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Microbial Risk Assessment

Microbial Risk Assessment

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Microbial Risk Assessment

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  1. Microbial Risk Assessment Envr 133 Mark D. Sobsey Spring, 2006

  2. Definition of Quantitative Microbial Risk Assessment Applications of the principles of risk assessment to the estimation of the consequences from anticipated or actual exposure to infectious microorganisms

  3. Relationship Between Exposure, Level of Protection and Microbial Risk = Confidence Region or Interval Risk  Exposure  Level of Technological Control

  4. Some Differences Between Chemical and Microbial Risks • A single microbe (one unit) is infectious • Microbes multiply: • In a host • In environmental media (some) • Secondary spread • Microbe infects a host from an environmental route of exposure (water, food, etc.) can • Then, it spreads to other hosts by person-to-person transmission • Some microbes cause a wide range (spectrum) of adverse effects

  5. (Adapted from: National Academy of Sciences - National Research Council framework)


  7. ILSI/EPA Risk Assessment Framework and Steps: Analysis Phase

  8. (Adapted from: National Academy of Sciences - National Research Council framework)

  9. Conducting Hazard Identification • Identify microbes as causative agent of disease • Develop/identify diagnostic tools to identify symptoms, infection and to isolate and identify causative microbe in host specimens • Understand the disease process from exposure to infection, illness (pathophysiology) and death • Identify transmission routes • Assess virulence factors and other properties of the microbe responsible for disease, including life cycle • Identify and apply diagnostic tools to determine incidence and prevalence in populations and investigate disease outbreaks • Develop models (usually animals to study disease process and approaches to treatment • Evaluate role of immunity in overcoming/preventing infection and disease and possible vaccine development • Study epidemiology associated with various exposures

  10. (Adapted from: National Academy of Sciences - National Research Council framework)

  11. Exposure Assessment • Purpose: determine the dose • Dose = number, quantity or amount of microorganisms corresponding to a single exposure (e.g., by ingestion) • Average or typical dose • a measure of central tendency • mean or median • Distribution of doses Described as a probability or frequency distribution; “probability density function”

  12. CHARACTERIZATION OF EXPOSUREELEMENTS INCLUDED IN PATHOGEN CHARACTERIZATION: OCCURRENCE (previous lecture) • Temporal distribution, duration and frequency • Concentration in food or environmental media • Spatial distribution • clumping, aggregation, particle-association, clustering • Niche • ecology and non-human reservoirs • potential to multiply/survive in specific foods or media • Survival, persistence, and amplification • Seasonality • Meteorological and climatic events • Presence of control or treatment processes • including their reliability and variability • Indicators/surrogates for indirect evaluation • predictive of pathogen

  13. ELEMENTS CONSIDERED IN PATHOGEN CHARACTERIZATION (previous lecture) • Virulence and pathogenicity of the microorganism • Pathologic characteristics and diseases caused • Survival and multiplication of the microorganism • Resistance to control or treatment processes • Host specificity • Infection mechanism and route; portal of entry • Potential for secondary spread • Taxonomy and strain variation • Ecology and natural history

  14. Pathogen Characteristics or Properties Favoring Environmental Transmission (previous lecture) Multiple sources and high endemicity in humans, animals and environment • High concentrations released into or present in environmental media (water, food, air) • High carriage rate in human and animal hosts • Asymptomatic carriage in non-human hosts • Ability to proliferate in water and other media • Ability to adapt to and persist in different media or hosts • Seasonality and climatic effects • Natural and anthropogenic sources

  15. Pathogen Characteristics or Properties Favoring Environmental Transmission (previous lecture) • Ability to Persist or Proliferate in Environment and Survive or Penetrate Treatment Processes • Stable environmental forms • spores, cysts, oocysts, stable outer viral layer (protein coat), capsule, etc. • Resistance to biodegradation, heat, cold (freezing), drying, dessication, UV light, ionizing radiation, pH extremes, etc. • Resists proteases, amylases, lipases and nucleases • Posses DNA repair mechanisms and other injury repair processes • Colonization, biofilm formation, resting stages, protective stages, parasitism • Spatial distribution • Aggregation, particle association, etc.

