Download Epidemiology of Infectious Disease

February 11, 2013
Initiative
Ostensible: 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?
The Epidemiology of
Infectious Disease
I. The Science of Epidemiology
 Epidemiology:
 The field of science concerned with the circumstances
under which diseases occur
 An epidemiologist works in this field
 Factors under investigation:
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Incidence (morbidity rate) and spread of infectious and noninfectious 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)
 Basic terms used in epidemiology
 Sporadic disease
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Occurs irregularly and only occasionally in a population
Example: Typhoid fever (Salmonella typhi)
 Endemic disease
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Occurs at regular intervals but at low levels
Example: Common cold (Rhinovirus)
 Hyperendemic disease
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When occurrence frequency rises, but not to epidemic
proportions
Example: Common cold in the winter months
 Epidemic
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Sharp increase in the incidence above the predicted/expected
level
 Reservoir
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Natural location of the organism
Can be animate or inanimate location
Examples:
 Rabies – Dogs, foxes, raccoons (zoonoses)
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Neisseria meningitidis (meningitis) – Humans
Malaria – Humans
Cryptococcus – Bird guano
 Source
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Immediate location from which infectious agent has
been transmitted
Examples:
 Neisseria gonorrhea
 Source = humans
 Reservoir = humans
 Salmonella typhi
 Source = food and water
 Reservoir = humans
 Hepatitis C
 Source = transfusion, blood products
 Reservoir = humans
 Carriers
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Hosts that harbor a pathogen without clinical
symptoms and are capable of transmitting the
infectious agent (sometimes unknowlingly)
Carrier state may be short (transient) or longterm (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
 Vector
 A biological or inanimate source that contributes to the
transmission of an infectious agent from one host to
another
 Examples:
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Arthropods
 Mosquitoes – Malaria, West Nile Virus
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Ticks – Lyme disease
Fleas – Bubonic plague
Flies - Trachoma
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Birds
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Lower vertebrates
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Parrots – Psittacosis
Pigeons – Cryptococcus
Frogs and turtles – Salmonella
Inanimate objects (fomites)
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Sporothrix schenkii (sporotrichosis)
February 20, 2013
Dependability
 Deviate: 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
 Index case - the first case in an epidemic
 Outbreak - an epidemic-like increase in
frequency, but in a very limited (focal)
segment of the population
 Rapid increase, usually localized
 Example: Legionnaire’s disease
 Pandemic - a long-term increase in frequency
in a large (usually worldwide or continental)
population
 Disease frequency rises on a large scale
geographically
 Epizootology
 Deals with animal diseases affecting animal populations
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Enzootic = moderate incidence
Epizootic = rapid increase
Panzootic = wide spread incidence
Zoonoses = if transferable to humans
 II. Equations for Determining Frequency of
Disease
 Statistical Analyses
 The mathematics of collection, organization, and
interpretation of numerical data (rate
acoomparisons, chi-square, SEMs)
 Used by state public health lab, CDC, WHO and
USPHS
 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
 Prevalence - number of individuals
infected at any one time per unit of
population
 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
 III. The Epidemiology of Infectious Disease
 Recognition of an Infectious Disease in a
Population
Factors Affecting the Cycle of Disease
 Causative agent
 Source/reservoir
 Method of Transmission
 Influence of host or environment in the spread of the
disease
Goal of the Epidemiologist
 Control the spread (dissemination)
 Eliminate etiological agent
Surveillance and Data Collection for Control
 Calculation of morbidity and mortality rates
 Case studies
 Field studies
 Review clinical records and lab reports
 Investigate source, reservoir and vectors
 Review treatments/success rates
 Employ demographic data to track the movement
of disease
 Signs versus symptoms:
 Sign = observable or measurable change in body function
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Diarrhea, rash, fever, vomiting
 Symptom = subjective
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Pain, appetite loss, lethargy, depression
  Disease syndrome - a set of signs and
symptoms that is characteristic of a disease
Phases of Infectious Disease Life Cycle
 Incubation period
 Variable length
 Prior to development of signs of symptoms
 Prodromal stage
 Beginning of signs and symptoms
 Often infectious/contagious
 Innate immune response “kick in” (first line of defenses)
 Illness stage
 Most severe phase
 Clear evidence of signs and symptoms
 Acquired immune responses begin
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Humoral – Antibodies and complement
Cell-mediated – T cells instruct destruction of infected cells
of destruction of intracellular bacteria
 Decline stage
 Alleviation of signs and symptoms
 Recover/convalescence
 IV. Two Major Types of Epidemic
Common Source Epidemic
 Sharp increase to a peak, then a rapid resolution
 Associated with common contaminated source
 Examples
 Food poisoning (food)
 Legionnaire’s disease (water – air conditioning)
Propagated Epidemic
 Extended rise with a gradual resolution
 Frequently observed when one individual = source
 Gradual dissemination
 All susceptible individuals succumb
 Examples
 Mumps, chickenpox
 # susceptible individuals eventually decreases due
to acquired immunity
 Agent loses the ability to disseminate through the
population
V. Herd Immunity
 Resistance a population acquires as a whole to infectious
disease
 The 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
 Opportunity for contact and transmission decreases as
the number of immune individuals increases
 Susceptible individuals benefit from an indirect
