Epidemiology of Infectious Disease

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Chapter 33

• Epidemiology of Infectious Disease

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Epidemiology

• science that evaluates occurrence, determinants,
  distribution, and control of health and disease
  in a defined human population
• health
• condition in which organism (and all its parts)
  performs its vital functions normally
• disease
• impairment of the normal state of an organism or
  any of its components that hinders the
  performance of vital functions
• epidemiologist
• one who practices epidemiology

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Figure 33.1
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Centers for Disease Control and Prevention (CDC)

• located in Atlanta, GA
• functions as national focus for
• developing and applying disease prevention and
  control
• environmental health
• health promotion and health education activities
  designed to improve the health of the people
• worldwide counterpart is the World Health
  Organization (WHO) located in Geneva, Switzerland

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Epidemiology Terminology

• sporadic disease
• occurs occasionally and at irregular intervals
• endemic disease
• maintains a relatively steady low-level frequency
  at a moderately regular interval
• hyperendemic diseases
• gradually increase in occurrence frequency above
  endemic level but not to epidemic level

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More Terms

• outbreak
• sudden, unexpected occurrence of disease
• usually focal or in a limited segment of
  population
• epidemic
• sudden increase in frequency above expected
  number
• index case first case in an epidemic
• pandemic
• increase in disease occurrence within large
  population over wide region (usually worldwide)

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Morbidity Rate

• an incidence rate
• number of new cases in a specific time period per
  unit of population
• new cases during a specific time
• individuals in population

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Prevalence Rate

• total number of individuals infected at any one
  time
• depends both on incidence rate and duration of
  illness

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Mortality Rate

• number of deaths from a disease per number of
  cases of the disease
• deaths due to given disease
• size of total population with disease

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Correlation with a Single Causative Agent

• after recognition of infectious disease in a
  population, outbreak correlated with specific
  pathogen
• clinical microbiologists help in isolation and
  identification of pathogen

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Figure 33.4
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Human Sources/Reservoirs

• carrier
• infected hosts who are potential sources of
  infection for others
• types of carriers
• active carrier has overt clinical case of disease
• convalescent carrier has recovered but continues
  to harbor large numbers of pathogen
• healthy carrier harbors the pathogen but is not
  ill
• incubatory carrier is incubating the pathogen in
  large numbers but is not yet ill

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Animal Reservoirs

• transmission to human can be direct or indirect
• vectors
• organisms that spread disease from one host to
  another

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More Types of Carriers

• casual, acute, or transient carriers
• convalescent, healthy, and incubatory carriers
  that harbor pathogen for brief time
• chronic carriers
• convalescent, healthy, and incubatory carriers
  that harbor pathogen for long time

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How Was the Pathogen Transmitted?

• Four main routes
• airborne
• contact
• vehicle
• vector-borne

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Airborne Transmission

• pathogen suspended in air and travels ? 1 meter
• droplet nuclei
• small particles (1-4 ?m diameter)
• can remain airborne for long time
• can travel long distances
• usually propelled from respiratory tract of
  source organisms by sneezing, coughing, or
  vocalization
• dust particles also important route of airborne
  transmission

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Figure 33.9
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Contact Transmission

• coming together or touching of source/reservoir
  and host
• direct contact (person-to-person)
• physical interaction between source/reservoir and
  host
• e.g., kissing, touching, and sexual contact
• indirect contact
• involves an intermediate (usually inanimate)
• e.g., eating utensils, bedding
• droplet spread
• large particles (gt5 ?m) that travel lt 1 meter

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Figure 33.8
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Vehicle Transmission

• vehicles
• inanimate materials or objects involved in
  pathogen transmission
• common vehicle transmission
• single vehicle spreads pathogen to multiple hosts
• e.g., water and food
• fomites
• common vehicles such as surgical instruments,
  bedding, and eating utensils

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Vector-Borne Transmission

• External (mechanical) transmission
• passive carriage of pathogen on body of vector
• no growth of pathogen during transmission
• Internal transmission
• carried within vector
• harborage transmission pathogen does not
  undergo changes within vector
• biologic transmission pathogen undergoes
  changes within vector

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Virulence and the Mode of Transmission

• Evidence suggests correlation between mode of
  transmission and degree of virulence
• direct contact ? less virulent
• vector-borne ? highly virulent in human host
  relatively benign in vector
• greater ability to survive outside host ? more
  virulent
• e.g., Mycobacterium tuberculosis and
  Corynebacterium
• diphtheriae can survive weeks to months outside
  human hosts

