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Epidemiology and WSH Infectious Diseases


Epidemiology and WSH Infectious Diseases Lecture 4 ENVR 890-2 Mark D. Sobsey Epidemiology - Definition The logic of observation and the methods to quantify these ... – PowerPoint PPT presentation

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Title: Epidemiology and WSH Infectious Diseases

Epidemiology and WSH Infectious Diseases
  • Lecture 4
  • ENVR 890-2
  • Mark D. Sobsey

Epidemiology - Definition
  • The logic of observation and the methods to
    quantify these observations in populations
    (groups) of individuals.
  • The study of the distribution of health-related
    states or events in specified populations and the
    application of this study to the control of
    health problems.
  • Epidemiology includes
  • 1) methods for measuring the health of groups and
    for determining the attributes and exposures that
    influence health
  • 2) study of the occurrence of disease in its
    natural habitat rather than the controlled
    environment of the laboratory (exception
    clinical trials) and
  • 3) methods for the quantitative study of the
    distribution, variation, an determinants of
    health-related outcomes in specific groups
    (populations) of individuals, and the application
    of this study to the diagnosis, treatment, and
    prevention of these states or events.

Infectious Disease Epidemiology Classical
  • the study of epidemics
  • the study of the dynamic factors involved in the
    transmission of infectious agents in populations
  • the natural history of infectious disease
  • how a disease spreads through groups or a
  • how a case of that disease develops in an

Basic Epidemiological Concepts and Terms
  • Incidence of new cases of disease/total at
  • Incidence rate Incidence/unit of time.
  • Prevalence cases (or with defined condition)
    existing at one time.
  • Prevalence rate of such cases/total at risk.
  • Epidemic
  • cases in excess of expected for population
  • the uncontrolled spread of a disease (or
    condition) in a community.
  • Immunity Inherited, acquired, or induced
    resistance to infection by a specific pathogen
  • Acquired resistance due to previous infection is
    from protective cellular and antibody responses
    in the host
  • Herd immunity cumulative of immune persons in
    population or of population immune.

Outbreaks or Epidemics
  • A disease or condition at involves many or an
    excessive number of people at the same time and
    the same place
  • The occurrence of a disease or condition at a
    frequency that is unusual or unexpected
  • increase above background or endemic level
  • Requirements for an outbreak or epidemic
  • (i) presence of an infected host or other source
    of infection.
  • (ii) adequate number of susceptibles
  • (iii) an effective method of contact for
    transmission to occur.

Transmission/Exposure Routes of Infectious
Agents Entry to and/or Exit From the Body
  • Sites or Portals of Exit or Entry
  • Respiratory
  • Enteric or Gastrointestinal
  • Skin especially if skin barrier is penetrated
  • Genitourinary
  • Eye

Routes or Methods of Entry
  • Direct Personal Contact Person
  • Indirect Personal Contact Droplet, Fomites,
    Other Vehicles
  • Water and Food (Gastrointestinal Tract)
  • Vector-borne often insects
  • Intrauterine or Transplacental
  • Organ Transplants, Blood and Blood Products

Transmission Routes of Infectious Agents
Infectious Diseases and the Process of Infection
  • Infection the growth/multiplication of a microbe
    in a host
  • Infection does not always result in injury of
    the host (disease)
  • Two main classes of infection by site
  • localized
  • generalized (disseminated systemic)
  • Some infections are usually localized but can
    sometimes spread to another site
  • Example Amoebic dysentery occurs in the
    intestines (colon) sometimes it spreads to the
    liver causing liver abscess

Localized Infections
  • Organism enters the body and reaches target site
    of infection
  • Organism adheres to or enters host cells and
    multiplies at site of infection
  • Infection spreads within the site (e.g.,
    respiratory tract intestines)
  • Symptoms of illness appear
  • Organism does not spread through the lymphatic
    system or reach the bloodstream
  • Infection subsides due to host defenses (e.g.,
  • Agent eliminated from the body infected cells
    replaced "cure"

Generalized Infections
  • Organism enters the body and reaches target site
    of initial infection
  • Organism adheres to or enters host cells and
    multiplies at initial site of infection
  • Infection spreads within site and to other sites
    via tissues, lymphatic system, bloodstream
    (bacteremia, viremia, etc.) and possibly other
  • Symptoms of illness may appear
  • Organisms infect other organs, tissues and cells
    more spread via bloodstream
  • Symptoms of illness become severe
  • Host defenses eliminate organisms leading to
    cure or disease continues, possibly leading to
    irreversible damage or death

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Factors Influencing Exposure and Infection Agent
(Microbe) Factors
  • Sources, Reservoirs, Transport and Persistence
    (in the Environment)
  • Ability to Enter a Portal in the Human or Other
  • Ability to Reach and Proliferate at Site(s) of
    Infection in the Host
  • Excretion of the Agent from the Host
  • Quantity and "Quality" (including virulence) of
    the Infectious

