or illnesses
Send out a letter, telephone or visit the facilities to collect
Through media alerts asking people to get checked
The information is collected by “line listing”.
A line list is like a nominal roll of the cases being reported
to the various health care establishments (like
dispensaries, general practitioners or admitted to the
Constitutes and updates a database of cases
Done by hand or by Excel.
The survey team will go for “door to door” survey
in the affected area and ask if any person had
suffered with symptoms fitting into case definition
(Rapid Household Survey)
If yes, their details were recorded on the
epidemiological case - sheet and required samples
are taken and dispatched to the hospital/ reference
Epidemiological case sheet = Case interview form
Detailed information from the case relevant to the
disease under study.
• Name, Age, Sex, Occupation, Social class
• Time of onset of disease, Signs & Symptoms
• Personal contact at home, work, school
• Travel history, attendance at large gatherings
• History of previous exposure/injections,
• Special events such as parties attended, foods
eaten, and exposure to common vehicles such as
water, food and milk
Active door-to-door collection of information is by
“ Rapid Household Survey”
Characterizing an outbreak by time, place and
person is called descriptive epidemiology.
Descriptive epidemiology is important because:
What is reliable and informative (e.g., similar exposures)
What may not be as reliable (e.g., many missing
responses to a particular question)
Provides a comprehensive description of an outbreak by
showing its trend over time, its geographic extent (place)
and the populations (people) affected by the disease
distribution of cases
according to host
(age, race, sex)
activities, use of
Count the cases in each age and sex groups
Obtain census denominators for each age and sex groups
Estimate the incidence for each:
An attack rate is the proportion of a well-defined
population that develops illness over a limited
period of time, such as during an epidemic or
What are the age and gender specific attack rates?
What age and gender groups are at highest and
lowest risk of illness?
In what other ways do the characteristics of the
cases differ significantly from those of the
Purpose => Identification of sub-group(s) at risk
Attack rate of measles by age and
sex, Cuddalore, Tamil Nadu,
Number of Population
Cases of Botulism
BOTULISM OUTBREAK IN CAIRO, EGYPT - APRIL1993
Weber JT, Hibbs RG Jr, Darwish A, et al. A massive outbreak of type E botulism associated with
traditional salted fish in Cairo. J Infect Dis 1993; 167: 451-454
What is the exact period of the outbreak?
What is the probable period of exposure?
Is the outbreak likely common source or
Drawing the epidemic curve based on time distribution of cases
to choose interval
1. Count cases by
time of onset
Interpretation of epidemic curve
Shape – type of epidemic
An early case in the curve may represent source of the
Give information about the time course of an epidemic
and what the future course might be
In a point-source epidemic of a known disease with a
known incubation period, epidemic curve provides
information to identify a likely period of exposure
Shape of epidemic curve illustrates type of epidemic.
Propagated source outbreak:
Number of cases
Increase in the number of cases with progressive peaks
7 10 13 16 19 22 25 28 31 34
The spatial relationships
of cases are shown best
on a spot map.
A spot map showing
the location of cases can
give an idea of the
source of infection like
maps show that the
cases occurred in
proximity to a body of
water, a sewage
treatment plant, or its
DRAWING A ROUGH SPOT MAP
Questions to be asked and answered:
What is the most significant geographic distribution
Place of residence? Workplace?
Do the attack rates vary by place?
Relation to any landmark or possible source?
Usually we generate hypotheses from the beginning of
the outbreak, however, at this point, the hypotheses
are sharpened and more accurately focused.
To consider what is known about the disease itself:
What is the agent’s usual reservoir?
B. How is it usually transmitted?
C. What vehicles are commonly implicated?
D. What are the known risk factors?
Talk to people who are ill: In-depth open
interviews, Group discussions
Hypotheses should be proposed in a way that they
can be tested.
After a hypothesis is formulated, one should be able to
all additional cases, lab data, and epidemiologic
evidence are consistent with the initial hypothesis
no other hypothesis fits the data as well
Observations that add weight to validity:
The greater the degree of exposure (or higher dosage
of the pathogen), the higher the incidence of disease
Higher incidence of disease in the presence of one risk
factor relative to other factors
Comparison of hypothesis with known/ established facts.
Analytic epidemiology to test the hypothesis
First method is used when the evidence is so strong
that hypothesis need not be tested
Example - A 1991 investigation of an outbreak of vitamin D
intoxication in Massachusetts.
All affected drank milk from local dairy.
Hypothesis - dairy was source, milk vehicle of excess vit D.
Visit to dairy, they quickly recognized that far more than
the recommended dose of vitamin D was inadvertently
being adding to the milk.
No further analysis was necessary.
Analytic epidemiology is used when cause is less
Case control studies
What to use?
Rare disease/ large community
Common disease/ small community
Complete population accessible
Large amount of resources
Case control study for an outbreak of Acute Gastroenteritis
following a dinner.
ATE and ILL
ATE and NOT ILL
These are additional studies undertaken to
corroborate the findings of the epidemiological
Environmental studies often help explain why an
outbreak occurred and may be very important in
Example: Site of contamination of irrigation canal with
cattle urine in an outbreak of Leptospirosis in
southeastern Washington, August 1964.
Pond connected to
Cattle around the site
Microbiological studies can clinch the relationship
between suspected source and outbreak.
Example: In the above outbreak of Leptospirosis, culture
of urine from the cattle, water of the canal and blood of
affected children yielded the same strain of L. pomona.
Also, the children who had recovered showed increased
anti leptospiral antibodies.
Entomological studies help identify the vector
responsible for the outbreak.
