february 11, 2013 initiative ostensible: representing or appearing in a certain way, but often not...
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February 11, 2013InitiativeOstensible: representing or appearing in a
certain way, but often not actually so; seeming
NO PHONES ALLOWED TODAY!!!Do Now:1. Identify 5 means of transferring the flu.2. What is the real name for “flu?”3. There was a flu epidemic during a war that
killed more people than the war. Which war was this?
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The Epidemiology of Infectious Disease
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I. The Science of EpidemiologyEpidemiology:
The field of science concerned with the circumstances under which diseases occur
An epidemiologist works in this fieldFactors under investigation:
Incidence (morbidity rate) and spread of infectious and non-infectious diseases
Prevention and control of infectious and non-infectious diseases
Effects of diseases on populations and individuals within a population (measured by death rate = mortality rate)
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Basic terms used in epidemiologySporadic disease
Occurs irregularly and only occasionally in a population
Example: Typhoid fever (Salmonella typhi)Endemic disease
Occurs at regular intervals but at low levels Example: Common cold (Rhinovirus)
Hyperendemic disease When occurrence frequency rises, but not to
epidemic proportions Example: Common cold in the winter months
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Epidemic Sharp increase in the incidence above the
predicted/expected levelReservoir
Natural location of the organism Can be animate or inanimate location Examples:
Rabies – Dogs, foxes, raccoons (zoonoses) Neisseria meningitidis (meningitis) – Humans Malaria – Humans Cryptococcus – Bird guano
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SourceImmediate location from which infectious
agent has been transmittedExamples:
Neisseria gonorrheaSource = humansReservoir = humans
Salmonella typhiSource = food and waterReservoir = humans
Hepatitis CSource = transfusion, blood productsReservoir = humans
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CarriersHosts that harbor a pathogen without
clinical symptoms and are capable of transmitting the infectious agent (sometimes unknowlingly)
Carrier state may be short (transient) or long-term (chronic carrier – e.g. tuberculosis, herpes, hepatitis B, typhoid)
Carrier state may also occur during:Incubation period (before symptoms appear)
Convalescent period (recovery)
Define Carrier State
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VectorA biological or inanimate source that
contributes to the transmission of an infectious agent from one host to another
Examples: Arthropods
Mosquitoes – Malaria, West Nile Virus Ticks – Lyme disease Fleas – Bubonic plague Flies - Trachoma
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Birds Parrots – Psittacosis Pigeons – Cryptococcus
Lower vertebrates Frogs and turtles – Salmonella
Inanimate objects (fomites) Sporothrix schenkii (sporotrichosis)
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February 20, 2013 DependabilityDeviate: to differ or move away from a
specified course or prescribed mode of behavior
Do Now: Make a 4 quadrant grid on your paper. You will be writing in this, so consider the size carefully. This is a “quadrant card.” Each quadrant will have something different in it.Topic of the Day Definition
Associated Terms Illustration
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Index case - the first case in an epidemic
Outbreak - an epidemic-like increase in frequency, but in a very limited (focal) segment of the populationRapid increase, usually localizedExample: Legionnaire’s disease
Pandemic - a long-term increase in frequency in a large (usually worldwide or continental) populationDisease frequency rises on a large scale
geographically
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EpizootologyDeals with animal diseases affecting animal
populations Enzootic = moderate incidence Epizootic = rapid increase Panzootic = wide spread incidence Zoonoses = if transferable to humans
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II. Equations for Determining Frequency of Disease
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Statistical AnalysesThe mathematics of collection,
organization, and interpretation of numerical data (rate acoomparisons, chi-square, SEMs)
Used by state public health lab, CDC, WHO and USPHS
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Morbidity - the number of new cases in a specific time period per unit of population
# new cases within a specified period x 100#individuals in a population
Indicator of new cases – critical for controlling spread of disease
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Prevalence - number of individuals infected at any one time per unit of population
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Mortality - number of deaths from a disease per number of cases of the disease
__# deaths ascribed to disease__ x 100# individuals affected by disease
Proportion of all deaths assigned to a single cause
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III. The Epidemiology of Infectious Disease
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Recognition of an Infectious Disease in a Population
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Factors Affecting the Cycle of DiseaseCausative agentSource/reservoirMethod of TransmissionInfluence of host or environment in the
spread of the disease
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Goal of the EpidemiologistControl the spread (dissemination)Eliminate etiological agent