Factors Influencing Exposure and Infection
Environmental Factors
  • Reservoirs where organisms can live, accumulate
    or persist outside of the host of interest could
    be another organism or the inanimate environment.
  • Vehicles inanimate objects/materials by which
    organisms get from one host to another includes
    water, food, objects (called fomites) and
    biological products (e.g., blood).
  • Amplifiers Types of reservoirs where organisms
    proliferate often applied to organisms
    transmitted by the airborne route.
  • Vectors Living organisms bringing infectious
    organisms to a host.
  • Mechanical vectors Microbes do not multiply in
    the vector
  • ex biting insects infected with the infectious
  • Biological vectors Microbes must propagate in
    the vector before they can be transmitted to a

Environmental Factors Influencing Survival or
Proliferation of Infectious Agents
  • Physical temperature, relative humidity,
    sunlight, moisture content or water activity,
    climate and weather, etc.
  • Chemical and Nutritional Antimicrobial
    chemicals, nutrients for microbial proliferation.
  • Biological Antagonistic activity by other
    organisms antimicrobial agents, parasitism,
    etc. presence and state of a vector

Factors Influencing Exposure and Infection Host
Factors and Host Susceptibility
  • Opportunities for host exposure
  • transmission routes
  • host availability
  • Susceptibility factors
  • Dosage (quantity) and "quality" of infectious
    organisms, including their "virulence"
  • age
  • immunity
  • nutritional status
  • immunocompetence and health status,
  • genetics
  • behavior (personal habits) of host.

Infectivity of Pathogenic Microorganisms and Risk
of Infection, Illness and Death
  • Infection
  • ?
  • Illness
  • ?? Sequelae
  • Death
  • Secondary Spread

Dose-Response and Infectious Dose (ID)
  • Probability of infection is dose-dependent
  • Higher dose ? higher probability of
    infection/illness dose-response relationship
  • Microbes differ in infectivity
  • Enteric and respiratory viruses infectious at
    very low doses
  • 1 cell culture ID50 has high probability of
    infecting an exposed human (1 HID50)
  • Norovirus HID50 is about 1 virion
  • Most enteric bacteria infective at moderate
    (10s-100s cells) to high (1,000 cells) doses
  • Protozoa can be infective at low doses
  • ID50 1-10 cysts of Giardia lamblia or oocysts
    of Cryptosporidium parvum

Outcomes of Infection
  • Microbes differ in their ability to produce the
    different outcomes of infection
  • (i) infection without illness
  • (ii) infection with illness (with or without
    long-term sequelae) and
  • (iii) infection, illness and then death

The Iceberg Concept As Applied to Virus
Transmission Dynamics of Infectious Diseases
Host States in Relation to Pathogen Transmission
Pathogen Exposure
? the rate or probability of movement from one
state to another
Mortality Rates for Different Viruses in Healthy,
Immunocompetent Humans (Rates Higher in the
  • VIRUSES Mortality
  • Adenovirus 0.01
  • Enteroviruses 0.001 (average)
  • Coxsackievirus B 0.59-0.94
  • Echoviruses 0.28
  • Hepatitis A virus 0.3
  • Norwalk virus 0.0001
  • Rotavirus 0.01

Mortality Rates for Different Pathogens in
Healthy, Immunocompetent Humans (Rates Higher in
the Immunocompromised)
  • BACTERIA Mortality
  • Campylobacter jejuni 0.1
  • E. coli 0.2
  • Salmonella spp. 0.1
  • Shigella spp. 0.2
  • Giardia lamblia 0.0001
  • Entamoeba histolytica 0.3

Transmission Categories of Water-Related Diseases
  • Water-borne
  • Water-washed
  • Water-based
  • Water-related/Insect vector-borne

  • Caused by ingestion of water contaminated by
    human or animal feces or urine containing
    pathogenic bacteria or viruses
  • Mostly enteric diseases transmitted by the
    fecal-oral route
  • Bacterial cholera, typhoid, amoebic and
    bacillary dysentery diseases
  • Viral Infectious hepatitis
  • Protozoan parasitic amoebic dysentery 
  • Some are due to organisms NOT fecally associated
    that proliferate in water
  • example Legionellosis (Legionella bacteria) via
    aerosols and droplets

Water-washed or Water Hygiene Diseases
  • Caused by poor personal hygiene and skin or eye
    contact with contaminated water
  • Diseases whose exposure is reduced by the use of
    water for personal and domestic hygiene
  • washing clothes, floors, other household chores
  • bathing and other personal hygiene
  • cleaning of cooking and eating utensils
  • Includes
  • many enteric organisms
  • diseases of the skin and eyes (ex trachoma)
  • insect infestations
  • Scabies caused by mites
  • Pediculosis caused by lice
  • Tick-borne diseases

  • Caused by parasites found in intermediate
    organisms living in contaminated water
  • Exposure by skin contact with infested water
  • Schistosomiasis
  • free-living larvae released from aquatic snails
    (the intermediate host) invade the skin
  • Dracunculiasis (Guinea Worm disease)
  • Other helminths 

