Upload
utpal-sharma
View
405
Download
1
Embed Size (px)
Citation preview
Investigation of an Epidemic
By Dr Utpal Sharma
Assistant ProfessorDepartment of Community Medicine
SMIMS, Gangtok, Sikkim
The concept….EndemicDisease occurring in a population regularly at an
usual level Eg Tuberculosis, malaria
Epidemic Unusual occurrence of disease clearly in excess
of its normal expectancy. In a geographical location At a given point of time.
Eg: Hepatitis, Measles, Typhoid
Outbreak versus epidemic Occurrence of cases of illness in excess of the expected
numbers
Measure Outbreak Limited to a small area, within a district or a few blocks
Epidemic Covers larger geographic areas Linked to control measure instituted in the district/ state
Cluster Merely a group of cases in a given area over a particular
period without regard to whether the number of cases is more than expected.
No formal difference, you are the one
who decide how much importance
you want to give to the event.
Investigating an epidemic….why????
Epidemics or outbreaks signals some significant shift between existing balance between agent, host and environment
Calls for thorough investigation to uncover the factors responsible and…
…..Institute control measures to prevent further spread of the disease
Role for research into a new laboratory method to diagnose diseases why some are more susceptible, what risk factors do they have?
To find out the limiting factor for outbreak of disease for which an existing health programme is there.Keep the public’s confidence
Objectives of epidemic investigation
Define the magnitude in terms of time, place and person
Determine the particular conditions and factors responsible
To identify cause/sources of infection and modes of transmission to…
….Institute measures necessary to control the epidemic
To make recommendations to prevent recurrence
Salient feature of an epidemic investigation
Epidemic investigation is meant for Description as well as Intervention.
Generally investigations are done at the peak of the epidemic….hence largely Retrospective
The steps for investigation are not necessary to be undertaken in the same sequence…..
1. Verification of the diagnosis Initial report is not in the form of particular diagnosis but rather
in the from of a “syndromic” constellation of symptoms and signs
It is therefore essential to verify the diagnosis on the spot, as quickly as possible.
Clinical examination of a sample of cases should be done.
Laboratory investigation, where ever applicable boosts the confirmation of diagnosis
Epidemiological investigation should not be delayed until lab results available
It is not necessary to examine or
investigate all the cases to arrive at a
diagnosis
2. Confirm the existence of an
epidemic Done by comparing the disease frequencies the same
period of previous years (generally past 3 years)
Epidemic confirmed when observed cases are clearly in excess (more than 2SE) of that of previous experiences.
Some of the epidemics are easy to confirm eg. cholera, food poisoning…..…..whereas it is difficult to establish in case of modern epidemics eg. HTN, CVD without comparison
Pragmatic considerations are also important as to whether to investigate or not…… …a single case of a disease may be enough to call for investigations eg. Polio, plague
3. Defining the population at risk (Initial line-listing)
a) Obtaining map of the area Should contain information regarding natural landmarks,
roads and location of dwellings. Areas divided according to the landmarks and the
dwellings numbered
b) Counting the population Denominator is calculated of the affected area Health workers can be engaged for this. This is done to estabilish the population size of the area Population censes helps in computing the attack rates in
the groups and rest of the population subsequently.
4. Rapid search for all the cases and their characteristics
a) Medical survey Done to identify all the cases under medical attention and
also the exposed. Trained Health workers employed to collect data by
administering “epidemiological case sheets”
b) Epidemiological case sheet Designed based on rapid preliminary inquiry Used to collect relevant information on
IdentificationOccupation and social classTravel and history of exposureTime of onset and Signs and symptomsPersonal contactsEvents like attending parties or gatherings History of reciept of any injections, blood products…etc
Cont….c) Searching for more cases Patients were enquired about other case in the vicinity
having onset within the incubation period of the index case
Cases admitted should also be taken into consideration
this data reveals the situation with finding of additional cases and also explains person-to-person spread
Search of new (secondary) cases should be carried out everyday, till the epidemic is over
This period is taken as the twice of the Incubation period of the disease since the last case.
5. Data analysis Data so collected should be analyzed on ongoing basis
relating to….
a) Time Epidemic curve designed and time clustering of cases sought
out, it suggests…..i. Time relation with the exposure to sourceii. Type of epidemic( common source/ propagated)iii. Trend of epidemic (cyclic/seasonal)
Cont…
b) Place “Spot maps” prepared and if possible their relation to
the source of infection eg. Water supply Geographic distribution (clustering) of cases
suggests source and spread of disease.
c) Person Analysis of data according to age, sex, occupation
etc. is suggestive of susceptibility. Attack rates/ case fatality rates for exposed and non-
exposed determined according to the host factors
Data analysis is to identify common events and delineate the group involved
6. Formulation of hypothesis On the basis of Time-Place-Person distribution or…
…..Agent-Host-Environment model hypothesis is formulated to explain….
Possible source Causative agent Environmental factors responsible
Formulation of tentative hypothesis guides further investigation
7. Testing of hypothesis
All reasonable hypothesis should be considered and weighed comparing attack rates with each suspected factor
This helps to ascertain the hypothesis most consistent with all known facts
Divergent hypothesis should also be checked out.
8. Evaluation of the ecological factors
Investigation of the circumstances should be made to prevent further transmission
Possible ecological factors responsible for the epidemic should be investigated… Sanitary status of the eateries Water and food supply Population migration Atmospheric changes like air and water pollution, temperature
and humidity Vector bionomics and animal reservoir
Evaluation of the ecological factors helps to relate disease to the environmental factors to know the sources of infection, reservoir and mode of transmission
Out break can be studied in a case
control manner
9. Further investigation of population at risk
This involves study o a population at risk or a sample of it for more information.
Involves medical examination, screening tests, biochemical examination, examination of suspected food, faeces or blood sample etc.
The approach may either be retrospective or prospective
Investigation of the healthy individuals will disclose the classification of all members to…a) Exposure to potential vehicleb) Whether ill or not
10.Writing the report
The report should be complete and convincing.
Final reporting should be done under following headings
Thank you….will meet soon