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Reasons for Increases in Emerging and Reemerging
Infectious Diseases

• crowding
• habitat disruption
• shift in distribution of nosocomial pathogens

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More Reasons

• excessive or inappropriate use of antimicrobial
  therapy
• medical practices that lead to immunosuppression
• rapid global transportation systems

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Control of Epidemics

• Three types of control measures
• reduce or eliminate source or reservoir of
  infection
• break connection between source and susceptible
  individual
• reduce number of susceptible individuals

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Reduce or Eliminate Source or Reservoir

• quarantine and isolation of cases and carriers
• destruction of animal reservoir
• treatment of sewage
• therapy that reduces or eliminates infectivity of
  cases

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Break Connection between Source and Susceptible
Individuals

• chlorination of water supplies
• pasteurization of milk
• supervision and inspection of food and food
  handlers
• destruction of insect vectors with pesticides

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Reduce Number of Susceptible Individuals

• raise level of herd immunity by
• passive immunity following exposure
• active immunity for protection

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Vaccines and Immunization

• Vaccine
• preparation of one or more microbial antigens
  used to induce protective immunity
• may consist of killed, living, weakened
  (attenuated) microbes or inactivated bacterial
  toxins (toxoids), purified cell material,
  recombinant vectors or DNA

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More on Vaccines and Immunization

• Immunization
• result obtained when vaccine stimulates immunity
• vaccines attempt to induce antibodies and
  activated T cells to protect host from future
  infection
• Vaccinomics is the application of genomics and
  bioinformatics to vaccine development

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Table 33.3
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Recombinant-Vector Vaccines

• pathogen genes that encode major antigens
  inserted into nonvirulent viruses or bacteria
  which serve as vectors and express the inserted
  gene
• released gene products (antigens) can elicit
  cellular and humoral immunity

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DNA Vaccines

• DNA directly introduced into host cell via air
  pressure or gene gun
• when injected into muscle cells, DNA taken into
  nucleus and pathogens DNA fragment is expressed
• host immune system responds to foreign proteins
  produced
• many DNA vaccine trials are currently being run

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Table 33.5
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Bioterrorism Preparedness

• Bioterrorism
• intentional or threatened use of viruses,
  bacteria, fungi, or toxins from living organisms
  to produce death or disease in humans, animals,
  and plants

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Examples of Intentional Uses of Biological Agents
for Criminal or Terror Intent

• 1984 in The Dalles, OR
• Salmonella typhimurium in 10 restaurant salad
  bars
• 1996 in Texas
• intentional release of Shigella dysentariae in a
  hospital lab break room
• 2001 in seven eastern U.S. states
• use of weaponized Bacillus anthracis spores
  delivered through U.S. postal systems
• all of above caused hospitalizations, the anthrax
  episode resulted in five deaths

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Table 33.6
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Choosing Biological Agents as Weapons

• biocrime when chosen as a means for a localized
  attack vs. bioterrorism when chosen for mass
  casualties
• characteristics that favor their use
• invisible, odorless, and tasteless
• difficult to detect
• take hours or days before awareness that they
  have been used
• fear and panic associated with the anticipation
  that they were used

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Nosocomial Infections

• result from pathogens that develop within a
  hospital or other clinical care facility and are
  acquired by patients while they are in the
  facility
• 5-10 of all hospital patients acquire a
  nosocomial infection
• usually caused by bacteria that are members of
  normal microbiota

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Source

• Endogenous pathogen
• brought into hospital by patient or acquired when
  patient is colonized after admission
• Exogenous pathogen
• microbiota other than the patients
• Autogenous infection
• caused by an agent derived from microbiota of
  patient despite whether it became part of
  patients microbiota following admission

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Control, Prevention, and Surveillance

• proper training of personnel in basic infection
  control measures
• e.g., handling of surgical wounds and hand
  washing
• monitoring of patient for signs and symptoms of
  nosocomial infection

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The Hospital Epidemiologist

• individual responsible for developing and
  implementing policies to monitor and control
  infections and communicable diseases
• reports to infection control committee or similar
  group

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Bibliography

• Lecture PowerPoints Prescotts Principles of
  Microbiology-Mc Graw Hill Co.
• http//en.wikipedia.org/wiki/Scientific_method
• https//files.kennesaw.edu/faculty/jhendrix/bio334
  0/home.html