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Dynamics of Disease Transmission

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Title: Dynamics of Disease Transmission


1
Dynamics of Disease Transmission
  • Jan Risser
  • PH 2610
  • Introduction to Epidemiology
  • October 2, 2002

2
Infectious Disease epidemiology
  • Original model for the study of epidemiology
  • Of considerable public health importance
  • Rift Valley fever
  • Yellow fever
  • Dengue
  • Lassa fever
  • West Nile Virus
  • HIV
  • TB
  • Hantavirus

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Learning objectives
  • By the end of this lecture you should be able to
  • 1. Describe the Epidemiologic Triangle
  • 2. Identify characteristics of
  • agent/host/environment
  • 3. State modes of infectious disease
    transmission

7
The epidemiologic triangle
Host Agent Environment
Host
Not to be confused with Person, place, time
VECTOR
Agent
Environment
8
Characteristics of host/agent/environment
9
The infectious disease process
  • Agents
  • what is causing the illness
  • Reservoirs
  • where the agents live
  • Portals of entry and exit
  • Transmission
  • how they get in
  • Host immunity
  • what factors affect disease progression

10
Agents
  • Biologic
  • worms, fungi and yeast, protozoa, bacteria,
    viruses, prions
  • Physical
  • noise, repetitive motion, violence
  • Chemical
  • tobacco, air pollutants, water pollutants
  • Nutritional
  • obesity, nutritional deficiencies

11
Characteristics of Biologic Agents
  • Infectivity likely to infect
  • Pathogenicity likely cause illness
  • Virulence severity of illness

12
Characteristics of Host
  • An agent must be present for an infectious
    disease to develop
  • But this is not a sufficient cause
  • Infection depends on
  • Agent factors and Host factors

13
4. Agent / Host Characteristics
14
Characteristics of Host
  • Immunity
  • all factors that alter the likelihood of
    infection and disease in the host once the
    agent is encountered.
  • Innate immunity barriers that prevent invading
    pathogens from entering the body
  • Acquired immunity resistance developed by the
    host as a result of previous exposure

15
Innate immunity
  • Physical barriers
  • Intact skin, mucosa lining, cilia in the
    respiratory tract, cough and gag reflex
  • Chemical barriers
  • Acidity of the stomach and vagina
  • Hydrolytic and proetolytic enzymes in saliva and
    intestines
  • Cellular and physiologic barriers
  • Macrophages, natural killer cells, inflammation,
    fever

16
Acquired immunity
  • The result of a highly specific and evolved
    response on the part of the host and begins
    when the host is exposed to a foreign pathogen
  • Cellular and humoral
  • Immunization
  • Active natural or artificial exposure to
    antigens
  • Passive maternal and therapeutic sources

17
Portals of Entry and Exit
  • Respiratory tract
  • Conjunctiva
  • Urogenital tract
  • Gastrointestinal tract
  • Skin
  • Placenta

18
Modes of transmission
  • Mode of transmission any mechanism by which an
    agent is spread to the host
  • Some bridging between hosts or environment and
    host
  • Contact Direct or Indirect contact
  • Vectors (animate objects)
  • Vehicles (inanimate objects)

19
Modes of transmission
  • Contact
  • Direct person to person
  • Indirect contact with relatively fresh bodily
    fluids or tissue

20
Modes of transmission
  • Common vehicle spread
  • Air, water, food, drugs
  • Serial transfer
  • Human to human,
  • human to animal to human,
  • human to environment to human in sequence

21
Modes of transmission
  • Airborne
  • respiratory disease
  • Food/Waterborne (fecal-oral )
  • enteric disease, polio, hepatitis A.
  • Bloodborne (parental, perinatal)
  • hepatitis, HIV

22
Modes of transmission
  • Sexually Transmitted
  • syphilis, gonorrhea, HIV
  • Insect-borne ( insect vector)
  • malaria, Lyme disease
  • Zoonosis (animal contact)
  • rabies , hantavirus infection

23
Modes of transmission
  • Vectors
  • Mechanical transmission
  • among other methods

24
Means of transmission
25
Characteristics of environment
  • Physical
  • habitat, climate
  • Biological
  • population density
  • flora
  • fauna
  • Socioeconomical
  • Occupation,urbanization , culture

26
Reservoirs
  • Reservoirs the normal habitat in which the
    agent lives, multiplies, and grows.
  • Symptomatic cases
  • Carriers
  • Animals (zoonoses)
  • Inanimate objects water, food, soil, air,
    fomites

27
West Nile Virus Transmission Cycle
Mosquito vector
Incidental infections
West Nile virus
West Nile virus
Incidental infections
Bird reservoir hosts
28
Iceberg concept of Infection
  • Iceberg concept of Infection
  • Tip of the iceberg
  • active clinical disease
  • Most people are subclinical
  • Substantial number of exposures may not lead to
    infection

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Clinical and Subclinical Disease
31
Endemic, Epidemic, Pandemic
  • Endemic
  • habitual presence of disease within a given
    geographic area
  • Epidemic
  • occurrence of disease clearly in excess of normal
    expectancy
  • Pandemic
  • Worldwide epidemic

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Determinants of Disease Outbreaks
  • Amount of disease in a population
  • depends on
  • number of infected
  • number not susceptible, or immune
  • Herd immunity

34
Herd Immunity
  • The resistance of a group to an attack by a
    disease to which a large proportion of the
    members of the group are immune.
  • Because disease spreads from person to person -
    the probability of reaching a susceptible person
    decreases as the proportion of immune increases

35
Herd Immunity
  • We do not have to immunize 100 of the population
    to be successful
  • For herd immunity to work
  • the disease agent must be restricted to a single
    host species
  • transmission must be relatively direct from one
    member to the other

36
Herd Immunity
  • If there is an outside reservoir
  • herd immunity will not operate because other
    means of transmission are available
  • infections must induce solid immunity
  • Herd immunity will only work if an infected
    person is random

37
Herd Immunity
  • What percent is necessary
  • 94 for measles

38
Epidemic
  • A bar-graph was made showing the number of cases
    vs. the time after which the meal was eaten.
  • To investigate the effect of each food on
    outbreak of disease, food-specific attack rates
    were calculated.

39
Top Ten Reported Infectious Diseases of 1999 in
U.S.A.
  • Chlamydia STD
  • Gonorrhea STD
  • AIDS STD / Blood
  • Salmonellosis Fecal/oral
  • Hepatitis A Fecal/oral
  • Shigellosis Fecal/oral
  • Tuberculosis Respiratory
  • Lyme Disease Arthropod
  • Hepatitis B STD / Blood
  • Syphilis STD

40
Prevention/Control Strategy
  • Host Prevention
  • Immunity - Immunization
  • Behavioral Change
  • Treatment - Contacts, Carriers
  • Environmental Control
  • Personal Hygiene
  • Food Protection
  • Water Supplies
  • Sanitation, Regulation

41
Prevention / Control Strategy
  • Vector Control
  • Mosquitoes, Insecticides
  • Animal Population Control

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Critical Reasoning
  • Lyme Disease (1975)
  • Legionellosis (1976)
  • AIDS (1981)
  • Hantavirus (1993)
  • DES exposure (1989)
  • EMS/ tryptophan (1970)
  • Toxic Shock Syndrome (1980)
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