Infectious Disease Epidemiology

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Infectious Disease Epidemiology

• Casey E. Barton, M.S.
• July 26, 2001

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REMINDERS!!!!!!!!!!

• TODAY by 5pm
• Submit all outstanding homeworks and redos
• July 31st
• Teaching-Learning Exam
• Course Evaluations
• Tutorial after class
• August 2nd
• FINAL EXAM
• Last day of class!!!!!!!

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Top 10 Reported Infectious Diseases 1999
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Epidemiologic Triangle
HOST
AGENT
ENVIRONMENT
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AGENTS

• An Agent MUST be present for an infectious
  disease to develop
• Epidemiologists describe
• Frequency of disease
• Mechanisms of transmission
• The causal agent

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AGENTS

• Bacteria
• Viruses and rickettsia
• Mycoses (fungi)
• Parasites
• Protozoa
• Helminths
• Arthropods

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AGENTS Bacteria

• Leading killers in 19th century
• Examples
• Tuberculosis
• Salmonellosis
• Streptococcal infections
• Problem emerging bacteria strains resistant to
  current antibiotics

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AGENTS Viruses and Rickettsia

• Viral infection include
• Hepatitis A
• Herpes simplex
• Influenza
• HIV
• Rickettsial infections include
• Q fever
• Rocky Mountain Spotted Fever

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Mycoses (Fungi)

• Examples
• Ringworm
• Aspergillosis
• Candidiasis

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AGENTS Parasites

• Protozoa
• Malaria
• Cyclosporiasis
• Giardiosis
• Chagas Disease
• Helminths (worms)
• Tapeworms
• Roundworms
• Schistosomiasis

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AGENTS Arthropods

• Insect vectors including
• Mosquitoes
• Ticks
• Flies
• Mites
• Other insects
• Arthropod-borne diseases include
• RMSF
• Malaria
• Encephalitis
• Lyme Disease

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Modes of Transmission

• Airborne or Contact
• Food- and Water-borne
• Blood-borne
• Sexually Transmitted
• Arthropod-borne
• Zoonotic

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Means of Transmission
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Means of Transmission

• Direct person to person
• Indirect through intermediate sources such as
  vehicles, fomites or vectors
• Horizontal transmission from one person to
  another (direct or indirect)
• Vertical mother to fetus either genetic or
  across placenta

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Characteristics of Infectious Disease Agents

• Infectivity
• Pathogenicity
• Virulence
• Toxigenicity
• Resistance
• Antigenicity

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Infectivity

• Capacity of the agent to enter and multiply in a
  susceptible host and thus produce infection or
  disease
• Examples measles and polio
• Measure of Infectivity Secondary Attack Rate
  (SAR)

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Pathogenicity

• Capacity of the agent to cause disease in the
  infected host
• Example measles
• Measure of pathogenicity proportion of
  individuals with clinically apparent disease

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Virulence

• Severity of the disease
• i.e. whether clinical manifestations are produced
• Example Rabies
• Measure of Virulence Case Fatality Rate (CFR)

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Toxigenicity

• Capacity of the agent to product a toxin or
  poison
• Examples botulism and shellfish poisoning
• Disease is caused by toxins produced by the
  agent, rather than by the actual agent

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Resistance

• Ability of the agent to survive adverse
  environmental conditions
• Examples parasite with spores or cysts,
  Hepatitis virus
• Resistance is also applied to the HOST

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Antigenicity

• Ability of the agent to induce antibody
  production in the host
• Agents may or may not induce long-term immunity
• Examples Measles re-infection is rare
• Immunogenicity
• Related term
• refers to an infections ability to produce
  specific immunity

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Host

• Subclinical infection ? Active Disease
• Hosts ability to fight off infectious agent
• Nonspecific defense mechanisms
• Disease-specific defense mechanisms

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HOST Non-specific defense mechanisms

• Age, Sex, Race, Genetics
• Religion, Customs, Occupation, Nutritional status
• Skin, Mucosal surfaces, Saliva,
• High pH of gastric juices
• Immune system (phagocytes and macrophages)

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HOST Disease-specific defense mechanisms

• Immunity resistance of host to an agent
• Active all or part of organism invokes an
  immunologic response
• Natural results from an infection
• Artificial results from immunization
• Passive preformed antibody invokes short term
  immunity
• Natural antibodies cross the placenta
• Artificial immuneglobulin after certain
  exposures

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ENVIRONMENT

• Physical weather, temperature, humidity,
  geologic formations, habitat
• Biological population density, flora, fauna,
  reservoirs
• Socioeconomical behavioral, personality,
  attitudinal, and cultural characteristics of a
  group of people