May also give useful insight into the life cycle of the
pathogen and the mode of transmission
affected villages of
Latur and Beed
Additional epidemiologic studies
What questions remain unanswered about the disease?
What kind of study used in a particular setting would
answer these questions?
When analytic studies do not confirm the hypotheses
new vehicles or modes of transmission
Further lab studies
Refine hypothesis till confirmation
Report provides a blueprint for action.
It also serves as a record of performance and a
document for potential legal issues.
It serves as a reference if the health department
encounters a similar situation in the future.
In public health literature serves the purpose of
contributing to the knowledge base of epidemiology
and public health
Daily interim reports and final report.
Contents of a Report:
1. Back ground
2. Historical data
3. Methodology of investigation
4. Analysis of data (clinical data, Epidemiological
data, modes of transmission, Lab data,
Interpretation of data)
5. Control measures
7. Preventive measures
Report to be submitted
officer/ team to the
next higher authority
within one week of
Appropriate control measures
based on Epidemiological, Clinical, Environmental
To prevent further spread of disease
The elements of epidemic control include:
1. Controlling the source of the pathogen (if known)
Remove or inactivate the pathogen
2. Interrupting the transmission.
Sterilize environmental source of spread; vector control
3. Modifying the host response to exposure.
Immunization; Prophylactic chemotherapy
Control measures should be implemented at
DO NOT wait for laboratory confirmation to
start control measures.
Follow up of outbreak
Detect last case
Detect and treat late complications
Evaluation of outbreak management including
investigations (by local authorities)
Genesis of outbreak
Early or late detection of outbreak
Preparedness for outbreak
Management of the outbreak
Control measures taken and their impact
Documentation and sharing the lessons learnt
Post outbreak seminar.
Feedback to local health authorities, RRTs and other
Developing case studies on selected outbreaks for
Focus of the
•Evidence that the
situation is under
POSSIBLE MATRIX FOR COMMUNICATION OF
INVESTIGATION RESULTS AND FINDINGS
An outbreak of fever, URI & loose motion among the
boarders of PTS, Jharoda kalan, Delhi was reported to
the MS of SJH by CDMO of SW district of Delhi on
Cause for concern – Panic d/t novel H1N1 cases in Delhi
RRT composition –
Epidemiologist, Physician, Microbiologist and other
Case definition: A person with acute onset of fever with
or without sore throat, diarrhea, headache, body ache
starting from 2nd July 2009 onward.
Line listing, Epidemiological case sheet (with travel
history), Lab analysis of samples for H1N1.
Time – Start = 2nd Jul, Peak = 7th Jul, Fall afterwards
Place – Start in Tent # 40, 25 & 8; then spread. clustered
around tent no 1,9,20, 22, 27, 36 &37.
Person – 61 cases. Mean age 22.2 yrs (20-49).
Environmental: Crowded, ill ventilated tents. Humid
environment with low temperature.
Lab: H1N1 negative. Influenza A +ve.
Reduce crowding, Improve ventilation
Increase staffing in dispensary and Proper record maintainance
Prompt identification and reporting of changes in disease
Outbreak of jaundice among the residents of Sector 8 of
RK Puram, New Delhi was reported to the MS of SJH by
CDMO of SW district of Delhi on 06.04.2011.
Initial report by DSO suggested sudden onset of
RRT – Epidemiologist, Physician, Microbiologist and
other personnel. To CGHS, dispensary on 06.04.2011
Case defn : A person with signs and symptoms of
jaundice with or without elevated serum
aminotransferase levels from 1st January 2011 onwards
Verification of outbreak by review of records of CGHS
Rapid survey, Line listing, Spot map, Clinical
examination, Epidemiological case sheet, Blood
samples, Environmental study
Time – Rise from 15th Jan, Peak 1st week March, Decline
afterwards. Max cases in March (11/21)
Place – Clustering around N block & adjacent to Palam rd
Person – 15-30 yrs (50%), M > F (58.3/ 41.7)
Lab: 3/5 recent cases +ve for Anti HEV IgM.
2/6 water samples – Fecal contamination
Environmental: Water & sewer lines running
close, Intermittent water supply – Booster
pumps, Latrine near water storage tank, Sewer lines not
de-silted – overflowing, Damaged water lines. Absent
Conclusion: Confirmed outbreak of jaundice. Lab results
Acute Hepatitis E. Damaged water lines and
contamination from sewer lines responsible.
Proper record maintenance in CGHS dispensary(diagnosis, S/s)
Monitoring and repairing of water lines
Sewer lines should not be close to water supply pipeline
Regular de-silting and cleaning of sewer line.
Proper chlorination of water supply.
1. A Dictionary of Epidemiology – 3rd ed; Last JM.
Park’s Textbook of Preventive and Social Medicine
– 21st ed; Park JE. 2010.
Epidemiology – 4th ed; Gordis L. 2004.
Checklist for CRRT for outbreak investigation, NICD
Oxford Textbook of Public Health – 4th ed; 2002
Mausner & Bahn Epidemiology: An Introductory
Text – 2nd ed; Mausner JS, Kramer S. 1985.
7. R Bonita, R Beaglehole, T Kjellström. Basic
Epidemiology: WHO;2nd Edition.
8. Outbreak Investigations Around The World: Case
Studies in Infectious Disease Field Epidemiology;
Mark S Dworkin. 2010
9. Steps of outbreak investigation; Epidemiology in
the classroom. Excite, CDC. From www.cdc.gov
10. Raut DK, Roy N, Nair D, Sharma R. Influenza A virus
outbreak in Police Training School, Najafgarh, Delhi
– 2009. Indian J Med Res; Dec 2010; 132: 731-732
LARGE OUTBREAKS FROM AROUND THE WORLD –