Introduction to Environmentally Transmitted Pathogens: Some Basic Concepts of Epidemiology, Infectio

1
Introduction to Environmentally Transmitted
Pathogens Some Basic Concepts of Epidemiology,
Infection, Disease, Environmental Transmission
and Waterborne-Exposure

• Lecture 3
• ENVR 421
• Mark D. Sobsey

2
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.

3
Infectious Disease Epidemiology Classical
Epidemiology

• 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
  population
• how a case of that disease develops in an
  individual

4
Basic Epidemiological Concepts and Terms

• Incidence of new cases of disease/total at
  risk.
• 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.

5
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.

6
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

7
Routes or Methods of Entry

• Direct Personal Contact Person
  (animal)-to-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

8
Transmission Routes of Infectious Agents
9
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

10
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.,
  immunity)
• Agent eliminated from the body infected cells
  replaced "cure"

11
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
  routes
• 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

12
(No Transcript)
13
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
  Host
• 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

14
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
  organism
• Biological vectors Microbes must propagate in
  the vector before they can be transmitted to a
  host.

15
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

16
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.

17
Infectivity of Pathogenic Microorganisms and Risk
of Infection, Illness and Death

• Infection
• ?
• Illness
• ?? Sequelae
• Death
• Secondary Spread

18
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 ID has high probability of
  infecting an exposed human.
• May still need exposure to many virions
• Most enteric bacteria infective at moderate
  (10s-100s cells) to high (1,000 cells) doses.
• Protozoa can be infective at low doses
• ID50 at 1-10 cysts of Giardia lamblia) or oocysts
  of Cryptosporidium parvum

19
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

20
The Iceberg Concept As Applied to Virus
Infections
21
Transmission Dynamics of Infectious Diseases
Host States in Relation to Pathogen Transmission
Pathogen Exposure
Susceptible
Infected
Resistant
?3
?1
?2
? the rate or probability of movement from one
state to another
22
Mortality Rates for Different Viruses in Healthy,
Immunocompetent Humans (Rates Higher in the
Immunocompromised)

• 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

23
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
• PARASITES
• Giardia lamblia 0.0001
• Entamoeba histolytica 0.3

24
Transmission Categories of Water-Related Diseases

• Water-borne
• Water-washed
• Water-based
• Water-related/Insect vector-borne

25
Waterborne

• 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

26
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

27
Water-based

• 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 

28
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)