Vm608 Epidemiology Terms

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Epidemiology

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The study of how disease is distributed in populations and the factors that influence or determine the distribution

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Objectives of epidemiology

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1. Identify the cause of disease
2. Determine the extent of disease
3. Study the natural history of disease
4. Evaluate preventative measures, therapies, etc.
5. Science-based public policy

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Test sensitivity

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How well does the test identify those with disease?

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Test specificity

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How well does the test rule out those who do not have disease?

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Primary prevention

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Protecting the non-infected

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Secondary prevention

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Detecting the pre-clinical

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Tertiary prevention

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Reducing the impact of clinical disease

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Endemic

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The usual occurrence of a disease within a given area

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Sporadic

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Separate or scattered disease incidents occurring at low frequency

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Epidemic/Outbreak

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More disease than expected for a given time and place

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Pandemic

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Epidemic affecting several continents

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Incubationary carrier

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Pre-clinical, many organisms being shed

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Convalescent carrier

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Continues to harbor and shed organisms for a variable period of time following recovery

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Transient carrier

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Subclinical but may shed organisms for variable periods

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Chronic carrier

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Organisms are shed for long periods of time

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Reservoir

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Any animal, 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

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Nidus

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A localized reservoir that persists over a very long time period

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Vehicle

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

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Vector

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A living creature which acquires a pathogen from one living host and transmits it to another

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Fomite

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An object or material that can transfer pathogens on its surface

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Incubation period

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The time between exposure to the pathogen and the onset of clinical signs

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Latent infection

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Inapparent infection that has the potential to develop signs of disease

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Source of infection

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The animal, person, or object from which an infectious agent passes immediately to the host

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Intrinsic host factors

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"From within" host factors
1. Species
2. Age
3. Breed
4. Gender
5. Physiological state
6. Disease history

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Extrinsic host factors

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"Outside" host factors
1. Use of animal/occupation in humans
2. Husbandry in animals/socio-economic status in humans

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Infectivity

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An agent factor
Ability of the agent to ledge and multiply within the host; minimum number of infectious particles required to establish an infectious

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Infectiousness

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An agent factor
The ease with which the agent is transferred to another

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Pathogenicity

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An agent factor
The ability to produce disease

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Virulence

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An agent factor
The degree of pathogenicity; the disease-evoking power of a microorganism in a given host

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Host range

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An agent factor
The range of hosts in which an agent can survive

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Viability

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An agent factor
Ability of an infectious agent to survive in the environment

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Environmental factors

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Physical aspects of the environment: temp, pressure, humidity, etc.
Biological aspects of the environment: presence of reservoirs, vectors, herd immunity

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Horizontal transmission

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Transmission of disease among peers

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Vertical transmission

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Transmission of disease between generations

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Direct transmission

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Person to person, animal to animal, person to animal

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Indirect transmission

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Transmission via a common vehicle or vector

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Expiratory droplet

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Direct transmission
Produced by a cough or sneeze and deposited in the respiratory tract according to size

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Droplet nuclei

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Indirect transmission
Truly airbourne, evaporated expiratory droplets and other fluids that are inhaled into the alveoli
Ten times more infectious than expiratory droplets

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Disease

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An abnormality of structure or function that interferes with the well-being of the individual animal or results in decreased production, growth, or efficiency

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Clinical disease

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Outward manifestation of disease (signs or symptoms)

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Pre-clinical disease

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Not yet clinically apparent but will be in the future

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Subclinical disease

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Not clinical and will not be in the future

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Herd immunity

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Resistance of an "at risk" population to an attack by a disease to which a large proportion of the group are immune

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Epidemic

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An increase in the number of cases in a given time and place over the number expected

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Index case

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The first recognized case of a disease in an outbreak

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Retrospective cohort study design

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Determine exposures among each individual in the entire population at risk and determines whether or not disease occurred in each individual

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Case-control study design

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Identify a group of individuals from the population at risk with the disease and find out about their exposures. Compare to a group of individuals from the same population at risk WITHOUT disease and find out about their exposures.

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Common source outbreak with point source exposure

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Common source outbreak with continuous exposure

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Common source outbreak with intermittent exposure

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Propagated outbreak