The Principles of Outbreak Epidemiology. By Dr Abhinav Sinha MBBS, MD. Author. Currently doing MD in Community Medicine at the Department of Community Medicine, NSCB Medical College, Jabalpur, India.
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Dr Abhinav Sinha MBBS, MD
During the last two and a half years of my postgraduation, I had an opportunity to witness and investigate 3 or 4 of different kinds of outbreaks ranging from Food poisoning to Gastrointestinal to Pyrexia of Unknown Origin to Cholera. What aroused an interest in Outbreak Investigation was the scientific cum artistic way of handling such outbreaks. It is more of an art than a pure science. When I came to know about the Supercourse, I decided to contribute or rather share my little experience with the world in the field of Outbreak Epidemiology. I hope you will enjoy this so called “lecture”.
The main motives behind this lecture are to develop the basic concept in investigating an epidemic, the need to recognize the urgency behind it and to orient the students toward the art and science of outbreak investigation and epidemiology.
After going through this lecture, the students should be able to perform the initial investigation of many smaller outbreaks that occur so much frequently in their vicinity that may often pass unrecognized. Also, they should be able to differentiate the epidemic and the endemic fluctuations in the frequencies of a disease.
Outbreak: Sudden occurrence of an epidemic in relatively limited geographic area. While an outbreak is usually limited to a small focal area, an epidemic covers larger geographical areas & has more than one focal point.
Outbreak Epidemiology: Study of a disease cluster or epidemic in order to control or prevent further spread of the disease in the population.
A definition has been proposed by
Goodman. The essential elements are:
The problem is unexpected
An immediate response may be necessary
Epidemiologists must travel to & work on location in the field
The extent of investigation is likely to be limited because of imperative for timely intervention
Primary- to control the spread of disease
To determine the causes of disease, its source & mode of transmission
To determine who is at risk
To determine what exposures predispose to disease
To know magnitude of the problem
6. To identify new agent
7. To determine the effectiveness of control measures
8. To identify methods for present & future prevention & control
9. Research & training opportunities
10. Public, Political and legal concerns
There is a pressure & urgency to conclude the investigations quickly which may lead to hasty decisions.
Data sources are often incomplete & less accurate.
Decreased statistical power due to analysis of small numbers.
Publicity surrounding the investigation – community members may have preconceived ideas.
There is often a strong tendency to collect what is “essential” in the field & then retreat to “home” for analysis. Such premature departure reflects lack of concern by the public, makes any further data collection or direct contact with the study population difficult. Once home, the team loses the urgency & momentum & the sense of relevancy of the epidemic. Don’t leave the field without final results & recommendations.
Clustering of cases/deaths in time/space
Unusual increase in cases/deaths
Shift in age distribution of cases
High vector density
Acute hemorrhagic fever or acute fever with renal involvement/altered sensorium
Severe dehydration following diarrhea in patients above 5 years age
Endemic diseases with epidemic potential – malaria, cholera, measles, hepatitis, meningococcal meningitis
Even a single case of diseases for which eradication/elimination goals have been set – polio, guineaworm and yaws
Rare but internationally important diseases with high case fatality rates – yellow fever
Outbreaks of unknown etiology
The basic general lines of action during epidemics include – Preparedness and Interventions (investigations). Success in dealing with an epidemic depends largely on the state of preparedness achieved in advance of any action. It would be an error to consider as an epidemic, a hitherto unrecognized endemic situation or a mere seasonal increase in the incidence of a disease.
Identify a nodal officer at state/district level
Strengthen routine surveillance system
Constitute rapid response teams
Train medical & other health personnel
Prepare a list of laboratories
List the “high risk” pockets
Establish rapid communication network
Undertake IEC activities
Ensure availability of essential supplies
Setup inter-departmental committees
Recognition & response to a request for assistance
Check initial information
Formulate a plan of action
Prepare for field work
Confirm the existence of epidemic
Verify the diagnosis
Identify & count cases/exposed persons
Orient data in terms of person, place & time
Choose a study design
Collect specimens for lab analysis
Conduct environmental investigations
Formulate & test hypotheses
Implement control measures
Conduct additional studies
Prepare a written report
Communicate the findings
If the local health officials request assistance, the regional epidemiologist should try to acquire as much information about the disease and the population at risk as possible. It is also important to find out why the request is coming – need extra hands?, unable to uncover the details of the disease in question?, share the responsibility?, or legal or ethical issues?