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Surveillance and Data Collection for ControlCalculation of morbidity and mortality
ratesCase studiesField studiesReview clinical records and lab reportsInvestigate source, reservoir and vectorsReview treatments/success ratesEmploy demographic data to track the
movement of disease
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Signs versus symptoms:Sign = observable or measurable change in
body function Diarrhea, rash, fever, vomiting
Symptom = subjective Pain, appetite loss, lethargy, depression
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Disease syndrome - a set of signs and symptoms that is characteristic of a disease
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Phases of Infectious Disease Life CycleIncubation period
Variable lengthPrior to development of signs of symptoms
Prodromal stageBeginning of signs and symptomsOften infectious/contagiousInnate immune response “kick in” (first line of
defenses)
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Illness stageMost severe phaseClear evidence of signs and symptomsAcquired immune responses begin
Humoral – Antibodies and complement Cell-mediated – T cells instruct destruction of
infected cells of destruction of intracellular bacteria
Decline stageAlleviation of signs and symptomsRecover/convalescence
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IV. Two Major Types of Epidemic
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Common Source EpidemicSharp increase to a peak, then a rapid
resolutionAssociated with common contaminated
sourceExamples
Food poisoning (food)Legionnaire’s disease (water – air conditioning)
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Propagated EpidemicExtended rise with a gradual resolutionFrequently observed when one individual
= sourceGradual disseminationAll susceptible individuals succumb
ExamplesMumps, chickenpox
# susceptible individuals eventually decreases due to acquired immunityAgent loses the ability to disseminate through
the population
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V. Herd ImmunityResistance a population acquires as a whole to
infectious diseaseThe number of individuals that must be immune to
prevent an epidemic outbreak of a disease is a function of:Infectivity of the disease (I)Duration of the disease (D)Proportion of susceptible individuals in the
population (S)When 70% of individuals in a population are
immune, the propagation from individual to individual is not sustained and epidemics do not occur
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Opportunity for contact and transmission decreases as the number of immune individuals increases
Susceptible individuals benefit from an indirect immunity (not self-made immunity)
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Acquisition of Herd Immunity through ImmunizationImmunization of large numbers of
susceptible individuals in a population can induce herd immunity
Necessary to achieve a balance between immune and susceptiblesDynamic
Births, deaths, migratory patterns
Immune individuals can become susceptible again if the pathogen mutates (antigenic shift or antigenic drift)
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VI. Antigenic Shift and Antigenic Drift Caused by MutationsMajor genetic changes in a pathogen =
Antigenic ShiftToo great to be the result of simple mutationsExample: Influenza strains derived from mixing
of different influenza serovars Can occur between animal and human virus (e.g.
human and avian influenza) Co-infection of same cell Genomes recombine (8 RNA strands/genome)
Mixing of gene pools, addition of new genes New serovar is generated No resistance in the population
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Influenza pandemic outbreak of 1918 (“Swine Flu”) Killed 20-40 million people
In the Far East, animal hosts for influenza viruses (ducks, chickens and pork) live close together and close to humans
Other examples: 1957 – “Asian Flu” 1968 – “Hong Kong Flu” 1977 – “Russian Flu” 1997 – All chickens killed in Hong Kong, 4 deaths,
new strain in chickens
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Antigenic DriftMinor genetic changes affecting critical
epitopes Point mutations in nucleic acids can cause single
amino acids to change in a protein Gradual and cumulative
Therefore, major changes are apparent only with time
Herd immunity will decrease as the number of susceptible individuals increases above a threshold density
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Example – Influenza virus – Types A, B and C (B and C are more stable) Inside of the virion
Nucleoprotein Matrix protein (under the envelope)
Outside of the virion Hemagglutinin spikes (HA) Neuraminidase spike (NA)
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RNA viruses have high rates of spontaneous mutations because RNA synthesis is not proof-read as well as DNA synthesis error prone (~1 base change per replication)
RNA viruses can adapt quickly to new environments Point mutations in NA and HA change the antigenic
structure Influenza A changes antigenic makeup often so
vaccines become ineffective
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VII. The Infectious Disease Cycle: Story of a Disease - Links in the
infectious disease chain
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Agent responsibleWhat pathogen caused the disease?Epidemiologists must determine the
etiology (cause) of a diseaseKoch’s postulates (or modifications of
them) are used if possibleThe clinical microbiology laboratory plays
an important role in the isolation and identification of the pathogen
Communicable disease - one that can be transmitted from one host to another
Transmittable?
