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Epidemiology II. ENVR 890-Sec. 003/ENVR 296-Sec. 003 Mark D. Sobsey With material from Prof. Jack Colford, UC-Berkeley Dr. Steve Luby, ICDDR,B Joseph Eisenberh, U. of Michigan. Using Epidemiology for Microbial Risk Analysis. Problem Formulation what’s the problem?

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Epidemiology ii l.jpg

Epidemiology II

ENVR 890-Sec. 003/ENVR 296-Sec. 003

Mark D. Sobsey

With material from

Prof. Jack Colford, UC-Berkeley

Dr. Steve Luby, ICDDR,B

Joseph Eisenberh, U. of Michigan


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Using Epidemiology for Microbial Risk Analysis

Problem Formulation

  • what’s the problem?

  • determine what infectious disease is posing a risk

  • its clinical features

  • causative agent

  • routes of exposure/infection

  • health effects

    (This is sort of like a preliminary QMRA, especially for exposure and health effects analyses)


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Using Epidemiology for Microbial Risk Analysis

Exposure Assessment

  • how

  • how much

  • when

  • where and why exposure occurs

  • vehicles

  • vectors

  • doses

  • loads


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Using Epidemiology for Microbial Risk Analysis

Health Effects Assessment

  • Human clinical trials for dose-response

  • field studies of endemic and epidemic disease in populations


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Using Epidemiology for Microbial Risk Analysis Risk Characterization

Epidemiologic measurements and analyses of risk:

  • relative risk

  • risk ratios

  • odds ratios

  • regression models of disease risk

  • dynamic models of population disease risk

  • Other disease burden characterizations:

    • relative contribution to overall disease burdens

    • effects of prevention and control measures and interventions

    • economic considerations (monetary cost of the disease, cost effectiveness of prevention and control measures)


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    Elements That May Be Considered in Risk Characterization

    • Evaluate health consequences of exposure scenario

      • Risk description (event)

      • Risk estimation (magnitude, probability)

    • Characterize uncertainty/variability/confidence in estimates

    • Conduct sensitivity analysis

      • evaluate most important variables and information needs

    • Address items in problem formulation (reality check)

    • Evaluate various control measures and their effects on risk magnitude and profile

    • Conduct decision analysis

      • evaluate alternative risk management strategies


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    Types of Epidemiological Studies that Have Been Used in Risk Assessment for Waterborne Disease


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    Some More Epidemiological Terms and Concepts

    • Outbreaks: two or more cases of disease associated with a specific agent, source, exposure and time period

    • Epidemic Curve (Epi-curve): Number of cases or other measure of the amount of illness in a population over time during an epidemic

      • Describes nature and time course of outbreak

      • Can estimate incubation time if exposure time is known

      • Can give clues to modes of transmission: point source, common source, and secondary transmission


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    Some More Epidemiological Terms and Concepts: Epidemic Curves

    # cases

    # cases

    Time

    Time

    Common Source

    Point Source


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    Additional Analyses of Health Effects:Health Effects Assessments

    • Health Outcomes of Microbial Infection

    • Identification and diagnosis of disease caused by the microbe

      • disease (symptom complex and signs)

      • Acute and chronic disease outcomes

      • mortality

      • diagnostic tests

      • DALYs

    • Sensitive populations and effects on them

    • Disease Databases and Epidemiological Data


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    Databases for Quantification and Statistical Assessment of Disease - USA

    • National Notifiable Disease Surveillance System

    • National Ambulatory Medical Care Survey

    • International Classification of Disease (ICD) Codes

    • Other Databases

      • Special surveys

      • Sentinel surveillance efforts

    • Resources for disease surveillance vary greatly by country.

      • WHO and other international health entities assist countries lacking capacity for disease surveillance to obtain such data in various ways

      • Tracking is poor for some diseases, such as gastroenteritis and its specific causative agents (etiologies)


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    Methods to Diagnose Infectious Disease

    • Symptoms (subjective: headache, pain) and Signs (objective: fever, rash, diarrhea)

    • Clinical diagnosis: lab tests

      • Detect causative organism in clinical specimens

      • Detect other specific factors associated with infection

    • Immune response

      • Detect and assay antibodies

      • Detect and assay other specific immune responses


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    Health Outcomes of Microbial Infection

    • Acute Outcomes

      • Diarrhea, vomiting, rash, fever, etc.

