10/10/2009Dr. Salwa Tayel1

10/10/2009Dr. Salwa Tayel2 Associate Professor Family and Community Medicine Department King Saud University By Infectious Disease Epidemiology

10/10/2009 Dr. Salwa Tayel 3 Learning Objectives At the end of this lecture you (will) be able to: Explain common definitions and basic concepts used in epidemiology. Apply these definitions in different situations. Identify the epidemiological approach to study health problems Describe natural history and spectrum of infectious diseases and their implications for public health.

10/10/2009 Dr. Salwa Tayel 4 Epidemiology is the study of distribution and determinants of health-related events in specified populations and the application of this study to prevent and control of health problems. Definitions

10/10/2009 Dr. Salwa Tayel 5 Person………. Place………….. Time…………… Describe disease frequency –Incidence Rate…… –Prevalence Rate…… Distribution

10/10/2009 Dr. Salwa Tayel6 The Epidemiologic Triad HOST AGENTENVIRONMENT Disease Determinants

10/10/2009 Dr. Salwa Tayel 7 The epidemiological approach: An Epidemiologist’ approach to study health- related events involves answering five questions: What? Who? Where? When? Why?

10/10/2009 Dr. Salwa Tayel 8 Infection: It is the entry, development and multiplication of an infectious agent in the body of man or animal. Outcome of infection varies. Infectious disease: A clinically manifest disease of man or animal resulting from infection. In-apparent infection: The infection does not become manifest at any stage.

10/10/2009 Dr. Salwa Tayel 9

10/10/2009 Dr. Salwa Tayel 10 An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent from reservoir to susceptible host. Communicable Disease (typhoid, influenza,.. Non- Communicable Disease (DM, cancer,.. Communicable Disease

10/10/2009 Dr. Salwa Tayel 11 Contamination: The presence of living infectious agents on the exterior surface of the body or on the clothes or articles of the person or on any inanimate object in the environment including water and food. Contagious diseases A disease that is capable of being transmitted from one person to another by contact or close proximity. e.g. scabies, trachoma and leprosy.

10/10/2009 Dr. Salwa Tayel 12 Endemic The constant presence of a disease or infectious agent within a given geographic area or population group; It may also refer to the usual prevalence of a given disease within such area or group. Epidemic The occurrence of more cases of a disease than expected in a given area or among a specific group of people over a particular period of time.

10/10/2009 Dr. Salwa Tayel 13 Pandemic An epidemic occurring over a very wide area (several countries or continents) at the same time and usually affecting a large proportion of the population. e.g. Influenza, cholera Outbreak A more or less localized epidemic affecting large number of a group, in the community e.g. outbreak of food poisoning

10/10/2009 Dr. Salwa Tayel 14 Sporadic Cases occur irregularly, haphazardly from time to time and generally infrequently. Cases are few and separated widely in space and time showing no connection to each other.

10/10/2009 Dr. Salwa Tayel 15 Nosocomial infection (Hospital acquired infection): It is an infection occurring in a patient while in a hospital or other health care facility It should not be present or incubating at the time of admission.

10/10/2009 Dr. Salwa Tayel 16 Elimination of disease Disease incidence is reduced to a minimal level at which the disease is no longer considered a public health problem, while infection may still occur Example: The aim of elimination of Neonatal tetanus is reduction of its incidence to less than one case/1000 live births.

10/10/2009 Dr. Salwa Tayel 17 Eradication:  It means worldwide disappearance of a disease (permanent reduction to zero level) with complete destruction of the agent.  The organism can be present only in laboratories and no need for interventions.  e.g. smallpox eradication from the world since 1979.

10/10/2009 Dr. Salwa Tayel 18 Natural history Natural history of disease refers to the progress of a disease process in an individual over time, in the absence of intervention. The natural history of a disease describes the course of the disease in an individual starting from the moment of exposure to the causal agents till one of the possible outcomes occurs.

10/10/2009 Dr. Salwa Tayel 19

10/10/2009 Dr. Salwa Tayel 20 Spectrum of disease The idea that an exposure can lead to varying signs, symptoms and severity of the same disease in the population is the spectrum of disease. Why do we have varying degrees of severity? Prognosis? The outcome will depend on the interactions of host, agent and environmental factors.