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Source or reservoir of pathogenInanimate or animateHuman or non-humanCarriers
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Carrier - an infected individual who is a potential source of infection for others Active carrier - a carrier with an overt clinical
case of the disease Convalescent carrier - an individual who has
recovered from the disease but continues to harbor large numbers of the pathogen
Healthy carrier - an individual who harbors the pathogen but is not ill
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Incubatory carrier - an individual who harbors the pathogen but is not yet ill
Casual (acute, transient) carriers - any of the above carriers who harbor the pathogen for a brief period (hours, days, or weeks)
Chronic carriers - any of the above carriers who harbor the pathogen for long periods (months, years, or life)
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Route of transmission to susceptible hostAirborneDirect contactIndirect contactVehicleVectors
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How was the pathogen transmitted?Airborne - suspended in air; travels a
meter or more Droplet nuclei - may come from sneezing,
coughing, or vocalization Dust particles - may be important in airborne
transmission because microorganisms adhere readily to dust
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Contact - touching between source and host Direct (person-to-person) - physical interaction
between infected person and host Indirect - involves an intermediate, such as
eating utensils, thermometers, dishes, glasses, and bedding
Droplets - large particles that travel less than one meter through the air
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Vehicle (fomite) - food and water, as well as those intermediates described for indirect contact
Vector-borne - living transmitters, such as arthropods or vertebrates External (mechanical) transmission - passive
carriage of the pathogen on the body of the vector with no growth of the organism during transmission
Internal transmission - carried within the vector Harborage - organism does not undergo
morphological or physiological changes within the vector
Biologic - organism undergoes morphological or physiological changes within the vector
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Immune status of host – susceptible?Depends on defense mechanisms of
the host and the pathogenicity of the organism
Release of pathogenActive escape - movement of
organism to portal of exitPassive escape - excretion in feces,
urine, droplets, saliva, or desquamated cells
Virulence and mode of transmission
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Virulence and the Mode of Transmission
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A virus that is spread by direct contact (e.g., rhinoviruses) cannot afford to make the host so ill it cannot be spread effectively
A virus that is vector-borne can afford to be highly virulent
Pathogens that do not survive well outside the host and that do not use a vector are likely to be less virulent while pathogens that can survive for long periods of time outside the host tend to be more virulent
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VIII. The Emergence of New Diseases and the Resurgence of Old Diseases
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New diseases have emerged in the past few decades such as AIDS, Hepatitis C and E, hantavirus, Lyme disease, Legionnaire’s disease, toxic shock E. coli 0157:H7, cryptosporidiosis and others
Systematic epidemiology focuses on the ecological and social factors that influence the development, emergence and resurgence (TB, diphtheria) of disease
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Systematic Epidemiology - FactorsRapid transportation systems
Aid in the spread of disease out of areas where they are endemic
Travelers to endemic areas should inquire about vaccination prior to travel
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MigrationLarge populations migrating due to
econimc distress of political conflictsImport/Export Commerce
Plants and animal trade – legal and illegal
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Damaged or altered ecosystemsDecreases in predationGeneration of new vectors
Compromised populations at risk to new diseaseDrug usersMalnourishedSexual promiscuityHIV
DeforestationNew hosts for pathogens
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IX. Control of Epidemics: Finding the Weakest Link in the Chain
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Reduce or eliminate the source or reservoir of infection through:Quarantine and isolation of cases and
carriersEradication of an animal reservoir, if one
exists (poisoning, trapping)Treatment of sewage to reduce water
contaminationTherapy that reduces or eliminates
infectivity of individuals
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Interrupt the interaction between source and susceptiblesSanitizationDisinfectionVector control (pesticides)Chlorination of water suppliesPasteurization of milkSupervision and inspection of food and
food handlersDestruction of insect vectors with
pesticides
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Increase resistant population, herd immunity and vaccination programsPublic Health Authorities =
Epidemiological guardians - a network of health professionals involved in surveillance, diagnosis, and control of epidemics
Passive immunity – antiserumActive immunity - vaccination
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Remote sensing and Geographic Information Systems (GIS)Disease dynamic related to mapped
environmental variables
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X. Acquisition of infectious in clinical settings: Nosocomial Infections
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Produced by infectious agents that develop within a hospital or other clinical care facility and that are acquired by patients while they are in the facility
Infections that are incubating within the patient at the time of admission are not considered nosocomial
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SourceEndogenous - patient’s own microbiotaExogenous - microbiota other than the patient’s,
animate or inanimate source Staff Other patients Visitor Food Catheters IV Respiratory aids Water systems
Autogenous – caused by patient’s own microbiota, even if acquired as a result of hospital stay cannot be determined whether it is endogenous or
exogenous
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Control, prevention, and surveillance should include:proper handling of the patient and the
materials provided to the patient, monitoring of the patient for signs of
infection
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The hospital epidemiologist (other terms are also used) is an individual (usually a registered nurse) responsible for developing and implementing policies to monitor and control infections and communicable disease usually reports to an infection control
committee or other similar groupThe CDC estimates that 5-10% of all hospital patients acquire some form of nosocomial infection – usually bacterial
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Morbidity and Mortality Weekly Report (MMWR)Lists the number of reportable diseases
within the last year and past 4 weeksCDC monitors ~50 different bacterial, viral ,
fungal and parasitic infections and intervenes with immunizations or control measures in epidemic situations