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Epidemiologic concepts for the prevention and control of microbial threats

Understanding strategies to prevent and control infectious diseases. Reduce contact rate (case finding

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Epidemiologic concepts for the prevention and control of microbial threats

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    1. Epidemiologic concepts for the prevention and control of microbial threats [INSTRUCTIONS ARE IN CAPITAL LETTERS. Talking points are in normal type.] This lecture covers the core epidemiologic concepts necessary for the investigation, prevention, and control of infectious diseases, including microbial threats.[INSTRUCTIONS ARE IN CAPITAL LETTERS. Talking points are in normal type.] This lecture covers the core epidemiologic concepts necessary for the investigation, prevention, and control of infectious diseases, including microbial threats.

    2. Understanding strategies to prevent and control infectious diseases Reduce contact rate (case finding & isolation, contact tracing & quarantine, behavior change) Reduce infectiousness (treatment, vaccination) Reduce susceptibility (vaccination, immune globulin) Interrupt transmission (infection control) Identify and control reservoir/source (pest/vector control, environmental disinfection) Reduce prevalence of infectious sources (identify and control infectious sources) Reduce duration of infectiousness (treatment, vaccination) Increase herd immunity (vaccination) Here is where we are going. These are the core public health and medical measures to prevent and control infectious disease threats. We will be covering the epidemiologic concepts that that provide the rationale for this measures. By end of this lecture we should be able to understand the core epidemiologic concepts supporting these control measures, and how how these control measures relate to each other. Here is where we are going. These are the core public health and medical measures to prevent and control infectious disease threats. We will be covering the epidemiologic concepts that that provide the rationale for this measures. By end of this lecture we should be able to understand the core epidemiologic concepts supporting these control measures, and how how these control measures relate to each other.

    3. Overview Traditional epidemiologic approach Infectious disease epidemiology concepts Transmission mechanisms Model for human-microbe interaction Chain model of infectious diseases Natural history of infection/infectiousness Transmission dynamics Reproductive number (R) Conditional infection rate (I) Application to a smallpox control strategy

    4. The epidemiologic approach: What is epidemiology? 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 First, let's review one definition of epidemiology. Epidemiology is ...First, let's review one definition of epidemiology. Epidemiology is ...

    5. The epidemiologic approach: What is epidemiology? Study Distribution (descriptive epidemiology) The who, what, where, when, and how many? Determinants (analytic epidemiology) The how and why? Health-related states or events Specified populations Applications Here is the same definition but now displayed by its major components. In descriptive epidemiology, we summarize the “distribution” of “health-related events” (what) by person (who), place (where), and time (when). Our goals includes measuring the burden of disease and injury, and generating ideas of what is causing or contributing to the observed differences. In analytic epidemiology, we design studies and conduct analyses to test causal hypotheses in order to direct prevention and control measures. “Determinants” is jargon for “causes.”Here is the same definition but now displayed by its major components. In descriptive epidemiology, we summarize the “distribution” of “health-related events” (what) by person (who), place (where), and time (when). Our goals includes measuring the burden of disease and injury, and generating ideas of what is causing or contributing to the observed differences. In analytic epidemiology, we design studies and conduct analyses to test causal hypotheses in order to direct prevention and control measures. “Determinants” is jargon for “causes.”

    6. The epidemiologic approach: Steps to public health action This slide summarizes the “epidemiologic approach.” Using descriptive vs. analytic approaches Using epidemiologic measures and methods Drawing valid conclusions (inferences) after considering and controlling for alternative explanations (chance, bias, confounding) Epidemiology supports the implementation of evidence-based measures targeting individual behaviors clinical interventions community-level interventions, and environmental/legislative approaches (e.g., laws to reduce exposure to environmental tobacco smoke). This slide summarizes the “epidemiologic approach.” Using descriptive vs. analytic approaches Using epidemiologic measures and methods Drawing valid conclusions (inferences) after considering and controlling for alternative explanations (chance, bias, confounding) Epidemiology supports the implementation of evidence-based measures targeting individual behaviors clinical interventions community-level interventions, and environmental/legislative approaches (e.g., laws to reduce exposure to environmental tobacco smoke).

