Unit 12-13:Infectious Disease

Epidemiology

Unit 12-13 Learning Objectives:

1. Understand primary definitions used in infectious disease epidemiology.

2. Differentiate between direct and indirect modes of infectious disease transmission.

3. Understand the concept of herd immunity.4. Understand the necessary conditions, goals,

and utility for outbreak investigations.5. Understand unique features for studying

infectious diseases from an epidemiological perspective.

6. Understand the major components of conducting an outbreak investigation.

Unit 12-13 Learning Objectives (cont.):

7. Interpret epidemic curves in terms of possible outbreak patterns and sources.

8. Recognize the primary epidemiological study designs used in outbreak investigations.

9. Understand measures of effect used in outbreak investigations.

10.Review a real-world example of an outbreak investigation.

DEFINITIONS

AND

BACKGROUND

DefinitionsDefinitionsEndemic: Habitual presence of a disease in a

given geographic area.

Epidemic: Occurrence of a group of illnesses of similar nature within a given community or region in excess of normal expectancy, and derived from a common or from a propagated source.

Pandemic: A worldwide epidemic.

Herd immunity: Resistance of a group of to an attack by a disease to which a large proportion of members of the group are immune.

Definitions (cont.)Definitions (cont.)

Virulence: Severity of the disease produced by the organism.

Carrier: Individual who harbors the organism but is not infected, as measured by serologic studies or evidence of clinical illness.

Classic Example: Typhoid Mary was a carrier of Salmonella typhi who worked as a cook in NYC in different households over many years – considered to have caused at least 10 typhoid fever outbreaks that included 51 cases and 3 deaths.

Definitions (cont.)Definitions (cont.)

Infectivity: Capacity of agent to enter and multiply in a susceptible host (hence produce infection/disease) (polio and measles have high infectivity)

Pathogenicity: Capacity of agent to cause clinical disease in the infected host (measles has high pathogenicity)

Toxigenicity: Capacity of agent to produce a toxin or poison (e.g. toxin produced by the microorganism rather than microorganism itself (such as botulism and shellfish poisoning))

Definitions (cont.)Definitions (cont.)

Resistance: Ability of agent to survive adverse environmental conditions (hepatitis agents generally very resistant whereas influenza viruses are typically fragile). Note: “resistance” is also applied to the host.

Antigenicity: Ability of agent to induce antibody production in the host (e.g. re-infection with measles virus is very rare). The related term “immunogenicity” refers to infection’s ability to produce specific immunity.

Modes of Disease TransmissionModes of Disease Transmission

Direct: Person-to-person contact (propagated)

Indirect:

a) Common vehicle such as contaminated air or water supply (can occur from single, multiple, or continuous exposure)

b) Vector such as a mosquito (e.g. West Nile virus)

The likelihood of disease transmission depends on several related factors (see upcoming slides).

Timelines for Infection and DiseaseTimelines for Infection and Disease

Dynamics ofinfectiousness latent

periodinfectious

period

noninfectious-- removed-- dead-- recovered

Dynamics ofdisease

susceptible

susceptible

Time of Infection

time

time

incubationperiod

symptomaticperiod

noninfectious-- dead-- recovered-- immune-- carrier

Timelines for Infection and DiseaseTimelines for Infection and DiseaseDefinitions from Previous Slide:

Latent period: time interval from infection to development of infectious (note: this definition differs from that used for non-infectious diseases).

Infectious period: time during which the host can infect another host.

Incubation period: time from infection to development of symptomatic disease.

Symptomatic period: period in which symptoms of the disease are present.

Host

Vector

Agent Environment

Factors Affecting Disease TransmissionFactors Affecting Disease Transmissionand Symptomatic Clinical Diseaseand Symptomatic Clinical Disease

-- Susceptibility-- Immune response-- Resistance--- Portal(s) of entry

-- Virulence Toxigenicity-- Infectivity Resistance-- Pathogenicity Antigenicity

VECTOR-- Prevalence-- Portal(s) of entry

-- Balance of immune to susceptible individuals--- Opportunity for exposure (e.g. crowding)

Characteristics of Herd ImmunityCharacteristics of Herd Immunity

-- If a large percent of the population is immune, the entire population is likely to be protected.

