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?

The Epidemiology of Infectious Disease 

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)

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

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

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

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

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

Birds Parrots – Psittacosis Pigeons – Cryptococcus

Lower vertebrates Frogs and turtles – Salmonella

Inanimate objects (fomites) Sporothrix schenkii (sporotrichosis)

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

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

EpizootologyDeals with animal diseases affecting animal

populations Enzootic = moderate incidence Epizootic = rapid increase Panzootic = wide spread incidence Zoonoses = if transferable to humans

II. Equations for Determining Frequency of Disease

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

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 DiseaseCausative agentSource/reservoirMethod of TransmissionInfluence of host or environment in the

spread of the disease

Goal of the EpidemiologistControl the spread (dissemination)Eliminate etiological agent

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

Signs versus symptoms:Sign = observable or measurable change in

body function Diarrhea, rash, fever, vomiting

Symptom = subjective Pain, appetite loss, lethargy, depression

Disease syndrome - a set of signs and symptoms that is characteristic of a disease

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)

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

IV. Two Major Types of Epidemic

Common Source EpidemicSharp increase to a peak, then a rapid

resolutionAssociated with common contaminated

sourceExamples

Food poisoning (food)Legionnaire’s disease (water – air conditioning)

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

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

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 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)

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

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

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

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)

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

VII. The Infectious Disease Cycle: Story of a Disease - Links in the

infectious disease chain

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?

Source or reservoir of pathogenInanimate or animateHuman or non-humanCarriers

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

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 hostAirborneDirect contactIndirect contactVehicleVectors

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 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 pathogenActive escape - movement of

organism to portal of exitPassive 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 - 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

MigrationLarge populations migrating due to

econimc distress of political conflictsImport/Export Commerce

Plants and animal trade – legal and illegal

Damaged or altered ecosystemsDecreases in predationGeneration of new vectors

Compromised populations at risk to new diseaseDrug usersMalnourishedSexual promiscuityHIV

DeforestationNew 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

carriersEradication of an animal reservoir, if one

exists (poisoning, trapping)Treatment of sewage to reduce water

contaminationTherapy that reduces or eliminates

infectivity of individuals

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

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

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

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

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 groupThe 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 weeksCDC monitors ~50 different bacterial, viral ,

fungal and parasitic infections and intervenes with immunizations or control measures in epidemic situations