    7. Infectious disease epidemiology concepts Mechanisms Model for human-microbe interaction Chain model of infectious diseases Natural history of infection/infectiousness Dynamics Reproductive number (R) Conditional infection rate (I) Infectious diseases differ in important ways from non-infectious diseases. Infectious diseases differ in important ways from non-infectious diseases.

    8. Epidemiologic concepts for the control of microbial threats

    10. A. Host-Agent-Environment model Most public health practitioners have been introduced to The Epidemiologic Triad (Agent-Host-Environment) as a model for understanding causation in infectious diseases. In this model, the environment influences the agent, the host, and the route of transmission of the agent from a source to the host. This simple model has served us well for many years. However, to better understand the factors that interact with the agent and host to permit or promote the emergence of microbial threats, the Institute of Medicine published the Convergence Model of Human-Microbe Interaction (next slide).Most public health practitioners have been introduced to The Epidemiologic Triad (Agent-Host-Environment) as a model for understanding causation in infectious diseases. In this model, the environment influences the agent, the host, and the route of transmission of the agent from a source to the host. This simple model has served us well for many years. However, to better understand the factors that interact with the agent and host to permit or promote the emergence of microbial threats, the Institute of Medicine published the Convergence Model of Human-Microbe Interaction (next slide).

    11. A. Convergence model for human-microbe interaction The “Convergence model” was published by the Institute of Medicine in March 2003. Think of this model as the updated version of the agent-host-environment model of infectious disease causation (also referred to as the “epidemiologic triad”). The model reminds us that the emergence of microbial threats is the result of the interaction of the agent, the host, and the physical-ecological-social-political-economic environment. The emergence of West Nile virus infection in the United States is a good example of the interaction of all these components. This broad approach helps us to understand the interrelated causes, and to understand the success or failure of prevention and control programs. The “Convergence model” was published by the Institute of Medicine in March 2003. Think of this model as the updated version of the agent-host-environment model of infectious disease causation (also referred to as the “epidemiologic triad”). The model reminds us that the emergence of microbial threats is the result of the interaction of the agent, the host, and the physical-ecological-social-political-economic environment. The emergence of West Nile virus infection in the United States is a good example of the interaction of all these components. This broad approach helps us to understand the interrelated causes, and to understand the success or failure of prevention and control programs.

    12. B. Chain model of infectious diseases For infection to occur, all the links in the chain must connect. In its natural settings, the agent resides in a reservoir (E coli O157:H7 in cattle). The source is where the agent is prior to infecting the host (meat contaminated with E coli O157:H7). Sometimes the source is the reservoir (children visiting a petting farm). The portal of exit is how the agent exits the reservoir or source (cattle fecally excrete E coli O157:H7). The mode of transmission is the mechanism by which the agent is transmitted from the source/reservoir to the host (contact, droplet, airborne, etc.). The portal of entry is how the agent enters the host (respiratory, gastrointestinal, mucous membranes, etc.). Of course, the host must be susceptible to infection by the agent for infection to occur.For infection to occur, all the links in the chain must connect. In its natural settings, the agent resides in a reservoir (E coli O157:H7 in cattle). The source is where the agent is prior to infecting the host (meat contaminated with E coli O157:H7). Sometimes the source is the reservoir (children visiting a petting farm). The portal of exit is how the agent exits the reservoir or source (cattle fecally excrete E coli O157:H7). The mode of transmission is the mechanism by which the agent is transmitted from the source/reservoir to the host (contact, droplet, airborne, etc.). The portal of entry is how the agent enters the host (respiratory, gastrointestinal, mucous membranes, etc.). Of course, the host must be susceptible to infection by the agent for infection to occur.

    13. Chain model of infectious diseases This illustration visually summarizes the chain model of infection.This illustration visually summarizes the chain model of infection.

