45672486 basic epidemiology

GENERAL OBJECTIVE:

To know the basic concepts and strategies of Epidemiology

SPECIFIC OBJECTIVES:At the end of the lecture, the student

must know the:

1. Definition of Epidemiology2. Concepts of Epidemiology

SPECIFIC OBJECTIVES:

3. Principles of Epidemiology4. Aims and purposes of Epidemiology5. Relationship of the other branches of Science

to Epidemiology6. Ecologic Concepts of diseases7. Types of Disease Agents8. Attributes of the Human host9. Attributes of the Environment10. Relationship among Agent, Host and

Environment


11. General and specific defense mechanisms of the body

12. Mode of Disease Transmission13. Nature of communicable diseases14. Classification of disease in human15. The stages of the natural history of

disease16. Levels of disease prevention17. Steps in scientific inquiry


18. Strategies of Epidemiology19. Methods of hypothesis formulation20. Description of the different study designs21. Concept of Descriptive Epidemiology in

relation to Person, Place and time22. Sources of Epidemiological data23. Description of disease frequency as to

counts, ratios, proportions and rates

DEFINITION OF EPIDEMIOLOGY

- The study of the distribution and determinants of disease frequency in man

- From the Greek word epi meaning ON or UPON and demos which means POPULATION

MODERN DEFINITION OF EPIDEMIOLOGY

- A branch of medical science concerned with the relationships of the various factors and conditions which influence the frequencies and distribute health, infectious disease process, defect, disability or death as it occurs in aggregations or groups of individuals in a population

CONCEPT OF EPIDEMIOLOGY

1. Concept of the “shrinking world“- The world is becoming smaller not

because of physical size but due to:

a). Marked improvement in transportationb). Better communication and

transportation makes easier exchange of information about diseases


2. Changes in nature of disease problema). Socio-economic conditionsb). Improvement and progress of medical

science 3. Much have been done to fight

communicable diseasesa). Better facilities for early Dx & Txb). Better control measures- vaccines


4. Epidemics are now a rarity - study more the endemic behavior of

disease rather than epidemic occurrences

5. Realization of the different applications of methods in Public Health both for infectious and non-infectious diseases

PRINCIPLES OF EPIDEMILOGY

1. Exact observation (strict, accurate, precise)

2. Correct interpretation ( free from error)

3. Rational instruction (by expert knowledge and technical skills)

AIMS & PURPOSES OF EPIDEMIOLOGY1. To analyze carefully the roles and

interactions of agents, host and environmental factors in the natural history of disease to discover gaps in the knowledge and contribute to preventive medicine

AIMS & PURPOSES OF EPIDEMIOLOGY 2. To describe and analyze disease

occurrence and distribution according to such variables as age, sex, occupation, temporal frequency, periodic fluctuation, long term trends and geographic distribution to make community diagnosis making an estimate of morbidity and mortality risks

AIMS & PURPOSES OF EPIDEMIOLOGY3. To aid in filling gaps in knowledge

about the causes of disease processes by observing the range, amplitude and group behavior of clinical syndromes in populations

AIMS & PURPOSES OF EPIDEMIOLOGY4. To study immediate and special

problems in the field of health. This would include the study of new diseases, endemic disease problems, epidemics and administrative problems

5. To measure the effectiveness of preventive and control programs in health

RELATIONSHIP OF OTHER BRANCHES OF SCIENCE TO EPIDEMIOLOGY

1. Clinical Medicine to obtain details of clinical diagnosis for epidemiological descriptions of the distribution of diseases or disorders

2. From Bacteriology, Entomology, Parasitology, Zoology for information about nature and characteristics, reservoirs, sources and modes of transmission of living agents of disease


3. From Demography on the composition and characteristics of the population

4. From Chemistry, Physics, Nutrition and Industrial Medicine for information about health hazards

5. From Anthropology and Sociology for information about habits, customs, cultural and social characteristics of the population