Step 2 – Check initial information: As soon as the initial information on an outbreak reaches, the regional health coordinator must determine whether the information is correct.
Step 3 – Formulate a plan of action: The plan should be based on situational analysis & taking technical, economical & political factors into account.
Step 4 – Prepare for field work: Identify the team members & assign responsibilities.
Are there cases in excess of the baseline rate for that disease & setting? The excess frequency should be found out with Epidemic Threshold Curve. The periodic frequency for previous 3 years is plotted on a graph. Another graph at mean + 2SD level is superimposed on it. Any fluctuations beyond these 2 graphs should be treated as epidemic fluctuations (method of moving averages).
The initial report may be spurious & arise from misinterpretation of the clinical features. This involves a review of available clinical & lab findings that supports the diagnosis. Do not apply newly introduced, experimental or otherwise not broadly recognized confirmatory tests at this stage. 15-20% of the suspected cases may be lab confirmed.
Identify additional cases not known or reported initially. The case definition must be precise but not too exclusive.. Persons who meet the case definition should be “line-listed”. Also, identify the population at risk or the exposed persons, places where the cases live, work & have traveled to, & the possible exposures that might have lead to the disease.
Search for the source of infection The main purpose here is to eliminate, terminate or isolate the source. The steps involved are – identify the time of disease onset, ascertain the range of incubation periods & look for the source in time interval between the maximum & the minimum IPs. In outbreaks with person-to-person transmission, all the contacts of the index case are to be searched (contact tracing).
Identify when patients became ill (time), where patients became ill (place) & what characteristics the patients possess (person). The earlier one can develop such ideas, the more pertinent & accurate data one can collect.
(a) Time: The epi-curvegives the magnitude of outbreak, its mode of spread & the possible duration of the epidemic. The unit of time on X-axis are smaller than the expected incubation period of the disease.
(b) Place: It provides major clues regarding the source of agent and/or nature of exposure. Spot maps show a pattern of distribution of cases.
(c) Person: Examine characters such as age, sex, race, occupation or virtually any other character that may be useful in portraying the uniqueness of case population.
The design (Case-control, Cohort & Case-cohort) is chosen based on size & availability of the exposed population, the speed with which the results are needed & the available resources. The study design that is chosen will then dictate the appropriate analysis & hypothesis testing.
It consists of collecting & testing appropriate specimens. To identify the etiologic agent, the collection need to be properly timed. Examples of specimens include - food & water, other environmental samples (air settling plates), and clinical (blood, stool, sputum or wound) samples from cases & controls.
A study of environmental conditions & the dynamics of its interaction with the population & etiologic agents will help to formulate the hypothesis on the genesis of the epidemic. Such actions assist in answering How? And Why? questions.
As soon as the preliminary data indicate the magnitude & severity of the outbreak, a hypothesis should be made regarding time, place and person; the suspected etiological agent & the mode of transmission. Risk specific attack rates are calculated & compared & relative risk/odds ratio is calculated.
1. Rare disease assumption: The OR & RR approximate each other if the attack rates is less than 5% but the attack rates are much higher in outbreaks.
2. To correct for multiple comparisons, the most effective approach is to lower the p-value according to the number of comparisons made.
Simultaneous to data collection & hypothesis formation, steps should be taken to contain the epidemic. These measures depend upon knowledge of etiologic agent, mode of transmission & other contributing factors. Protective measures are necessary for patients (isolation & disinfection), their contacts (quarantine) and the community (immunization, etc).
Because there may be a need to find more patients, to define better the extent of the epidemic, or because a new lab method or case finding method may need to be evaluated, the epidemiologists may want to perform more detailed & carefully executed studies.
The final responsibility of the investigative team is to prepare a written report to document the investigations, findings and the recommendations. The written report should be submitted, in a standardized format, to the public health authorities including the ministry of health & remain confidential until it has been given official permission.
Communicating the investigative findings clearly is essential. All public health officers will benefit if the experience acquired by the investigative team is shared by the publication of an account of the outbreak. As a rule, the epidemiologist informs those who reported the first cases of the epidemic first.
The efficacy of control measures should be assessed day by day during the outbreak, a final assessment being made after it has ended. This will provide a logical basis for post-epidemic surveillance & preventive measures aimed at avoiding the repetition of similar outbreaks.
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