    14. Chain model of infectious diseases: Reservoir Human Symptomatic illness Carriers Asymptomatic (no illness during infection) Incubatory (pre-illness) Convalescent (post-illness recovery) Chronic (persistent infection) Animal (zoonoses) Environmental Reservoirs for microbes are either human, animal, or environmental. Examples of diseases where humans are the reservoir for the agent include: polio, hepatitis A, B, & C, measles, mumps, rubella, varicella, and smallpox (before eradication), malaria, etc Examples of diseases where animals are the reservoir for the agent include: West Nile virus disease (migratory birds), Lyme disease (rodents), E coli O157:H7 (cattle), cryptosporidiosis (cattle) Examples of diseases where the environment are the reservoir for the agent include: legionellosis (water), leptospirosis (water), mycobacterium avium complex (soil, water), coccidioidomycosis (soil dust).Reservoirs for microbes are either human, animal, or environmental. Examples of diseases where humans are the reservoir for the agent include: polio, hepatitis A, B, & C, measles, mumps, rubella, varicella, and smallpox (before eradication), malaria, etc Examples of diseases where animals are the reservoir for the agent include: West Nile virus disease (migratory birds), Lyme disease (rodents), E coli O157:H7 (cattle), cryptosporidiosis (cattle) Examples of diseases where the environment are the reservoir for the agent include: legionellosis (water), leptospirosis (water), mycobacterium avium complex (soil, water), coccidioidomycosis (soil dust).

    15. C. Modes of transmission for an exogenous agent Contact Direct (touch, kissing, sex) Droplet and respiratory secretions Indirect (intermediate object, usually inanimate) Vertical transmission (before, during, & after) Airborne (droplet nuclei, dust) Vehicle (ingestion, instrumentation, infusion) Vector-borne (mechanical, biologic) Modes of transmission have implications for interrupting transmission, disease control, and infection control. To the extent the mode of transmission is unknown, it becomes the focus of an investigation. Contact-direct: sexually transmitted diseases Contact-droplet: SARS, smallpox, pneumonic plague Contact-indirect: contaminated surfaces and fomites (SARS, smallpox) Airborne: TB, measles, varicella Vehicle: Ingested food or water; intravenous infusion Vector-borne: mosquitos transmitting West Nile virus, malariaModes of transmission have implications for interrupting transmission, disease control, and infection control. To the extent the mode of transmission is unknown, it becomes the focus of an investigation. Contact-direct: sexually transmitted diseases Contact-droplet: SARS, smallpox, pneumonic plague Contact-indirect: contaminated surfaces and fomites (SARS, smallpox) Airborne: TB, measles, varicella Vehicle: Ingested food or water; intravenous infusion Vector-borne: mosquitos transmitting West Nile virus, malaria

    16. Good infection control starts with common sense: cover the source!

    17. Disease scare at San Jose airport 5 on flight from Asia examined -- none found with SARS, SF Chronicle April 2, 2003

    20. Natural history of infection, infectiousness, and disease

    21. Natural history of infection, infectiousness, and disease

    22. Natural history of infection, infectiousness, and disease

    23. Incubation & infectious distributions of SARS and other infections

    24. D. Basic reproductive number (R0)

    25. Basic reproductive number (R0) (perspective of infectious agent)

    28. Effective reproductive number (R)

    29. Effective reproductive number (R)

    34. E. Conditional infection rate (perspective of susceptible hosts)

    36. Understanding strategies to prevent and control infectious diseases Reduce contact rate (case finding & isolation, contact tracing & quarantine, behavior change) Reduce infectiousness (treatment, vaccination) Reduce susceptibility (vaccination, immune globulin) Interrupt transmission (infection control) Identify and control reservoir/source (pest/vector control, environmental disinfection) Reduce prevalence of infectious sources (identify and control infectious sources) Reduce duration of infectiousness (treatment, vaccination) Increase herd immunity (vaccination) Here is where we are going. These are the core public health and medical measures to prevent and control infectious disease threats. We will be covering the epidemiologic concepts that that provide the rationale for this measures. Here is where we are going. These are the core public health and medical measures to prevent and control infectious disease threats. We will be covering the epidemiologic concepts that that provide the rationale for this measures.

    38. Epidemiologic concepts for the control of microbial threats

    39. Summary Traditional epidemiologic approach Infectious disease epidemiology concepts Transmission mechanisms Model for human-microbe interaction Chain model of infection Natural history of infection/infectiousness Transmission dynamics Reproductive number (R) Conditional infection rate (I) Application to a smallpox control strategy

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