-- Once a high proportion of all people in the community are immune, the likelihood is small that an infected person will encounter a susceptible person.

-- Due to herd immunity, highly protective immunization can occur without requiring 100% immunization rates (estimated 94% immunity for measles to interrupt the chain of transmission).

Characteristics of Herd ImmunityCharacteristics of Herd Immunity

-- For herd immunity, disease agent must be restricted to a single host species, and transmission must be relatively direct from one member of host species to another (e.g. no reservoir outside the human host in which the organism can exist).

-- Herd immunity operates optimally when there is random mixing of the population.

Conditions for an OutbreakConditions for an Outbreak

Agent and susceptible hosts are present in sufficient number

The agent can effectively be conveyed from a source to susceptible hosts

Why do outbreaks occur?– Increase in amount or virulence of agent– Recent introduction of the agent into a new

setting– Enhanced mode of transmission, exposing

more susceptible individuals– Factors that increase host exposure or

involve introduction through new portals of entry

Outbreak InvestigationOutbreak Investigation

GOALS:

1.Rapidly identify the source and reservoir of the outbreak.

2. Implement interventions to control and eliminate the outbreak.

3.Develop policies to prevent future outbreaks.

Note: It is important to maintain an open mind when investigating the source of an outbreak – natural biologic toxins, heavy metals, and chemical poisoning may mimic signs and symptoms of infectious agents.

Outbreak InvestigationOutbreak InvestigationUtility of Performing Outbreak Investigations:

1.Has goal of determining cause of outbreak, eliminating or interrupting transmission, and providing post-exposure prophylaxis.

2.Uncovers new infectious agents and diseases.

3. Identifies spread of a known virus to a new geographic area (e.g. West Nile encephalitis in New York City).

4. Improves epidemiological understanding by uncovering new means of disease transmission.

5.Leads to public health regulations/recommendations to prevent future disease outbreaks.

Unique Features of Infectious DiseasesUnique Features of Infectious Diseases 1. A case may also be a source:

For most non-infectious (e.g. “chronic”) diseases, a person’s risk of disease in not influenced by the disease status of others (e.g. a person’s risk of CHD is not influenced by the fact that his/her neighbor experienced a myocardial infarction).

However, for infectious diseases such as influenza, a person’s risk is greatly affected by the number of influenza patients around, and if many of the people one meets have been vaccinated.

Unique Features of Infectious DiseasesUnique Features of Infectious Diseases

2. Some people may be immune:

For most non-infectious risk factors (e.g. toxins or radiation), there will be levels when all exposed will become ill.

However, for some infectious diseases, such as measles, once a person has had the disease, he/she will never get it again, even if exposed in the middle of an epidemic.

Thus, in terms of measuring incidence, not everyone is “at risk” of developing the disease.

Unique Features of Infectious DiseasesUnique Features of Infectious Diseases

3. There is sometimes a need for urgency:

Many “chronic” diseases are concerned with the impact of environmental (e.g. lead exposure) and behavioral (e.g. smoking) risk factors that require big time-consuming public health programs.

However, with outbreaks of some infectious diseases, such as Ebola or Legionnaires disease, the time frame for investigation and preventive action may be a matter of hours or days – this may give little time for elaborate analyses.

Unique Features of Infectious DiseasesUnique Features of Infectious Diseases4. Preventive measures (often) have a clear

scientific basis.

For many “chronic” diseases such as CHD, the relative impact of factors such as diet and cholesterol on disease etiology (and hence prevention), remains equivocal and a matter of debate.

However, for many infectious diseases, the causative agent and characteristics of transmission are well established – this leads to clear (but not always easy) targets for disease prevention.