Bms1026 Epidemiology

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Infectious disease

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Diseases caused and transmitted by microorganisms.

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Non-infectious disease

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Diseases not caused by microorganisms so not transmittable.

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Contagious disease

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Diseases easily transmitted through contact with an infected individual or surfaces touched by them.

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Non-contagious disease

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Diseases that require a mode of transmission (eg vector or bodily fluid transmission)

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Incidence of a disease

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Number of people in a population who develop a disease at a particular time (new cases - how fast it spreads)

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Prevalence of a disease

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Number of people in a population with a disease at a particular time (new and old cases - how long the disease lasts)

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Epidemic

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A disease when it occurs in an unusually high number of individuals in a population at a particular time.

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Pandemic

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A wide spread, usually global epidemic.

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Endemic

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A disease that is constantly present in the population usually at low incidence.

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Outbreak

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A number of cases occur in a short period of time in an area with usually only isolates cases.

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Carrier

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Individuals who are infected but down develop symptoms.

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Epidemiology

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The study of distribution and determinants of disease frequency in specified populations, and the application of this study to the control of health problems.

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Descriptive study

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Collecting all the data that describes occurrences of disease (ask everyone infected or not)

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Analytical study

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Determines causes by case study (diseased vs non diseased) or cohort method (population in contact with disease vs population not in contact)

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Chain of infection

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Reservoir - animate (animal/human - need living host to survive) inanimate (soil, water, food - living host incidental to lifecycle.
Transmission - direct/indirect contact, droplet, vector, airborne etc.
Susceptible host - nose, mouth, skin, airways, bodily fluid, genitals.

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Why does a pathogen spread?

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- allows offspring to find new habitat to grow,
- allows population to grow and survive as a whole,
- current environment no longer viable (eg lack of nutrients, abiotic stress, competition etc).

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Contact transmission - Direct contact

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Animal - Animal
Animal - Person
Person - Person

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Contact transmission - Indirect contact

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Reservoir to susceptible host via non living object eg bedding, clothes, doorhandles, toys etc

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Contact transmission - Droplet

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Spread in droplet nuclei that travel a max of 1m in air eg coughing, talking, sneezing.

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Vehicle transmission - Food

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Disease agents are generally transmitted in food that incompletely cooked, poorly refrigerated or prepared under unsanitary conditions.

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Vehicle transmission - Water

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When water is contaminated eg by untreated/poorly treated sewage.

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Vehicle transmission - Airborne

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By droplet nuclei that travel more than 1m in air and can remain airborne for prolonged periods of time.

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Vector transmission - Mechanical

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Passive transport of the pathogen on the insects feet or other body parts.

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Vehicle transmission - Biological

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Active process through insect bite, pathogens reproduce in the vector and transmitted to host during feeding.

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Life cycle of pathogen transmission

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1) transmission to host,
2) colonisation and invasion,
3) damage to host by toxins/mechanisms,
4) exit from host (straight transmission or survival outside host until transmission).

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Infectivity

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Ease of transmission of pathogen (whether exposure will cause infection).

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Pathogenicity

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Ability of pathogen to cause disease.

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Virulence

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How serious the disease outcome (whether its likely to be prolonged or cause death).

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Public health measures - Reduce reservoir

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a) Human - immunisation, quarantine or treatment of all exposed/infected individuals.
b) Animals - Culling, immunisation to reduce infected animals (harder with wild animals/insects)

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Public health measures - Reduce transmission

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a) Food/Waterborne - treatment of food/water to eliminate pathogen (purification, pasteurisation etc)
b) Airborne - can purify/filter small areas, wear face masks, but otherwise harder to control

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Public health measures - Reduce host susceptibility

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a) Active immunity - develops antibodies in response to infection/vaccine etc
b) Passive immunity - gains antibodies by maternal transfer, vaccine etc

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Public health measures - Herd immunity

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When the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity.