Epidemiology 2: Disease Transmission

Primary

Which type of prevention?:

Preventing initial development of a disease

Examples: vaccination; emissions regulations

Secondary

Which type of prevention?:

Early detection of health conditions to reduce duration and severity of condition

Examples: child development assessments; mental health or suicide screening

Tertiary

Which type of prevention?:

Treatment, therapy, rehabilitation

Reduce impairment, complications, and associated disability

Examples: relapse prevention

John Snow

Father of epidemiology

Did retrospective study of an outbreak of cholera in London

Smith and Kilbourne

Hypothesized that cattle ticks cause Texas Fever in cattle

Curtice

Established the life cycle of the cattle tick to help link it to Texas Fever

Cattle tick (Texas fever)

First arthropod that was definitively linked with the transmission of an infectious disease

Endemic

Usual (habitual) occurrence of a disease within a given area

Sporadic

Separate or scattered disease incidents occurring at a low frequency

Epidemic or outbreak

More disease than expected for a given time and place

Pandemic

Epidemic affecting several continents

Reservoir

Any animal (including human), arthropod, plant, soil, or inanimate matter in which an infectious agent normally lives and multiplies or on which it depends primarily for survival and reproduces in such a way that it can be transmitted to a susceptible host

Nidus

Localized reservoir that persists over a very long time period

Vehicle

Object, substance, or non-receptive living being serving as an intermediary in transmitting a pathogen from the organism hosting it to a receptive host

Vector

Living creature which, because of its ecological relationship to others, acquires a pathogen from one living host and transmits it to another

Fomite

Object or material that is not in itself harmful but on which pathogens may be conveyed

Carrier

Individual who harbors the organism; can infect others; not clinically ill

Clinical signs, duration of shedding

How are carriers classified?

Incubationary, convalescent

Categories of carriers in relation to clinical signs

Incubationary

Pre-clinical; many organisms being shed

Convalescent

Animal that continues to harbor and shed organisms for a variable period of time following recovery

Transient, chronic

Categories of carriers in relation to duration of shedding

Transient

Subclinical but may shed organisms for variable periods (ex. stress)

Chronic

Animals that shed organisms for long periods of time

Incubation period

Time between exposure to pathogen and onset of clinical signs

Latent infection

Inapparent infection that has the potential to develop signs of disease

Triggering mechanism may be an altered host response

Source of infection

Animal, person, or object from which an infectious agent passes immediately to the host (not necessarily synonymous with reservoir)

Species, age, breed, sex, physiological state, disease history

Types of intrinsic host factors

Use (animal) / occupation (human), husbandry (animal) / socio-economic status (human)

Types of extrinsic host factors

Pathogenicity

Ability to produce disease

Virulence

Degree of pathogenicity; disease-evoking power of a microorganism in a given host

Case-fatality rate (CFR)

Number of deaths among the cases

Used to assess virulence

Host range

Range of hosts in which an agent can survive

Viability

Ability of an infectious agent to survive in the free state (environment)

Infectivity

Ability of the agent to lodge and multiply within the host; minimum number of infectious particles required to establish an infection

Infectiousness

Ease with which the agent is transmitted from one host to another

Basic reproduction number (R0)

Measure of infectiousness of an agent in a totally susceptible population

Number of new cases of disease a single case will generate during its infectious period

Inherent characteristic of an infectious agent

Effective reproduction number (R)

Average number of new cases generated by a single case in a population that consists of both immune and non-immune individuals

1.0

If R is less than ___, sustained transmission within a population cannot occur.