6. Genetics and Psychology for characteristics of persons which may influence occurrence of diseases

7. Meteorology for data on weather and climate in relation to occurrence of diseases

8. Biostatistics – analytic and descriptive9. Vital statistics containing data on

natality, morbidity and mortality

ECOLOGIC CONCEPT OF DISEASEThe ecologic concept of disease is based

on the 3 premises of the biologic laws:1. The disease results from an imbalance

between disease agents and man2. That the nature and extent of

imbalance depend on the nature and characteristics of the host and the agent

ECOLOGIC CONCEPT OF DISEASE3. That the characteristics of agent

and host and their interaction are directly related to and depend largely on the nature of physical, social, economic and biologic environment

DISEASE AGENT

- May be defined as an element, a substance or a force, either animate or inanimate, the presence or absence of which, may following effective contact with susceptible human host under proper environmental conditions, serve as stimulus to initiate or perpetuate a disease process

TYPES OF DISEASE AGENTS

1. Biologic2. Nutrients3. Chemical4. Physical5. Mechanical6. Psychological

ETIOLOGIC FACTORS OF AGENTS OF DISEASE1. Biologic – living plants and animals, parasites2. Nutritive agents – cholesterol, vitamins,

proteins, fats3. Chemical – poisons, allergens, metals, drugs4. Physical – climate, season, weather,

radiation5. Mechanical – machines, cars, engines6. Psychological/ social – mental stress

CHARACTERISTICS OF THE LIVING AND NON-LIVING AGENTS

1. Inherent nature and characteristics in morphologic agents

2. Viability and resistance3. Infectivity and pathogenicity to man4. Reservoirs and sources of infection5. Vehicles and condition of

dissemination

CHARACTERISTICS OF LIVING AND NON-LIVING AGENTS

LIVING AGENTS

Morphology

Mortality

Physiology

Reproduction

Metabolism

Nutrition

Toxic products

NON-LIVING AGENTSDustCrystalsSolutionsInsoluble substancesCorrosive & non-

corrosive substances

THE INCUBATION OF NON-INFECTIOUS AGENTS DEPENDS ON:

1. Strength of stimulus2. Repeated increments of the agent3. Period of exposure4. Tissue involved

DISEASE AGENTS MAY:

1. Fail to lodge in the body, being expelled or dislodged by the non-specific outer defenses of the host

2. In-apparent infection – lodge, multiply and yet produce no discernible action on the host

3. Sub-clinical cases- reactions may be so mild to escape detection

Whatever the original reaction is, the end result may be:

1. Elimination of the agent leading to complete recovery

2. Clinical recovery without elimination of the agent (carrier state)

3. Death, defect or disability of the host

ATTRIBUTES OF THE HUMAN HOST1. IMMUNE RESPONSE2. HABITS & CUSTOMS3. AGE, SEX & RACE4. MARITAL FACTORS5. OCCUPATIONAL FACTORS6. OTHERS: Constitution, heredity,

psychological factors

GENERAL & SPECIFIC DEFENSE MECHANISMS

Resistance: the sum total of the defense mechanism of the host

1. Non-specific resistancea). Skin & mucous membranesb). Phagocytosisc). Reticuloendothelial systemd). Hormonese). Different reflexes or physiologic

mechanisms

Different reflexes or physiologic mechanisms:

Winking reflex Tears Sneezing Coughing vomiting

Diarrhea Urination Genito-urinary

discharge Sweating

Specific Resistance or immunity of the human host- Possession of antibodies for a specific

disease1. Passive immunity – either attained

by maternal transfer or by inoculation of specific antibodies; brief duration of immunity

2. Active immunity – natural immunity and artificial immunity; vaccines

3. Latent immunity – developed due to the giving of small doses for a long period of time

The human host may be :

1. Susceptible – a person not possessing resistance against a particular pathogenic agent

2. Immune – a person who possesses antibodies that are specific and protective

CHARACTERISTICS OF AGENTS IN RELATION TO HOST:

1. INFECTIVITY –the ability of an agent to invade and adapt itself to the human host

2. PATHOGENICITY – the measure of the ability of an agent when lodged in the body to set up either a local or general tissue reaction

3. VIRULENCE – the measure of the severity of the reaction produced

4. ANTIGENIC PROPERTIES – the activity to stimulate the host to produce agglutinins, opsonins, presipitins, antitoxins, complement fixing, neutralizing and sensitizing antibodies

ATTRIBUTES OF THE ENVIRONMENT:

