B ASIC C ONCEPTS OF E PIDEMIOLOGY

What is Epidemiology? Greek roots: Epi : Upon. Demos: Human population. Logia: Science. “Science that deals with the distribution and. determinants of diseases in human populations”. Mac Mahon & Plug, 1970

Epidemiology is the study of the distribution and determinants of disease in human population

Epidemiology… Medicine is focus in the individual Epidemiology is focus in communities, groups, population. Integrating science : links to clinical & laboratory biostatistics, health economics

Background Relatively new science – emerged in 19th century Development during the 20th century for chronic diseases (heart diseases, cancer, stroke, etc) Concerned with epizootic diseases and all other forms

JOHN SNOW John Snow is credited by many with developing the modern field of epidemiology

S NOW I NDEX C ASE

THE GREAT EXPERIMENT Lambeth Company : water intake upstream of London sewage outfall into Thames Southwark & Vauxhall Company : water intake downstream of sewage outfall

THE GREAT EXPERIMENT HousesDeaths Deaths/ 100,000 S and V40, Lambeth26,

Concerns Detecting existence of a disease Identifying causes & modes of transmission Obtaining information on ecology & natural history Planning & evaluating possible disease control strategies Monitoring & surveillance to prevent further disease episodes Assess economic effects of disease & control programmes

Rationale Diseases do not occur by chance: there are always determinants for the disease to occur. Diseases are not distributed at random: Distribution is related to risks factors that might need to be studied.

Disease investigation Thalidomide baby : เด็กแขนขาพิการเป็นผล มาจากแม่กินยา Thalidomide ขณะตั้งครรภ์ Tampons and toxic shock syndrome Legionnaires disease ทหารผ่านศึกที่ล้มป่วย จำนวนมากภายหลังมาประชุมที่โรงแรมแห่ง หนึ่งเกิดจากการติดเชื้อผ่านระบบ เครื่องปรับอากาศ

วัตถุประสงค์การศึกษาทางระบาด วิทยา เพื่อทราบขนาดการกระจายโรคที่ เกี่ยวกับบุคคล เวลา สถานที่ เพื่อทราบสาเหตุโรค เพื่อทราบธรรมชาติการเกิดโรค ปัจจัย การเกิดโรค และวิธีการถ่ายทอดโรค เพื่อเป็นแนวทางการวางแผนการ ควบคุมและป้องกันการเกิดโรค

การดำเนินงานทางระบาด วิทยา Epidemiological Surveillance Epidemiological Investigation Epidemiological Study

R OUTES OF TRANSMISSION Direct  Skin-skin  Herpes type 1  Mucous-mucous  STI  Across placenta  toxoplasmosis  Through breast milk  HIV  Sneeze-cough  Influenza Indirect  Food-borne  Salmonella  Water-borne  Hepatitis A  Vector-borne  Malaria  Air-borne  Chickenpox (www)www

Modes of Disease Transmission

No infection Clinical Sub-clinical Carrier Death Carrier Immunity No immunity Outcome (www)www Exposure to Infectious Agents

Infection Susceptible Dynamics of infectiousne ss Dynamics of disease Incubatio n period Symptom atic period Non- diseased Latent period Infectious period Non-infectious Infection Tim e (www)www Timeline for Infection

Cases  Index – the first case identified  Primary – the case that brings the infection into a population  Secondary – infected by a primary case  Tertiary – infected by a secondary case P S S T Susceptible Immune Sub-clinical Clinical S T (www)www Transmission

Data from Dr. Simpson’s studies in England (1952) MeaslesChickenpoxRubella Children exposed Children ill attack rate Attack rate = ill exposed (www)www Person-to-Person Transmission

Disease is the result of forces within a dynamic system consisting of: agent of infection host environment Epidemiologic Triad

Agent Host Environment Age Sex Genotype Behaviour Nutritional status Health status Infectivity Pathogenicity Virulence Immunogenicity Antigenic stability Survival Weather Housing Geography Occupational setting Air quality Food (www)www Factors Influencing Disease Transmission