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Endemic vs.Epidemic

• Endemic constant presence of a disease or agent
  in a defined geographical area
• Epidemic excessive occurrence of disease (above
  endemic level) in a defined geographical area
• 2 Types
• Common Source
• Propogated Source

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Epidemic

• Common Source Epidemic event or exposure comes
  from a single source that all persons in the
  group had a chance to encounter
• Propogated Epidemic caused by either direct or
  indirect transmission of infectious disease from
  one individual to another and can have multiple
  sources from which disease can be transmitted

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Endemic versus Epidemic
ENDEMIC
Point EPIDEMIC
Propagating EPIDEMIC
NOTE X axis Time Y axis new cases
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Reservoir (Niche)

• Fosters survival of infectious disease agent
• Types of reservoirs
• Human
• Plant
• Animal
• Arthropod
• Physical environment

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Inapparent infection

• Subclinical infection absence of clinically
  obvious signs and symptoms
• Asymptomatic individuals can transmit disease
• Carrier status
• Serologic evidence of infection

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Incubation Period

• Time interval between exposure to an infectious
  agent and the appearance of the first signs of
  disease
• During this time, the infectious agent replicates
  within the host
• Can be hours, days, weeks, or years
• Applies only to clinically apparent cases of
  disease

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Generation Time

• Time between lodgment of an infectious agent in a
  host and the maximal communicability of the host
• May or may not be equivalent to incubation period
• Applies to both clinically apparent and
  inapparent cases of disease
• Utilized for describing the spread of infectious
  agents that have a large proportion of
  subclinical cases

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Herd Immunity

• A population may become immune to an infectious
  agent after a large proportion of individuals
  have become immune
• i.e. through past infections or vaccination
• Can occur when immune persons prevent the spread
  of disease to unimmunized individuals and confers
  protection to the population even though not
  every single individual has been immunized

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Effect of Herd Immunity on Spread of Infection
(from Jekel et al. 1996, p. 12)
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Absence of Herd Immunity
Presence of 50 Herd Immunity
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Iceberg Concept of Disease
Death Clinical illness
Visible Cases
Invisible Cases
Asymptomatic cases Carriers
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Zoonoses

• World Health Organization (WHO)
• diseases and infections which are naturally
  transmitted between vertebrate animals and man
• Examples Rabies, roundworms, Lyme disease,
  Brucellosis, Cryptosporidiosis
• Epizootic and enzootic but refers to diseases in
  animals

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Emerging Infectious Diseases

• Changes in climate global warming
• Human activities farming, reforestation
• Technologic changes air travel, organ
  transplantation
• Demographic changes migration to cities

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Host Prevention

• Immunity immunizations
• Behavioral change
• Treatment of contact and carriers
• screening

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Control

• Environmental
• Personal hygiene
• Food protection and safety
• Water supplies
• Sanitation, regulation
• Vectors
• Mosquitoes, insecticides
• Animal population control

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Measures of Disease Outbreaks

• Attack Rate
• Secondary Attack Rate
• Case Fatality Rate

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Attack Rate

• Type of incidence rate
• AR
• Ill___ 100 during a time period
• Ill Well
• Can calculate food or exposure specific AR

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Secondary Attack Rate

• Yields an index of the spread of disease within a
  circumscribed unit, i.e. household, dorm
• Index case case that first comes to the
  attention of public health authorities
• SAR
• new cases in group initial cases 100
• susceptible persons in group initial cases

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Case-Fatality Rate

• Refers to the number of deaths caused by a
  specific disease among those who have the disease
• Provides index of deadliness of a particular
  disease within a specific population
• CFR deaths due to disease X 100
• cases disease X
• DIFFERENT than crude death rate

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Centers for Disease Control and Prevention (CDC)

• National Center for HIV,STD, and TB Prevention
• National Center for Infectious Diseases
• AIDS, STD and TB Laboratory Research
• Division of Bacterial and Mycotic Diseases
• Division of Parasitic Diseases
• Division of Vector-borne Infectious Diseases
• Division of Viral and Rickettsial Diseases
• National Immunization Program

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Infectious Disease Information

• Emerging Infectious Diseases Journal
• www.cdc.gov/ncidod/eid/
• Morbidity and Mortality Weekly Report (MMWR)
• www.cdc.gov/mmwr/
• Program for monitoring Emerging Diseases (Promed)
• www.promedmail.org/pls/promed/promed.home

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THE END!!!
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EXAMPLE CALCULATIONS
Which food item appears to be the most probable
vehicle for the food-borne infection assocaited
with illness at this picnic?