- Influence existence of the agent- Influence exposure or susceptibility to

agenta). Physical environment –geology,

climateb). Biologic environment – human

population density, crowding index - flora and fauna – sources of food,

vertebrate hosts, arthropod vectorsc). Socio-economic – occupation,

urbanization, pollution, disruption (wars, floods, earthquakes)

RELATIONSHIP AMONG AGENT, HOST & ENVIRONMENT

A H

E

MODE OF DISEASE TRANSMISSION

Infectious diseases can spread through human population by:

1. Common vehicle epidemics - the etiologic agent is transmitted by water, food, air or inoculation

a). Vehicle – water, food, soilb). Vector – snails, mosquitoes2. Epidemics propagated by serial transfer

from host to hosta). Airborne droplets, anal, oral, genital

routeb). Infected blood or sera

NATURE OF COMMUNICABLE DISEASESCOMMUNICABLE DISEASES - an illness

due to specific infectious agents or its toxic products

CONTAGIOUS DISEASES – implies transmission of the disease through direct contact

Periodicity of Communicable Disease Occurrence:

1. EPIDEMIC2. Endemic – a disease constantly

occurring in a geographical area3. Pandemic – epidemic occurring within

more than one country or territory4. Sporadic – occasional or infrequent

occurrence of a disease5. Epizootic or enzootic – diseases

occurring in animals6. Zoonotic – disease of animals

transmissible to man

Biologically, disease agents are classified according to decreasing sizes into the following seven categories:1. Arthropods – scabies, lice, ticks

2. Helminths – schistosoma, ascaris3. Protozoa – plasmodium4. Fungi – yeast and molds5. Bacteria – spirochetes, streptococci,

TB bacilli6. Ricketssiae – typhus7. Viruses – flu, measles, Hepa B, HIV

Host factors in the occurrence and distribution of communicable diseases

1. Reservoir of agents – man, animals, plants, soils, inanimate organic matter

2. Portals of entry and exit – interrupt the natural history before man is infected

- by detecting agents, abolishing reservoir, interfering with transmission, barriers against infection

CLASSIFICATION OF DISEASES

Communicable or Infectious

Bacteria

Virus

Ricketsiae

Arthropods

Helminths

Protozoa

Fungi

Non-communicable or non-infectious

OccupationalEnvironmentalLife-style diseasesFamilial or

hereditary

THE NATURAL HISTORY OF DISEASE

1. Stage of susceptibility a). Portal of entry2. Stage of pre -symptomatic disease a). Incubation period3. Stage of Clinical Disease a). Clinical horizon4. Stage of disease outcome a). Complete recovery b). Disability or defect c). Carrier state d). death

STAGE OF SUSCEPTIBILITY

- The disease has not developed but the groundwork has been laid by the presence of risk factors which favors its occurrence

- Portal of entry is a pre-requisite for successful infection

STAGE OF PRESYMPTOMATIC DISEASE

- There is no manifest disease but usually through the interaction of factors, pathogenic changes have started to occur

- INCUBATION PERIOD – the interval between the time of entry of agent into the host and the onset of signs and symptoms

STAGE OF CLINICAL DISEASE

- Sufficient end-organ changes have occurred so that signs and symptoms of the disease can be recognized

- Classification and Staging of Disease

STAGE OF DISEASE OUTCOME

- Some diseases run their course and then resolve completely either spontaneously or under the influence of therapy

- Some diseases will give rise to residual defect of short or long duration with disability

NATURAL HISTORY OF DISEASE DISEASE

INCUBATION PERIOD

CLINICAL CASE

RECOVERY DEATH

COMPLETE CARRIER

DISABILITY OR DEFECT

LEVELS OF DISEASE PREVENTION

1. PRIMARY LEVEL- Pre-pathogenesis- Isolation, quarantine- Health education, food sanitation,

proper waste disposal, eradication of animal reservoir, specific protection

- Active and passive immunization

LEVELS OF DISEASE PREVENTION2. SECONDARY LEVEL- PATHOGENESIS period- Early diagnosis and prompt

treatment of the disease to prevent its spread

- Example: killing of rabid dogs, cows with MCD, pigs with FMD

- Screening tests for diseases , periodic examinations, case-finding, adequate treatment and follow-up

