Investigation of Epidemic NOTES

INVESTIGATION

OF

 EPIDEMIC

CONTENTS

INTRODUCTION
DEFINITION
OBJECTIVES
STEPS IN INVESTIGATION OF EPIDEMIC
CONCLUSION

INTRODUCTION

• Epidemiology is a scientific study of factors, conditions related to disease as they occur in people.
• The word epidemic (epi = among, demos= people).
• Epidemiology has an important role to play in the investigation of an epidemic.
• Epidemic investigation is a set of procedures used to identify the cause i.e, the infectious agent responsible for the diseases, it is also used to identify the people affected.

DEFINITION

• PARKIN (1873) defines epidemiology as “the branch of medical science which deals with the treatment of epidemics”.

• Epidemic is an outbreak of disease that attacks many people at about same time and may spread through one or several communities.

OBJECTIVES OF INVESTIGATION OF EPIDEMIC

• To define the magnitude of the epidemic outbreak or involvement in terms of time, place and person.
• To determine the particular conditions and factors responsible for the occurrence of the epidemic.
• To identify the cause, source of infection, and modes of transmission to determine measures necessary to control the epidemic; and
• To make recommendations to prevent recurrence.

STEPS IN INVESTIGATION OF EPIDEMIC

• Verification of diagnosis 
• Confirmation of existence of an epidemic
• Defining the population at risk
• Rapid search for all cases and their characteristics
• Data  analysis
• Formulation of hypotheses
• Testing of hypotheses
• Evaluation of ecological factors
• Further investigation of population at risk
• Writing the report

VERIFICATION OF DIAGNOSIS

• Verification of diagnosis: First step in an epidemic investigation. 
• Spurious, and arise from the misinterpretation of signs and symptoms by the lay public. 
• Not necessary to examine all the cases to arrive at a diagnosis.
• A clinical examination of a sample of cases may well suffice. 
• Laboratory investigations wherever applicable, are most useful to confirm the diagnosis but the epidemiological investigations should not be delayed until the laboratory results are available.

CONFIRMATION OF EXISTENCE OF AN EPIDEMIC

• The next step is to confirm if epidemic exists.
• This is done by comparing the disease frequencies during the same period of previous years.
• An epidemic is said to exist when the number of cases (observed frequency) is in excess of the expected frequency for that population, based on past experience. 
• Often the existence of an epidemic is obvious needing no such comparison, as in the case of common-source epidemics of cholera, food poisoning, and hepatitis A.
• These epidemics are easily recognized. In contrast, the existence of modern epidemics (e.g., cancer, cardiovascular diseases) is not easily recognized unless the comparison is made with previous experience.

DEFINING THE POPULATION AT RISK

A) Obtaining a map of the area :

• Before beginning the investigation, it is necessary to have a detailed and current map of the area.
• If this is not available, it may be necessary to prepare such a map. 
• It should contain information concerning natural landmarks, roads and the location of all dwelling units along each road or in isolated areas.
• The area may be divided into segments, using natural landmarks as boundaries.
• This may again be divided into smaller sections.
• Within each section, the dwelling units (houses) may be designated by numbers. 

B) Counting the population :

• The denominator may be related to the entire population or sub-groups of a population.
• It may also be related to total events.
• For this purpose lay health workers in sufficient numbers may be employed.
• Using this technique it is possible to establish the size of the population.
• The population census will help in computing the much-needed attack rates in various groups and subgroups of the population later on.
• Without an appropriate denominator of “population at risk” attack rates cannot be calculated. 

RAPID SEARCH FOR ALL CASES AND THEIR CHARACTERISTICS

A)Medical survey :

• Concurrently, a medical survey should be carried out in the defined area to identify all case including those who have not sought medical care, and those possibly exposed to risk.
• Ideally, the complete survey will pick up all affected individuals with symptoms or signs of the disorder. 
• Lay health workers may be trained to administer the "epidemiological case sheet or questionnaire to collect relevant data. 

B)Epidemiological case sheet:

• The epidemiologist should be armed with an "epidemiological case sheet" for collecting data from cases.
• The epidemiological case sheet or "case interview form" should be carefully designed to collect relevant information.
• The information collected should be relevant to the disease under study.

C)Searching for more cases:

• The patient may be asked if he knew of other cases in the home, family, neighborhood school, work place having an onset within the incubation of the index case. 
• Cases admitted to the local hospitals should also be taken into consideration.
• This may reveal not only additional cases but also person-to-person spread. 

DATA ANALYSIS

• The data collected should be analysed on ongoing basis using the classical epidemiological parameters

1. time
2. place
3. person

Time: 

• Prepare a chronological distribution of dates of onset and construct an "epidemic curve".
• An epidemic curve may suggest

1. Time relationship with exposure to a suspected source
2. Whether it is a common-source or propagated epidemic.
3. Whether it is a seasonal or cyclic pattern suggestive of a particular infection.

Place :

• Prepare a "spot map" (geographic distribution of cases, and if possible, their relation to possible sources of infection.
• Clustering of cases may indicate a common source of infection. 
• Analysis of geographic distribution may provide evidence of the source of disease and its mode of spread. 
• This was demonstrated by John Snow in the cholera outbreak in the Golden Square district, London. 

Person:

• Analyse the data by age, sex, occupation and other possible risk factors.
• The purpose of data analysis is to identify common event or experience and to delineate the group involved in the common experience. 

FORMULATION OF HYPOTHESIS

On the basis of time, place and person distribution or the agent-Host-Environment model, formulate hypotheses to explain the epidemic in terms of
a) possible source 
b) causative agent 
c) possible modes of spread
d) the environmental factors which enabled it to occur

• Formulation of a tentative hypothesis should guide further investigation.
• Testing of hypotheses: All reasonable hypotheses need to be considered and weighed by comparing the attack rates in various groups for those exposed and those not exposed to each suspected factor.
• This will enable the epidemiologist to ascertain which hypothesis is consistent with all the known facts.
• Therefore it is instructive to turn back to arguments which have been tested by the subsequent course of events. 

EVALUATION OF ECOLOGICAL FACTORS

• An investigation of the circumstances involved should be carried out to undertake appropriate measures to prevent further transmission of the disease. 
• Ecological factors which have made the epidemic possible should be investigated such as sanitary status of eating establishments, water and milk supply:   atmospheric changes such as temperature, humidity and air pollution, population dynamics of insects and animal reservoirs. 
• Further investigation of population at risk:

1. A study of the population at risk or a sample of it may be needed to obtain additional information. This may involve the medical examination, screening tests, examination of suspected food, feces or blood samples, biochemical studies, assessment of immunity status, etc. 
2. The approach may be retrospective or prospective. 

WRITING THE REPORT

• The report should be complete and convincing
• The report provides local public health departments an explanation of the parameters and the epidemics cause which enables timely and effective public health action.
• Report should identify the risk factors that resulted in the epidemic.
• Also report can be distributed to other public health professionals to help prevent a future similar problem. 

CONCLUSION

• 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. 

REFERENCES

1. SOBEN PETER
2. PARK

Updated post:
Investigation of Epidemic NOTES (Repost)