    • Chronic Outcomes

      • Paralysis, hemorrhagic uremia (HUS), reactive arthritis, Guillain-Barre Syndrome, etc.

    • Hospitalizations

    • Deaths


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    Outcomes of Infection Process to be Quantified

    Exposure

    Infection

    Asymptomatic Infection

    Advanced Illness, Chronic Infections and Sequelae

    Disease

    Acute Symptomatic Illness: Severity and Debilitation

    Sensitive Populations

    Mortality

    Hospitalization


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    Morbidity Ratios for Salmonella (Non-typhi)


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    Acute and Chronic Outcomes Associated with Microbial Infections


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    Health Effects Outcomes: E. coli O157:H7


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    Health Effects Outcomes: Campylobacter


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    Sensitive Populations

    • Infants and young children

    • Elderly

    • Immunocompromised

      • Persons with AIDs

      • Cancer patients

      • Transplant patients

    • Pregnant

    • Malnourished


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    Mortality Ratios for Enteric Pathogens in Nursing Homes Versus General Population


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    Impact of Waterborne Outbreaks of Cryptosporidiosis on AIDS Patients


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    Mortality Ratios Among Specific Immunocompromised Patient Groups with Adenovirus Infection


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    Waterborne Outbreak Attack Rates- USA


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    Waterborne Outbreak Hospitalizations - USA


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    Predicted Waterborne Cryptosporidiosis in NYC in AIDS Patients Compared to the General Population

    Perz et al., 1998, Am. J. Epid., 147(3):289-301


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    Assessing Risk from Environmental Exposure to Waterborne Pathogens: Use of Dynamic, Population-Based Analytical Methods and Models

    The following material is based on a lecture prepared by Prof. Joe Eisenberg, formerly of the University of California-Berkeley and now at the University of Michigan

    Used with his permission


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    Overview

    • Role of water in disease burden

      • Water as a route of disease transmission

    • Methods of risk estimation

      • Direct: intervention trials

      • Indirect: risk assessment

    • Population-level risks

      • Example: the Milwaukee outbreak


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    Importance of Waterborne Pathogens

    Domestic: U.S. interest in water quality

    • 1993 Cryptosporidium outbreak

    • Increasing number of disease outbreaks associated with water

    • Congressional mandates for water quality

      • (Safe Drinking Water Act)

    • Emphasis on risk assessment and regulation


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    Pathways of Transmission

    • Person-person

      • Mediated through fomites (e.g., phone, sink, etc.)

      • Often associated with hygiene practices

    • Person-environment-person

      • Mediated through water, food, or soil

      • Contamination can occur through improper sanitation (example: sewage inflow into drinking water source or lack of latrines)

      • Animals are often sources (Zoonotic pathogens)

      • Exposure can occur through improper treatment of food or water


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    The Disease Transmission Process

    Risk estimation depends on transmission dynamics and exposure pathways

    Transport to other water sources

    Agricultural

    Runoff

    Drinking

    Water

    Recreational Waters

    or

    Wastewater reuse

    Animals

    Trans.

    Food


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    Approaches to Risk Estimation

    • Direct approach: The intervention trial

      • Can be used to assess risk from drinking water and recreational water exposures

      • Problems with sensitivity (sample size issue)

      • Trials are expensive

        • less so in developing world settings

    • Indirect approach: Mathematical models

      • Must account for properties of infectious disease processes

      • Pathogen specific models

      • Uncertainty and variability may make interpretation difficult.


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    Approaches to Risk Estimation

    • Combining direct and indirect approaches

      • Models can define the issues and help design studies.

      • Epidemiology can confirm current model structure and provide insight into how to improve the model


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    Approaches for Risk Estimation: Direct estimates of waterborne infectious illnesses

    • Surveillance: count waterborne infectious illnesses

      • How can a waterborne disease outbreak be distinguished from other outbreak causes (food, fomites, etc.)?

      • What about endemic disease?

    • Observational

      • Ecologic studies (e.g., sero-survey comparing communities with and without filtration)

      • Time series (e.g., correlation between turbidity and hospitalization data)


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    Approaches for Risk Estimation:Distinguishing waterborne GI disease from other GI diseases

    • Methods for addressing the question

      • In a single community: a randomized, blinded, placebo-controlled trial

      • design provides an estimate of the effectiveness of a drinking water intervention.

    • Basic study design: two groups

      • “Exposed” group = normal tap water.