LEVELS OF DISEASE PREVENTION3. TERTIARY LEVEL- Stage of Disease outcome- Disability limitation- Rehabilitation- Intensive, periodic follow-up and treatment

Health Screening

For CVS diseases: Chest x-ray & ECG Blood sugar for diabetes Pap’s smear for cervical cancer Digital rectal examination Newborn Screening Hearing Screening Mantoux Test

STEPS OF SCIENTIFIC INQUIRY 1. Examining existing facts and

hypotheses and identifying gaps in knowledge

2. Formulating a new and a more specific hypothesis

3. Obtaining additional information to test the acceptability of the new hypothesis

4. Evaluating the new evidence and deriving appropriate conclusions

The Epidemiologic Approach or Strategy 1. identify the problem 2. review of related literature 3. identify critical knowledge gaps

about the problem 4. special data collection activities 5. hypothesis formulation 6. hypothesis testing

Features of a good hypothesis: Formulating the Hypothesis The population to whom the

hypothesis will apply (the target population)

The cause being investigated is usually a particular environmental exposure. It could be a physical, chemical, biologic or psychological factor

The outcome is usually the disease or condition of interest

Features of a good hypothesis: Formulating the hypothesis The dose response relationship is the

amount of exposure necessary for the disease or condition to develop

The time-response relationship is the time period between the exposure and the development of the outcome. This concept is synonymous to the incubation period for infectious diseases and to the latency period of non-infectious diseases

Some considerations in formulating hypothesisa). New hypotheses are commonly

formed by relating observations from several different fields

b). The stronger the statistical association between the exposure and the disease, the more likely it is to suggest a causal hypothesis

Some consideration in formulating hypothesisc). Observed changes in the frequency of

disease can lead to very productive hypothesis

d). An isolated or an unusual case should receive particular attention in forming hypothesis

e). Observations that appear in conflict or those which present a paradox should be considered

Study Designs

Descriptive studies – case studies, case series and cross-sectional studies

Observational studies – cohort studies, case-control studies

Experimental studies – clinical trials, RCT’s, community trials

Sources of Data

Two Types of Data according to Source:1). Primary Data – obtained by the

investigatorExample: interviews

physical examinationlaboratory examinations

- More accurate and up-to-date but more expensive and difficult to obtain

Sources of Data

2). Secondary Data – data actually gathered by other individuals or agencies

Example: published reportsclinical/hospital

recordscensus

- More readily available but incomplete

- Confidentiality of information

Surveillance

In most health departments, routinely collected statistics provide the key data for monitoring morbidity and mortality trends

Surveillance System – includes a functional capacity for data collection, analysis and dissemination linked to public health programs

Epidemiologic Surveillance Has been defined by the Centers for

Disease Control (CDC) as the ongoing systematic collection, analysis and interpretation of health data essential to the planning, implementation and evaluation of public health practice, closely integrated with the timely dissemination of these data to those who need to know

Specific Data that are Useful in Epidemiologic Studies

1. Data on vital events – birth, death, marriages, divorces, adoptions, total births/deaths, deaths by specific causes, mortality rate, case fatality rate, etc.

2. Disease statistics – prevalence and incidence of specific diseases


3. Data on physiologic or pathologic conditions – prenatal Hgb levels, blood sugar levels among diabetics, BP readings

4. Statistics on Health Resources and Services – number of hospital beds, vaccine vials consumed, number of health center staff


5. Statistics pertaining to the environment – number of households with sanitary water source, number of snail breeding places, amount of pollution in the air, level of noise in the factory, workers’ protective gears

6. Demographic data – total number of population, age groups, gender, rural-urban residence, occupation, income

7. Socio-cultural data – knowledge, attitude, practices of people regarding health

Sources of Data

1. Registers of births and deaths2. Certificates of death3. Disease registers4. Disease notifications5. Census6. Clinic/hospital records7. Surveys

10 Key Sources of Data for Surveillance Systems Designated by the WHO:1. Mortality registration2. Morbidity reporting3. Epidemic reporting4. Laboratory investigation5. Individual case investigations6. Epidemic field investigations7. Surveys8. Animal-reservoir and vector

distribution studies9. Biologic and drug utilizations10. Knowledge of the population and the

environment

Other Sources of Surveillance Data1. Hospital and medical care statistics2. Panels of cooperating physicians3. Public health laboratory reports4. Absenteeism from work or school5. Telephone and household surveys6. Newspaper and news broadcasting

reports

Data can readily be available from1. National Center for Health Statistics2. Centers for Disease Control &

