Dynamics of Disease Transmission

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Dynamics of Disease Transmission

• Jan Risser
• PH 2610
• Introduction to Epidemiology
• October 2, 2002

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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

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The epidemiologic triangle
Host Agent Environment
Host
Not to be confused with Person, place, time
VECTOR
Agent
Environment
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Characteristics of host/agent/environment
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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

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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

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Characteristics of Biologic Agents

• Infectivity likely to infect
• Pathogenicity likely cause illness
• Virulence severity of illness

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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

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4. Agent / Host Characteristics
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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

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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

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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

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Portals of Entry and Exit

• Respiratory tract
• Conjunctiva
• Urogenital tract
• Gastrointestinal tract
• Skin
• Placenta

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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)

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Modes of transmission

• Contact
• Direct person to person
• Indirect contact with relatively fresh bodily
  fluids or tissue

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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

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Modes of transmission

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

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Modes of transmission

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

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Modes of transmission

• Vectors
• Mechanical transmission
• among other methods

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Means of transmission
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Characteristics of environment

• Physical
• habitat, climate
• Biological
• population density
• flora
• fauna
• Socioeconomical
• Occupation,urbanization , culture

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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

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West Nile Virus Transmission Cycle
Mosquito vector
Incidental infections
West Nile virus
West Nile virus
Incidental infections
Bird reservoir hosts
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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
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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

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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

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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

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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

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Herd Immunity

• What percent is necessary
• 94 for measles

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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.

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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

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Prevention/Control Strategy

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

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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)