      • “Treated” group = use a water treatment device to provide water as pathogen-free as technically possible


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    Approaches for Risk Estimation: A Tap Water Intervention Trial

    • Enroll 1000 subjects

    • 500 receive an active home water treatment device (and carry drinking water to work, etc. when practical)

    • 500 receive a “placebo” home water drinking device (does nothing to change the water)

    • Follow the subjects for one year with daily logs of GI illness

    • Alternative design: Each household changes device type after 6 months.


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    Approaches for Risk Estimation: A Tap Water Intervention Trial

    • Placebo (control) group (tap water):

      • 90 illnesses over course of the study

      • “Rate” = 90 / 500

        Rate in placebo group = 0.18 per person per year

    • Treated (intervention) group (active device):

      • 60 illnesses in the treated group (active device)

      • “Rate” = 60 / 500

        Rate in treated group = 0.12 per person per year


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    Approaches for Risk Estimation: Epidemiologic Measures

    • Relative Risk (RR)

      Incidence in exposed group

      Incidence in unexposed group

    • Interpretation: the risk of disease in the tap water group is 1.5 times higher than that of the treated group


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    Approaches for Risk Estimation: Epidemiologic Measures

    • Attributable Risk (AR)

      Incidence in exposed – Incidence in unexposed

      Interpretation: There are 6 excess cases of disease per 100 subjects receiving tap water


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    Approaches for Risk Estimation: Epidemiologic Measures

    • Attributable Risk Percent (AR%)

      Excess cases in exposed

      Incidence in exposed

      Interpretation: 33% of the cases of disease in the tap water group are due to water


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    Approaches for Risk Estimation: Epidemiologic Measures

    • To generalize beyond the cohort, need an estimate of the community incidence.

    • PAR: population attributable risk

    • PAR%: population attributable risk %

    • AR compares completely protected group with completely unprotected group.

    • PAR incorporates intermediate exposure


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    Approaches for Risk Estimation: Epidemiologic Measures

    • Population attributable risk

    • Incidence in the community–incidence in the unexposed

      Interpretation: In the community, 2 excess cases of disease per every 100 subjects in the community


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    Approaches for Risk Estimation: Epidemiologic Measures

    • Population attributable risk percentage

      Excess cases in the community

      Incidence in the exposed

      Interpretation: 14% of the cases of disease in the community are due to tap water


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    Approaches for Risk Estimation: Tap Water Intervention Trials: Developed World

    Trials in immunocompetent populations

    • Canada (Payment)--challenged surface water

      • AR = 0.35 (Study 1), 0.14-0.4 (Study 2)

    • Australia (Fairley)--pristine surface water

      • No effect

    • Walnut Creek (UCB) – pilot trial

      • AR = 0.24 (non-significant effect)

    • Iowa (UCB)--challenged surface water

      • No effect

        Trials in sensitive populations

    • HIV+ in San Francisco (UCB)--mixed sources

    • Elderly in Sonoma (UCB)--intermediate quality surface


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    Approaches for Risk Estimation: Tap Water Intervention Trials

    • Davenport, Iowa study

      • Comparing sham vs. active groups

      • AR = - 365 cases/10,000/year (CI: -2555, 1825)

      • Interpretation: No evidence of a significantly elevated drinking water risk

      • Is the drinking water safe?


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    Approaches for Risk Estimation: Risk Assessment vs. Intervention Trial

    Comparing estimates from a risk assessment to randomized trial results (Eienberg et al. AJE, submitted)

    • Data collected during the intervention trial

      • Self-report illnesses from participants: Weekly diaries

      • Source water quality: Cryptosporidium, Giardia, enteric viruses

      • Drinking water patterns: RDD survey

      • Water treatment: B. subtilis, somatic coliphage


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    Approaches for Risk Estimation: Risk Assessment Model


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    Approaches for Risk Estimation: Risk Assessment Model


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    Approaches for Risk Estimation: Risk Assessment Results

    Overall risk estimate: 14 cases/10,000/yr


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    Approaches for Risk Estimation: Comparison/Conclusions

    Table 3. Comparison of risk assessment and intervention trials


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    Conclusions

    Risk assessments should use models that can integrate relevant information

    • Health data

      • Epidemiology

      • Basic biology

    • Environmental data

      • Water quality

      • Fate and transport

    • Need a population perspective

      • Model-based approach


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