Prevention3. Birth Defects Monitoring Program4. Metropolitan Atlanta Congenital Defects5. Cancer Surveillance, Epidemiology and

End Results (SEER) Program6. Morbidity and Mortality Weekly Report

from the CDC7. Medical Specialty Societies8. World Health Organization

Locally, data may be available from:1. Department of Health2. National Institutes of Health3. Medical Specialty Societies4. National Congenital Defects

Registry5. Newborn Screening Program6. Specialty Hospitals7. National Census and Statistics

Office

Descriptive Epidemiology: Person, Place and TimeDescriptive Epidemiology – the study

of the amount and distribution of disease within a population by person, place and time.

WHO (person) is affected?WHERE (place) do the cases occur?WHEN (time) do the cases occur?

PERSON

Characteristics of PERSON1. age 7. family variables2. sex 8. other personal3. ethnic group variables:4. social class Blood type5. occupation Environmental exposures6. marital status Personality traits

AGE

- The most important determinant among the personal variables

- Death rate is fairly high in infancy- Lowest point is between 5-14 years

old- Doubling in rate from 40 and every

decade of life

AGE

- Chronic conditions tend to increase with age whereas the relation of age

to acute infectious diseases is less consistent

- Age is related to the frequency and severity of infectious diseases

- High rate of injury in particular age group

SEX

- Death rates are higher for males than females, but morbidity rates are higher for females

- In utero and neonatal death rates are also higher for males

- The higher death rates for males throughout life maybe due to sex-linked inheritance, differences in hormonal balance, environment or habit patterns

SEX

- The higher mortality rate for men are not paralleled by higher rates of illness

- Women have more episodes of illness and more physician contacts than men have

- Rate of attempted suicide is higher in women but completed suicides are more common in men

- Toxic shock syndrome

SEX

Possible explanations for the relatively high morbidity and low mortality in women:

1). That women seek medical care more freely and perhaps at an earlier age of disease

2). That the same disease will tend to have less lethal course in women than in men

ETHNIC GROUP & RACE

Blacks have higher rates of deaths caused by CHVD, CVA, TB, SY

Whites have higher rates of death from suicide, leukemia and atherosclerotic disease

Many differences in rates of diseases and death reflect at least in part, differences in various environmental exposures, in lifestyle and in the extent and quality of medical care

SOCIAL CLASS

Difference in wealth, power, prestige: difference in access to medical care and facilities

Poverty affects utilization of medical services

More common cases of mental illness in lower strata

OCCUPATION

This influence may occur thru a variety of exposures – unfavorable physical conditions (heat, cold, changes in atmosphere), chemicals, noise, stress in work

Silica (pulmonary fibrosis), asbestos (lung cancer), aniline dyes (bladder cancer)

Injury, trauma, social and psychological climate of the job or workplace

MARITAL STATUS

Marital status is associated with level of mortality for both sexes

Psychological and physical support from the spouse

For women, marital status may also be related to health through differences in sexual exposure, pregnancy, childbearing and lactation

FAMILY VARIABLES

Family size: larger families – especially if they are poor, children may be in a disadvantage: higher rates of fetal, neonatal and infant deaths, higher childhood mortality, and a tendency to poorer intellectual performance

Birth order: first borns tend to be more healthy and better educated

Personal Variables

Maternal age: etiologic importance in congenital malformations

Parental deprivation – psychiatric, psychosomatic disorders, TB incidence, attempted suicides and accident repeaters

Blood type A- gastric CA, Type O –doudenal ulcer

Environmental exposurePersonality traits – medical advice,

compliance

PLACE

Frequency of disease can be related to place of occurrence in terms of areas set off either by natural barriers or by political boundaries

Frequency of disease may be related to temperature, humidity, rainfall, altitude, mineral content of soil or water supply

Lack of iodine, mottled dental enamel Rural-urban differences Migrants in national and international

borders

TIME

Disease occurrence is usually expressed on a monthly or annual basis

Secular trends – refers to changes over a long period of time, years, decades

Cyclic change – refers to recurrent alterations in the frequency of disease

If there are enough cases of a particular disease annually for stable rates, cases for several years around a census may be combined

Measures of Disease Frequency Count – the basic measure of disease

frequency Ratio – a measure that shows the

relationship between quantities Proportion – the numerator in the

proportion is part of the denominator Rate – the most common measure

Measures of Morbidity

Prevalence – the proportion of individuals with the disease during a a given point in time

- the probability that a person randomly chosen from the population will have a disease at the time he was examined.