Infectivity (ability to infect) (number infected / number susceptible) x 100 Pathogenicity (ability to cause disease) (number with clinical disease / number infected) x 100 Virulence (ability to cause death) (number of deaths / number with disease) x 100 All are dependent on host factors Epidemiologic Triad-Related Concepts

Iceberg Concept of Infection

A host that carries a pathogen without injury to itself and serves as a source of infection for other host organisms (asymptomatic infective carriers) (www)www Reservoirs

Vectors A host that carries a pathogen without injury to itself and spreads the pathogen to susceptible organisms (asymptomatic carriers of pathogens)

Arthropod Vectors Pathogen - Vector Viruses (Arbovirus) - Mosquitoes Bacteria (Yersinia) - Fleas Bacteria (Borrelia) - Ticks Rickettsias (R. prowazeki) - Lice, ticks Protozoa (Plasmodium) - Mosquitoes Protozoa (Trypanozoma) -Tsetse flies Helminths (Onchocerca) - Simulium flies

Ecological Factors in Infections Altered environment {Air conditioning} Changes in food production & handling {intensive husbandry with antibiotic protection; deep-freeze; fast food industry} Climate changes {Global warming} Deforestation Ownership of (exotic) pets Air travel & Exotic journeys / Global movements Increased use of immunosuppressives/ antibiotics

ENDEMIC - EPIDEMIC - PANDEMIC  Endemic  Transmission occur, but the number of cases remains constant  Epidemic  The number of cases increases  Pandemic  When epidemics occur at several continents – global epidemic Time Cases R = 1 R > 1 R < 1 (www)www

Endemic Epidemic Number of Cases of a Disease Time Endemic vs Epidemic

Levels of Disease Occurrence Sporadic level: occasional cases occurring at irregular intervals Endemic level: persistent occurrence with a low to moderate level Hyperendemic level: persistently high level of occurrence Epidemic or outbreak: occurrence clearly in excess of the expected level for a given time period Pandemic: epidemic spread over several countries or continents, affecting a large number of people (www)www

Basic Measures of Disease Frequency Incidence – Rate at which new cases of disease occur. Prevalence – Measures both new and existing cases in a population with and without disease. – Proportion of people who have a specific condition at any specified time. Attack rate – Measures the cumulative incidence of disease during an epidemic or outbreak.

A TTACK R ATE SourcecasesPopulationAR per 1000 A689, B142, C1264, D42734, Total63850,

Ratio, Proportion, and Rate A sample with 2 categories, X and Y (e.g.males and females) X : Y = Ratio (Male : Female Ratio) X/(X+Y) % = Proportion A population P with Z events (cases) Z/P per 1,000 (constant) = Rate

M EASURES OF R ISK Relative Risk Exposed Unexposed RR :CA lung with SMK CA lung without SMK : 50/10 : 5 เท่า Incidence among exposed / Incidence among unexposed

A TTRIBUTABLE R ISK Incidence among exposed - Incidence among unexposed

Study Types Descriptive Epidemiological Studies – Information is only available about the occurrence of disease or associations among exposures, demographic characteristics, and disease rates in population groups Analytical Studies – Test hypotheses to evaluate exposure-disease information Experimental Studies - Clinical trial

Analytical Studies Cohort study Prospective : เหตุ ไปหาผล Relative risk Case control study Retrospective : ผลหาเหตุ Odd ratio

Reporting of Study Results The likelihood of a positive association is caused by random error can be estimated by calculating the level of statistical significance (p value) or the confidence interval (CI). Epidemiologists typically use the confidence interval. – CI provides a range of possible values of the risk estimate.

Fundamental Assumptions in Epidemiology Disease doesn’t occur at random Disease has causal and preventive factors Disease is not randomly distributed throughout a population Epidemiology uses systematic approach to study the differences in disease distribution in s ubgroups