The early days…John Snow and the Broad Street Pump

BaSiCs OF EpidEmioLoGy

-Dr Preetam ‘08 BATCH

Definition of epidemiology

It is defined as “ study of distribution & determinants of health related states or events in a specified population, & application of this study to control of health problems”.

Aims of EpidemiologyDescribe distribution &

magnitude of health & diseaseTo identify etiological factors in

pathogenesis of diseaseTo provide data for planning

implementation & evaluation of services for the prevention control & treatment of diseases

Epidemiology Vs Clinical medicine.Basic difference between

epidemiology & clinical med is UNIT OF STUDY is ‘Defined population’ & case or ‘patient’

Epidemiology is thus concerned with both sick and healthy.

Clinician statistician & epidemiologist

In clinical med the physician seeks a diagnosis from which he derives the prognosis & prescribes specific treatment

In epidemiology the epidemiologist has data from which he derives the aetiology & determines the disease trend and gives various control measures.

In clinical med the patient comes to the doctor in epidemiology the investigator goes to the communityto find risk factors or etiology.

Epidemiological Aproach

Asking Questions Making Comparisons

Basic Measurements in Epidemiology

Measurements of MortalityMorbidityDisabilityNatalityMeasurements of demographic

variablesMeasurement of health care

facilities, utilization of health services.

Tools of measurementRatesRatios &ProportionsRates comprises of numerator

denominator time specification and a multiplier Death rate=no of deaths in 1 yr/mid yr population* 1000

Ratio- is relation in size between two random quantities. Wbc:rbc, sex ratio etc

Proportion

Is a ratio which indicates relation in magnitude of a part of the whole.

Types and sources of data1. “Primary data”- data collected by an

investigator in the field, through cross section or longitudinal survey.

2. “Secondary data”- data obtained from published or unpublished sources eg. Medical records , private practitioners & survey reports namely National Family Health Survey, Reproductive & Child Health Report, textbooks , journals, annual & other reports published by various health agencies.

Community Diagnosis

It is defined as “ process of appraising the health status of a community, including assembly of vital & other health related statistics & of information pertaining to determinants of health, such as prevalence of tobacco smoking & examination of these determinants of health in the specified community”.

The community diagnosis is based on collection & interpretation of the relevant data such as

1. Age & sex distribution of population by social groups.

2. Vital statistical rates such as birth rate & death rates.

3. The incidence & prevalence of the imp. Diseases of the area

Clinical vs Community Diagnosis CLINICAL

DIAGNOSIS

1. Individual is the unit.

2. Made on the basis of clinical history, signs & symptoms & lab. Investigations.

3. It is made in terms of anatomical, physiological & pathological condition of pt.

COMMUNITY DIAGNOSIS

1. Study populations or subgroups of the population is the unit.

2. Made by collection, compilation & interpretation of data gathered from community.

3. Made in terms of rate or ratio , etc.

DISEASE TRENDS

Secular or long term trendIt implies changes in occurrence of disease

over a long period of time generally several years or decades.

A secular trend implies a consistent tendency to change in a particular direction or definite movement in one direction eg coronary heart ds. , lung cancer & DM, which have shown a consistent upward trend.

And declining trend of T.B, typhoid, & diphtheria during past 50 yrs.

Cyclical trend

Some diseases occur in cycles spread over short period of time which may be days, weeks, months or years.

Eg- measles in pre-vaccination era appeared in cycles with major peaks every 2-3 yrs.

Seasonal trend

Seasonal variation is well known characteristic of many communicable diseases eg- URI increased during winter, GIT infections in summer & malaria during rainy season.

INVESTIGATION OF AN EPIDEMIC

Need for epidemic investigation

The primary objective of an epidemic investigation is to control the outbreak, limit its spread to other areas & assess how preventive strategies can be further strengthened to reduce or eliminate the risk of such outbreaks in future.

Diseases requiring investigations1. Endemic diseases with the potential of

causing focal or large outbreaks eg- malaria , cholera

2. The diseases for which eradication or elimination goals have been set. A single case such disease should be treated as an outbreak eg- polio, guinea worm.

3. Rare but internationally important diseases with higher case fatality rates eg- yellow fever.

Definition of an OutbreakAn outbreak or epidemic is defined as

the occurrence in a community of an illness clearly is excess of expected numbers.

Outbreak is usually limited to a small focal area, an epidemic covers larger geographic areas & has more than one focal point

The number of cases which is exceeded to be called an outbreak, varies according to several factors. It depends on past historical patterns of the disease , case fatality, complication rates & potential for spread to other areas

For some diseases even a single case (polio, guinea worm) constitutes an outbreak.