  16. Virulence Properties of Pathogenic Bacteria Favoring Environmental Transmission (previous lecture) Virulence properties: structures or chemical constituents that contribute to pathophysiology: • Outer cell membrane of Gram negative bacteria: endotoxin (fever producer) • Exotoxins • Pili: for attachment and effacement to cells and tissues • Invasins: to facilitate cell invasion • Effacement factors Spores: • highly to physical and chemical agents and • very persistent in the environment Others: • plasmids, lysogenic bacteriophages, etc.

  17. Pathogen Characteristics or Properties Favoring Environmental Transmission (previous lecture) Genetic properties favoring survival and pathogenicity • Double-stranded DNA or RNA • DNA repair • Ability for genetic exchange, mutation and selection • recombination • plasmid exchange, transposition, conjugation, etc. • point mutation • reassortment • gene expression control • Virulence properties: expression, acquisition, exchange • Antibiotic resistance

  18. Role of Selection of New Microbial Strains in Susceptibility to Infection and Illness (previous lecture) • Antigenic changes in microbes overcome immunity, increasing risks of re-infection or illness • Antigenically different strains of microbes appear and are selected for over time and space • Constant selection of new strains (by antigenic shift and drift) • Partly driven by “herd” immunity and genetic recombination, reassortment , bacterial conjugation, bacteriophage infection and point mutations • Antigenic Shift: • Major change in virus genetic composition by gene substitution or replacement (e.g., reassortment) • Antigenic Drift: • Minor changes in virus genetic composition, often by mutation involving specific codons in existing genes (point mutations) • A single point mutation can greatly alter microbial virulence

  19. Pathogen Characteristics or Properties Favoring Environmental Transmission (previous lecture) Ability to Cause Infection and Illness • Low infectious dose • Infects by multiple routes • ingestion (GI) • inhalation (respiratory) • cutaneous (skin) • eye • etc.

  20. Microbe Levels in Environmental Media Vary Over Time - Occurrence of Giardia Cysts in Water: Cumulative Frequency Distribution Previous lecture

  21. CHARACTERIZATION OF EXPOSUREELEMENTS CONSIDERED IN EXPOSURE ANALYSIS (previous lecture) • Identification of water, food or other media/vehicles of exposure • Units of exposure • Routes of exposure and transmission potential • Size of exposed population • Demographics of exposed population • Spatial and temporal nature of exposure (single or multiple; intervals) • Behavior of exposed population • Treatment, processing, and recontamination

  22. (Adapted from: National Academy of Sciences - National Research Council framework)

  23. CHARACTERIZATION OF HUMAN HEALTH EFFECTSELEMENTS CONSIDERED IN HOST CHARACTERIZATION (previous lecture) • Age • Immune status • Concurrent illness or infirmity • Genetic background • Pregnancy • Nutritional status • Demographics of the exposed population (density, etc.) • Social and behavioral traits

  24. CHARACTERIZATION OF HUMAN HEALTH EFFECTSELEMENTS CONSIDERED IN HEALTH EFFECTS (previous lecture) • Duration of illness • Severity of illness • Infectivity • Morbidity, mortality, sequelae of illness • Extent or amount of secondary spread • Quality of life • Chronicity or recurrence

  25. Characteristics or Properties of Pathogens Interactions with Hosts (previous lecture) • Disease characteristics and spectrum • Persistence in hosts: • Chronicity • Persistence • Recrudescence • Sequelae and other post-infection health effects • cancer, heart disease, arthritis, neurological effects • Secondary spread

  26. Elements That May be Included in Dose-Response Analysis • Statistical model(s) to analyze of quantify dose-response relationships • Human dose-response data • Animal dose-response data • Utilization of outbreak or intervention data • Route of exposure or administration • Source and preparation of challenge material or inoculum • Organism type and strain • including virulence factors or other measures of pathogenicity • Characteristics of the exposed population • age, immune status, etc. • Duration and multiplicity of exposure

  27. Dose-Response Dataand Probability of Infection for Human Rotavirus

  28. Dose-Response Models and Extrapolation to Low Dose Range • Most dose-response data for microbes are for high doses of microbes and few hosts • due practicalities and cost limits • Real world exposures to microbes from water, food and air are often to much lower microbial doses • It becomes necessary to extrapolate the dose-response relationship to the low dose range where there are no experimental data points • a best-fit modelling approach is employed