Prevalence = no. of existing cases of a disease

-----------------------------------------

Total population examined

Measures of Morbidity

Incidence – refers to the proportion of the population who developed the disease in a given interval time

- the numerator is the count of new cases of disease in the population

- a measurement of the risk of developing the disease in the population at risk of the disease

Cumulative incidence – no. of new cases of disease----------------------------------population at risk

Relationship of Incidence and Prevalence The higher the number of new

(incident) cases, then there would be a greater number of existing (prevalent) cases

Study of Epidemics

Two Types of Epidemics:1. Common Vehicle Epidemic2. Propagated or Progressive

Type

Common Vehicle Type of Epidemic Single exposure, single source or

point epidemic It indicates simultaneous exposure of

the population to a common source Example: food or chemical

poisoning- Multiple exposure- Water-supply epidemic (Dysentery)

Propagated or Progressive Type of Epidemic Also called contact epidemic,

propagated by:a). Person to person spreadb). Arthropodc). Vector reservoir

Characteristics of Epidemic1). Type of onset or manner of onset

a). Sudden, abrupt or explosive – time factor is the period of incubation

b). Insidious or gradual – most cases start after the incubation period

2). Types of infectiona). Mass infection – more of

primary casesb). Progressive infection – more

of secondary cases

Types of Epidemic Curves

1). Classical Epidemic Curve –short ascending and descending limbs- picture of common source- rapid transmission due to big dose of organism- longer descending limb is due to the development of secondary cases- more deaths on the ascending limb because of heavier dose of the organism and less resistance

Classical Epidemic Curve

Inverted Epidemic Curve

- Long ascending and short descending limb

- Indicates that the transmission is more complex and the disease has a longer incubation period

- Person to person spread, insect-borne diseases (malaria, DHF)

Inverted Epidemic Curve

Bell-shaped Epidemic Curve- Rapid ascending and rapid

descending limb- The spread is rapid and the

transmission is simple so there is rapid elimination of susceptible

- Measles and poliomyelitis

Bell-shaped Curve

Investigation of Epidemics1. DEFINITION OF THE PROBLEM

- define clearly the nature and extent of the problem

a). Verify the diagnosis through clinical signs and symptoms of the disease aided by laboratory examinations

b). Verify whether epidemic exists. Compare the incidence of the disease with its usual incidence in the community

Investigation of Epidemics2. APPRAISAL OF EXISTING

INFORMATIONa). Orient as to time, the chronologic

order of the disease (epidemic curve)b). Orient as to place, determine

geographic distribution of casesc). Orient as to the person’s

characteristics (age, sex, occupation, socioeconomic conditions, etc.)

Investigation of Epidemics

3. FORMULATE TENTATIVE HYPOTHESIS

What could be the cause/ causes?Classify according to mode of transmission:

3.1. Common Vehicle – single exposuremultiple or continuous exposure

3.2. Propagated – person to person arthropod vector reservoir

Investigation of Epidemics4. TESTING THE HYPOTHESIS

a). Epidemiological investigation of all cases or representative sampleb). Search for additional informationc). Analyze detailed data – attack rates, common source or vehicled). test various hypotheses

e). Formulate conclusions as to source, mode of transmission and all features of the epidemic which require explanations

Investigation of Epidemics

5. CONCLUSIONS & PRACTICAL APPLICATIONS- evaluate the results in terms of local situations

- conclusions are based on pertinent evidence to control present outbreak and prevent future similar epidemics

- reports should be simple, clear and honest

- collection of data must be made in a scientific manner which requires exact observation, correct interpretation and rational explanation

45672486 basic epidemiology

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