Warning signals for outbreakClustering of cases of deaths in time and /

or space.Unusual increase in deaths.Acute hemorrhagic fever.Acute fever with altered sensorium, renal

involvement.Acute flaccid paralysis in a child.Even a single case of measles or any

other epidemic prone disease from a tribal or other poorly accessible area.

Natural disasters.

Steps of Investigation1. Verification of diagnosis by-

a) Clinical examination

b) Lab. Diagnosis.

2. Confirmation of the existence of an epidemic by –

a) Cases finding through active surveillance & community surveys.

b) Visits or telephonic calls to the medical facilities or priv. pract. , NGOs

c) Active surveillance should be maintained even after the outbreak is over

d) House to visits.

3) Line listing ,defining & counting of cases

Sr no.

Name of the patient.

Father/husband’s name

address

age sex Symptoms of illness

Date of onset of symptoms

Outcome(still ill/recoverd/died)

remarks

Data analysis with respect to time place & person.

Formulation of hypothesisEvaluation of ecological factorsFurther evaluation of population

at risk.Writing a report

Surveillance

Surveillance is data collection for action. Surveillance data are required for planning disease activities & for evaluating impact .

Disease surveillance data are also required to identify high risk areas or high risk case specific & other groups who require special attention.

Early warning signals will be missed in the absence of an effective surveillance system.

To plan any disease control prog. It is imp. To know the following-

1. Who get the diseases

2. How many get them.

3. Where they get them

4. When they get them

5. Why they get them

Steps in surveillance procedure-

1. Collection of data

2. Compilation of data

3. Analysis & interpretation

4. Follow up action

5. Feedback

Prerequisites for effective surveillance-

1. Use of standard case definition

2. Ensuring regularity of reports

3. Action on the reports

Methods of data collection Sentinel surveillance

Active surveillance

Vector surveillance

Laboratory surveillance

Sample surveys

Outbreak investigations

Special studies

Sentinel surveillance

A hospital, health center, lab. Rehabilitation center which caters to a relatively large number of cases of the disease, can be considered as sentinel center. It can provide information on one or more disease.

Sentinel center helps in providing

1. Line lists of cases of selected diseases.

2. Early warning signals which trigger action for outbreak investigation.

Active surveillance Done under following circumstances-

1. During outbreaks to determine its extent &keep mortality rates by initiating early T/t.

2. As the number of cases of disease decline to negligible level it becomes important to receive info. On every single case so that future transmission is interrupted. Eg-AFP

Vector surveillance Important to monitor the existing

factors.

Increase in density of vector on their breeding site should be taken as early warning signals for vector borne outbreak

Eg- the clues mosquitoes breed in dirty water whereas anopheles prefers clean water for breeding.

Laboratory surveillance Testing water samples for

coliform organisms is a measure to determine the risk of water borne outbreaks.

Checking the chlorination level of water is also important especially during monsoon & post monsoon period.

IDSP IDSP is a decentralized, state based surveillance

programme in the country. Launched in 2004 by Ministry of Health and Family

Welfare with World Bank assistance. Intended to detect early warning signals of impending

outbreaks. Helps to initiate an effective response in a timely

manner. Also expected to provide essential data to monitor

progress of on-going disease control programmes All the outbreaks cannot be predicted or prevented . But

precautionary measures can be taken within the existing health infrastructure and service to reduce the outbreaks

Diseases and core condition under IDSP

Regular Surveillance∗: Vector Borne Disease : 1. Malaria Water Borne Disease : 2. Acute Diarrhoeal Disease

(Cholera) : 3. Typhoid : 4. Jaundice Respiratory Diseases : 5. Tuberculosis

: 6. Acute Respiratory Infection Vaccine Preventable Diseases : 7. Measles Diseases under eradication : 8. Polio Other Conditions : 9. Road Traffic Accidents

(Linkup with police computers) Other International commitments : 10. Plague, Yellow fever Unusual clinical syndromes : 11. Menigoencephalitis/ Respiratory

(Causing death/hospitalization) Distress, Haemorrhagic fevers, other undiagnosed conditions

Types of Surveillance in IDSP1. Syndromic – Diagnosis made on the basis of symptoms/clinical pattern by paramedical personnel and members of the community.2. Presumptive (Clinical) – Diagnosis made on typical history and clinical examination by Medical Officers.3. Confirmed(Laboratory) – Clinical diagnosis confirmed by an appropriate laboratory test.