10/10/2009 Dr. Salwa Tayel 21 The Epidemiologic Triad HOST AGENTENVIRONMENT Disease Determinants

10/10/2009 Dr. Salwa Tayel 22 Agent factors refer to an infectious microorganism— virus, bacterium, parasite, or other microbes.Agent factors refer to an infectious microorganism— virus, bacterium, parasite, or other microbes. They are necessary but not always sufficient alone to cause disease.They are necessary but not always sufficient alone to cause disease. Host factors are intrinsic factors that influence an individual’s exposure, susceptibility, or response to a causative agent.( age, nutrition, race,…Host factors are intrinsic factors that influence an individual’s exposure, susceptibility, or response to a causative agent.( age, nutrition, race,… Environmental factors are extrinsic factors which affect the agent and the opportunity for exposure.Environmental factors are extrinsic factors which affect the agent and the opportunity for exposure.

10/10/2009Dr. Salwa Tayel23 Factors Affecting Disease Transmission and Symptomatic Clinical Disease Host Vector AgentEnvironment -- Susceptibility -- Immune response -- Resistance --- Portal(s) of entry -- Virulence Toxigenicity -- Infectivity Resistance -- Pathogenicity Antigenicity VECTOR -- Prevalence -- Portal(s) of entry -- Balance of immune to susceptible individuals --- Opportunity for exposure (e.g. crowding)

10/10/2009 Dr. Salwa Tayel 24 Classification of diseases according to clinical severity (spectrum)

10/10/2009 Dr. Salwa Tayel 25 Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS Class A: Inapparent infection

10/10/2009 Dr. Salwa Tayel 26 Examples: Measles, Chickenpox Class B: Classic cases

10/10/2009 Dr. Salwa Tayel 27 Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg viruses. Class C: Severe or Fatal infections

10/10/2009 Dr. Salwa Tayel 28 Implications for public health

10/10/2009 Dr. Salwa Tayel 29 The relation of severity of illness to disease statistics.

10/10/2009 Dr. Salwa Tayel 30 The pyramid and iceberg of disease 1Diseased, diagnosed & controlled 2Diagnosed, uncontrolled 3Undiagnosed or wrongly diagnosed disease 4Risk factors for disease 5Free of risk factors Diagnosed disease Undiagnosed or wrongly diagnosed disease

10/10/2009 Dr. Salwa Tayel 31 Cases of illness correctly diagnosed by clinicians in the community often represent only the “tip of the iceberg.” Many additional cases may be too early to diagnose or may remain asymptomatic. Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS. The risk is that persons with in-apparent or undiagnosed infections may be able to transmit infection to others. Iceberg phenomenon

10/10/2009 Dr. Salwa Tayel 32 Timelines for Infection and Symptomatic Disease Dynamics of infectiousness latent period infectious period noninfectious -- removed -- dead -- recovered susceptible time Dynamics of disease susceptible time incubation period symptomatic period -- dead -- recovered -- immune -- carrier E

10/10/2009 Dr. Salwa Tayel 33 The pyramid and iceberg of disease 1Diseased, diagnosed & controlled 2Diagnosed, uncontrolled 3Undiagnosed or wrongly diagnosed disease 4Risk factors for disease 5Free of risk factors Diagnosed disease Undiagnosed or wrongly diagnosed disease

10/10/2009 Dr. Salwa Tayel 34 Applications of the concepts of natural history and spectrum of disease Persons with in-apparent or undiagnosed infections can transmit infections to others. Control measures must be directed toward all infections capable of being transmitted to others; –both clinically apparent cases and –those with in-apparent or undiagnosed infections.

10/10/2009 Dr. Salwa Tayel 35 Further reading: Principles of EPIDEMIOLOGY in Public Health Practice Third Edition An Introduction to Applied Epidemiology and Biostatistics U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention (CDC) Office of Workforce and Career Development Atlanta, GA ( free Online)

10/10/2009 Dr. Salwa Tayel 36 EPIDEMIOLOGY IN MEDICINE Charles H. Hennekens, M.D., Dr.P.H. Julie E. Buring, Sc.D. Park’s textbook of PREVENTIVE AND SOCIAL MEDICINE John E. Park

10/10/2009 Dr. Salwa Tayel 37 The End Thank You Website