Epidemiology: An Overview

Dr. Bhoj R Singh, Principal Scientist (VM)I/C Epidemiology; Centre for Animal Disease Research and DiagnosisIndian Veterinary Research Institute, Izatnagar-243122, Bareilly, UP,

India. TeleFax +91-581-2302188

EpidemiologyFather of Epidemiology: Hippocrates, 400 BCThe first person known to have examined the

relationships between the occurrence of disease and environmental influences.

He coined the term ‘epidemic’ in his essay entitled “On Airs, Waters, and Places,”

Clinicians are concerned with the health of an individual; epidemiologists are concerned with the collective health of the population in one or other area.

History• Hippcrates (400 BC) an essay entitled “On Airs, Waters, and Places,”.• Another early contributor to epidemiology was John Graunt, a London

haberdasher who published his landmark analysis of mortality data in 1662. He was the first to quantify patterns of birth, death, and disease occurrence, noting male-female disparities, high infant mortality, urbanrural differences, and seasonal variations.

• In 1854, William Farr wrote a book ‘Cholera’. He began to systematically collect and analyze Britain’s mortality statistics. Farr, considered the father of modern vital statistics and surveillance, developed many of the basic practices used today in vital statistics and disease classification. He extended the epidemiologic analysis of morbidity and mortality data, looking at the effects of marital status, occupation, and altitude. He also developed many epidemiologic concepts and techniques still in use today.

• Epidemiology= (Epi=upon/ among+demos=people/ district+logos=discourse/ study)

• Epizootiology= epi=upon+zoo=animal+logos=discourse)• Epidemic= outbreak of disease in human population• Epizootic= outbreak of disease in animal population• Epornitics= outbreak of disease in bird population• Epidemein= to visit a community, I.e., a diseases which comes periodically, not

present continuously• Endemein= resides within (the diseases continuously present in a population)

• Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population.

• “Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.” (Last JM, 1988)

More Definitions• Mayer inferentially acknowledges the epidemiologic character of

population phenomena, for as its etymology indicates, (epi, upon; demos, people; logos, study), epidemiology is the study of what “comes upon” groups of people. More specifically epidemiology is concerned with the distribution of disease and death, and with their determinants and consequences in population groups.

• Epidemiology is: a) a quantitative basic science built on a working knowledge of probability, statistics, and sound research methods; b) a method of causal reasoning based on developing and testing hypotheses pertaining to occurrence and prevention of morbidity and mortality; and c) a tool for public health action to promote and protect the public’s health based on science, causal reasoning, and a dose of practical common sense. (Cates WJ. 1982)

• the study of the distribution and determinates of health related states in specified populations and the application of this study to control health problems (Peterson DR 1970)

Veterinary Epidemiology, Epizootology

• Investigation and assessment of health and productivity related events in animal population.

Objectives of Epidemiology

1. To determine origin of disease in a population when etiology is known

2. To investigate and control a disease where etiology is obscure or little understood

3. To determine ecology and natural history of a disease

4. Planning and monitoring of disease control programme

5. Assessment of economic effects of a disease and cost/benefit of a control programme

Uses of EpidemiologyEpidemiology assists • to identify the etiology or cause of a disease and the risk factors." • "to study the natural history and prognosis of disease". • "to evaluate new models of health care delivery." • "to provide a scientific foundation for regulatory decisions relating to

health or environmental problems.• "to provide a clue to changes taking place over time…" • "to identify subgroups in the population who are at high risk for

disease." • to determine the best or most appropriate types of primary and

secondary prevention.'– primary prevention prevents disease in healthy individual (vaccination). – Secondary prevention limits disease by early detention usually through

screening programs.

Types of Epidemiological investigations

• DATE – Descriptive, – Analytical, – Theoritical, – Experimental

Disciplines of Epidemiology

• Computational epidemiology• Genetic epidemiology• Molecular epidemiology• Environmental epidemiology• Nutritional epidemiology• Applied Epidemiology

– Clinical epidemiology– Micro-epidemiology (Study of disease in small group of individuals

as study of feline AIDS in kittens and then application to understand human AIDS)= Comparative epidemiology

– Macro epidemiology (Study of disease in population in national and international perspective)

Descriptive Epidemiology

• The science of characterization of the distribution of health-related states or events is one broad aspect of epidemiology called descriptive epidemiology. Descriptive epidemiology provides the What, Who, When, and Where of health-related events.

• We temporarily classify a case as suspect or probable until laboratory results are available.

• When we receive the laboratory report, we then reclassify the case as either confirmed or “not a case,” depending on the lab results.

Analytic epidemiology

• Analytic epidemiology attempts to provide the Why and How of such events by comparing groups with different rates of disease occurrence and with differences in demographic characteristics, genetic or immunologic make-up, behaviours, environmental exposures, and other so-called potential risk factors. Under ideal circumstances, epidemiologic findings provide sufficient evidence to direct swift and effective public health control and prevention measures.

Applied epidemiology

• The term applied epidemiology is sometimes used to describe the application or practice of epidemiology to address public health issues. Examples of applied epidemiology include the following:

• the monitoring of reports of communicable diseases in the community

• the study of whether a particular dietary component influences risk of developing a disease

• evaluation of the effectiveness and impact of a health awareness or production improvement program

• analysis of historical trends and current data to project future health and production resource needs

Concepts of Disease causation• Demons: (Placation- sacrifice, exorcism, evasion-millet seeds,

transference- scapegoats, Gadarene swine, amulets, talismans, fetishes and icons.

• Divine wrath: From Old testament, Persian and Aztec writings.• Metaphysical: Stars, planets (treated with foul medicine,

swallowing of toads, fishes, moles roasted alive).• Miasmata:

– Derangement of four humours of body (Geeks; Heat, moisture, dryness and cold; air, earth, water and fire) creating mismata (Malaria).

– Vedic theory, three humors and tridosha (Vat, pitta, cough)• Contagion (Contagium animatum- Edward Jenner, 17th Century;

contagium vivum fluidum- virus, Beijerinck, 1998-99)

Studies in Epidemiology

1. Experimental (Full control of investigator on formation of groups, clinical trials, nutritional trials)

2. Cross-sectional: Investigator tries to find a relation between occurrence of disease with a hypothesized cause)

3. Case-Control: Investigator compares groups of diseased with healthy population with respect to exposure to a hypothesized causal agent.

4. Co-hort: Investigator compares two groups one exposed to hypothesized cause and other not exposed for occurrence of disease (Prospective)

Methods in epidemiology• Survey: Examination of aggregates of units of disease in population i.e., record of events• Types of Surveys– Sample Survey (On a sample of population)– Census (On total population)– Cross sectional (record of events a defined point of time)– Longitudinal (record of events over a period of time): Prospective (present to future),

Retrospective (records of past events)– Screening: A kind of survey for identification of undiagnosed cases of a disease often with some

rapid test/ examination.• Monitoring: Routine observation on health/ production of a population and

environmental factors, without much emphasis on identification of a diseased animals. (Regular meat/ milk recording, abattoir examination, meat inspection etc.)

• Surveillance: More intensive form of monitoring often associated with a control programme, identification of an individual with disease in view to detect changes in population and its environmental health.

• Sentinel units: Units selected for monitoring and surveillance as abattoirs, some clinics, farms etc), horses may be sentinel for VEE, stray dogs for Canine distemper)

• Study in epidemiology means comparison of groups for occurance of disease/ production, thus survey is not a study.