Water-related, Insect Vector-borne
  • Caused by insect vectors, especially mosquitoes,
    that breed in water water habitat "insect
    vector" diseases
  • Insect vectors breed in or near water
  • Examples
  • Dengue (virus)
  • Filariasis (nematode worms)
  • Malaria (protozoan)
  • Onchocerciasis river blindness (filarial worm)
  • Trypanosomiasis (parasite)
  • Yellow Fever (virus) 

Analytical Epidemiology-Types of Studies
  • Descriptive studies
  • Intended to describe the distribution of cases of
    disease in time, place and person
  • Descriptive studies used in WSH
  • Ecological study
  • Time series study
  • Analytical studies
  • Case control
  • cohort type
  • In both, individuals/groups are compared on the
    basis of something, often a risk or risk factor
  • Intervention studies
  • experimental studies to observe impacts of a
    certain intervention (introduced change in/on
    people populations) on the risk of illness
  • Example WSH intervention POU-HH water treatment

Types of Epidemiological Studies Ecological
  • Description
  • Determines relationship between disease and risk
  • Compares incidence of disease in different
    communities with varying exposure to risk factors
  • Advantages/Disadvantages
  • Relatively inexpensive to do if data are
    available on disease rates and risk factors
  • Data available only for groups, so not known if
    individuals with disease are exposed to risk
  • Good for hypotheses generation can not be used
    as evidence of epidemiological proof

Types of Epidemiological Studies Time Series
  • Description
  • Determines relationship between disease incidence
    in population and variation in a risk factor over
  • A kind of ecological study
  • Advantages/Disadvantages
  • As a kind of ecological study, with the same
    advantages and disadvantages

Types of Epidemiological Studies Case-Control
  • Description
  • Determines the relationship between disease and
    risk factors
  • Compares disease incidence in exposed individuals
    to matched controls
  • Advantages/Disadvantages
  • Relatively inexpensive to carry out
  • Generates data on individuals exposed to the risk
    factors in comparison with healthy individuals
  • Easy to compare diseased and healthy individuals
    in relation to possible risk factors

Retrospective Case-Control Studies
  • Used to determine if a particular personal
    characteristic or environmental factor is related
    to disease occurrence.
  • Cases persons who have a specific illness or
  • Controls, those who do not have the illness or
  • Select both.
  • Selection may seek to "match" for other
    variables, such as age, race, sex, wealth, etc.
  • Query cases and controls to determine if their
    exposure to environmental hazards have been
    similar or different
  • Determine if cases were more likely to have
    been exposed to something that controls were
    not, and that this exposure is consistent with
    the health effect observed in the cases.
  • Foodborne outbreak investigations rely on
    case-control methods
  • Compute food-specific attack rates for a
    suspected foodborne illness

Retrospective Case-Control Studies
  • Useful in disease outbreaks where it is possible
    to determine if certain activities or exposures
    were related to the disease or illness under
  • Example, cases of cholera and their matched
    controls are asked about their past activity with
    respect to food consumption, drinking water and
    swimming events
  • Results of questioning may show that consuming a
    certain drinking water source is more likely to
    have occurred with cholera cases than with
  • This indicates a potential association between
    drinking water and the disease.
  • John Snows investigation of cholera in London
    was partly a case-control study. (It was an
    intervention study, too - he took off the pump
    handle and cholera cases stopped)
  • Snow on Cholera - (If you have not read this,
    you really must do that soon!)
  • The linkage between disease and exposure can be
    determined, but it is seldom possible to
    determine the magnitude of the exposure.

Types of Epidemiological Studies Cohort
  • Description
  • Compares disease rate in two, or more,
    populations with different levels of exposure
    over a specific time period on randomly selected
  • Advantages/Disadvantages
  • Relatively expensive
  • Generates risk factor data in populations by
    comparing two or more groups of randomly selected

Types of Epidemiological Studies Prospective
Cohort Applied Recreational Water Exposure
Illness Risks
  • Recruit people immediately before or, more
    commonly, after, participating in recreational
    water activity (exposure)
  • Recruit a control group similarly
  • Follow both cohorts for a period of time, T
  • he exposure status of the bath.
  • Acquire data on illness symptoms experienced by
    the two cohorts
  • Use questionnaire interviews, in person or by
    means of telephone
  • Quantify recreational water quality on the day of
  • Quantify a relationship between "exposure day"
    water quality and resulting disease in an
    exposure-response curve predicting illness from a
    measure of water quality
  • Enterococcus spp. bacteria concentration in water
    relative to risks of GI illness is swimmers
    (compared/normalized to non-swimmers)

Types of Epidemiological Studies Intervention
  • Description
  • Compares disease rates in two or more groups
    (cohorts) of randomly chosen individuals after
    intervening to change the exposure level
  • Advantages/disadvantages
  • Gold standard for epidemiological proof
  • Can be time consuming and costly
  • Less costly in developing countries where disease
    burdens are high and a single type of WSH
    intervention can be studied for small cohorts

Intervention Study Effect of Household Water
Chlorination on Diarrhea Disease Risk
Sobsey,M.D., T. Handzel and L. Venczel (2004)
Chlorination and safe storage of household
drinking water in developing countries to reduce
waterborne disease. Wat. Sci. Tech, 47(3)
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