immunity (not self-made immunity)
Acquisition of Herd Immunity through
Immunization
 Immunization of large numbers of susceptible
individuals in a population can induce herd
immunity
 Necessary to achieve a balance between immune
and susceptibles
 Dynamic
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Births, deaths, migratory patterns
 Immune individuals can become susceptible again
if the pathogen mutates (antigenic shift or
antigenic drift)
VI. Antigenic Shift and Antigenic Drift
Caused by Mutations
 Major genetic changes in a pathogen = Antigenic
Shift
 Too great to be the result of simple mutations
 Example: Influenza strains derived from mixing of
different influenza serovars
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Can occur between animal and human virus (e.g. human and
avian influenza)
Co-infection of same cell
Genomes recombine (8 RNA strands/genome)
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Mixing of gene pools, addition of new genes
New serovar is generated
No resistance in the population
 Influenza pandemic outbreak of 1918 (“Swine Flu”)
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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:
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1957 – “Asian Flu”
1968 – “Hong Kong Flu”
1977 – “Russian Flu”
1997 – All chickens killed in Hong Kong, 4 deaths, new strain
in chickens
 Antigenic Drift
 Minor genetic changes affecting critical epitopes
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Point mutations in nucleic acids can cause single amino acids
to change in a protein
Gradual and cumulative
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Therefore, major changes are apparent only with time
 Herd immunity will decrease as the number of
susceptible individuals increases above a threshold
density
 Example – Influenza virus – Types A, B and C (B and C
are more stable)
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Inside of the virion
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Nucleoprotein
Matrix protein (under the envelope)
Outside of the virion
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Hemagglutinin spikes (HA)
Neuraminidase spike (NA)
 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
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Point mutations in NA and HA change the antigenic
structure
Influenza A changes antigenic makeup often  so vaccines
become ineffective
 VII. The Infectious Disease Cycle: Story of a
Disease - Links in the infectious disease chain
 Agent responsible
 What pathogen caused the disease?
 Epidemiologists must determine the etiology
(cause) of a disease
 Koch’s postulates (or modifications of them) are
used if possible
 The 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?
 Source or reservoir of pathogen
 Inanimate or animate
 Human or non-human
 Carriers
 Carrier - an infected individual who is a potential
source of infection for others
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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)
 Route of transmission to susceptible host
 Airborne
 Direct contact
 Indirect contact
 Vehicle
 Vectors
 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
 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
 Vehicle (fomite) - food and water, as well as those
intermediates described for indirect contact
 Vector-borne - living transmitters, such as
arthropods or vertebrates
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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
 Immune status of host – susceptible?
 Depends on defense mechanisms of the host
and the pathogenicity of the organism
 Release of pathogen
 Active escape - movement of organism to
portal of exit
 Passive escape - excretion in feces, urine,
droplets, saliva, or desquamated cells
 Virulence and mode of transmission
 Virulence and the Mode of Transmission
 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
 VIII. The Emergence of New Diseases and the
Resurgence of Old Diseases
 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
Systematic Epidemiology - Factors
 Rapid 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
 Migration
 Large populations migrating due to econimc
distress of political conflicts
 Import/Export Commerce
 Plants and animal trade – legal and illegal
 Damaged or altered ecosystems
 Decreases in predation
 Generation of new vectors
 Compromised populations at risk to new
disease
 Drug users
 Malnourished
 Sexual promiscuity
 HIV
 Deforestation
 New hosts for pathogens
 IX. Control of Epidemics: Finding the
Weakest Link in the Chain
 Reduce or eliminate the source or reservoir of
infection through:
 Quarantine and isolation of cases and carriers
 Eradication of an animal reservoir, if one exists
(poisoning, trapping)
 Treatment of sewage to reduce water
contamination
 Therapy that reduces or eliminates infectivity of
individuals
 Interrupt the interaction between source and
susceptibles
 Sanitization
 Disinfection
 Vector control (pesticides)
 Chlorination of water supplies
 Pasteurization of milk
 Supervision and inspection of food and food
handlers
 Destruction of insect vectors with pesticides
 Increase resistant population, herd immunity
and vaccination programs
 Public Health Authorities = Epidemiological
guardians - a network of health professionals
involved in surveillance, diagnosis, and control of
epidemics
 Passive immunity – antiserum
 Active immunity - vaccination
 Remote sensing and Geographic Information Systems
(GIS)
 Disease dynamic related to mapped environmental
variables
 X. Acquisition of infectious in clinical
settings: Nosocomial Infections
 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
 Source
 Endogenous - patient’s own microbiota
 Exogenous - microbiota other than the patient’s, animate or
inanimate source
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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
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cannot be determined whether it is endogenous or exogenous
 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
 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 group
 The CDC estimates that 5-10% of all hospital
patients acquire some form of nosocomial
infection – usually bacterial
Morbidity and Mortality Weekly Report
(MMWR)
 Lists the number of reportable diseases within the last
year and past 4 weeks
 CDC monitors ~50 different bacterial, viral , fungal and
parasitic infections and intervenes with immunizations
or control measures in epidemic situations