  29. Models Typically Applied in Microbial Dose-Response Analyses • Exponential model: Pinfection = 1 - e-r where r = probability of infection and  = mean concentration/dose • assumes organisms are distributed randomly (Poisson) and • probability of infection = r • approaches a linear model at low doses • Exponential (linear) model; two populations: • one-hit kinetics, but • two classes of human susceptibility to microbe • Beta-Poisson: a distributedthreshold model • assumes Poisson distribution of microbes and a Beta-distributed probability of infection • r is not a constant but a probability distribution (Beta-distribution) • two variables in the model

  30. Probabilities of Exposure and Infection • Pexp (j Dose) = Probability of having j pathogenic microbes in an ingested dose • Pinf (j Inf) = Conditional probability of infection from j pathogens ingested

  31. Probability of Exposure

  32. Exponential Dose-Response Model

  33. Beta-Poisson Dose-Response Model

  34. Rotavirus Dose-Response Relationships:Experimental Data, Exponential Model and Beta-Poisson Model

  35. Daily and Annual Risks of Various Outcomes from Exposure to Water Containing 4 Rotaviruses per 1000 Liters

  36. Volunteer Dose-Response Data for Norwalk Virus* Dose (ml)No. Dosed No Ill % Ill 4 16 11 69 1 21 14 67 0.01 4 2 50 0.0001 4 0 0 *"1st passage NV": Dolin et al. 1972; Wyatt et al., 1974.

  37. Norwalk Virus Dose-Response Analysis Using Alternative Models

  38. Dose-Response Relationships for Various Waterborne Pathogens: Downward Extrapolation to Low-Dose Range

  39. Comparing Risks of Disease Agents • Comparing chemical to microbial risks as well as among agents of each type • Effects vary widely in severity, mortality rates and time scale of exposure • Need to protect both quality and quantity of life • Drinking water policy needs to be linked to overall public health policy • Decision making process needs to take social and economic factors into account

  40. Desirable attributes of an integrated measure of risk • Address probability, nature and magnitude of adverse health consequences • Incorporate age and health status of those affected

  41. DALYs as unit measuresfor health • Conceptually simple: • health loss = N x D x S • N = number of affected persons • D = duration of adverse health effect • S = measure for severity of the effect • Disability Adjusted Life Years • mortality: years of life lost (YLL) • morbidity: years lived with disability (YLD) • DALY = YLL + YLD

  42. Hypothetical example Premature death Acute (infectious) disease Residual disability

  43. Key Question: define health? • ‘a state of complete physical, mental and social well-being, and not merely the absence of disease or infirmity’ (WHO charter, 1946) • ‘the ability to cope with the demands of daily life’ (the Dunning Committee on Medical Cure and Care, 1991) • the absence of disease and other physical or psychological complaints (NSCGP, 1999)

  44. Deriving severity weights • Global Burden of Disease Project • Define 22 indicator conditions • Use Person Trade Off method to elicit severity weights • Panel of physicians and public health scientists • Use scale of indicator conditions to attribute severity weights to other conditions • Methodology also applied in other studies

  45. 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 and health effects • Exposure Assessment: How, how much, when, where and why exposure occurs; vehicles, vectors, doses, loads, etc. • Health Effects Assessment: • Human clinical trials for dose-response • field studies of endemic and epidemic disease in populations • Risk characterization: Epidemiologic measurements and analyses of risk: relative risk, risk ratios, odds ratios; regression models of disease risk; dynamic model of disease risk • other disease burden characterizations: relative contribution to overall disease burdens; effects of prevention and control measures; economic considerations (monetary cost of the disease and cost effectiveness of prevention and control measures

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

  47. Some More Epidemiological Terms and Concepts # cases • 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 Point Source Time # cases Common Source Time

  48. Databases for Quantification and Statistical Assessment of Disease • National Notifiable Disease Surveillance System • National Ambulatory Medical Care Survey • International Classification of Disease (ICD) Codes • Other Databases • Special surveys • Sentinel surveillance efforts