MGY277H1 Lecture Notes - Lecture 9: Epidemiology, Contact Tracing, Vertically Transmitted Infection

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Unit 9
Epidemiology and Disease Transmission
Epidemiology
Epidemiologythe study of disease patterns in populations
Epidemiologists (health detectives)
Collect/Compile data about sources of disease and risk factors
Design infection control strategies
Prevent or predict spread of disease
Use expertise in diverse disciplines including
ecology/microbiology/sociology/statistics/psychology
Influences our daily lives (hand washing, waste disposal, restaurant/food
inspection/water treatment)
Principles of Epidemiology
Communicable (contagious) disease
Transmitted from one host to another
o Measles,colds, influenza
Transmission determined by interactions between environment+ pathogens
+ host
Control of any of these factors may break infection cycle
o Improved sanitation, antimicrobial medication, vaccination
Non-communicable disease
DONOT spread host to host
Microorganisms most often arise from individual’s normal microbiota or
environment (e.g. Clostridium tetani)
Legionnaire’s disease cause outbreaks and has an infection cycle that
must be broken (find and clean the contaminated water supply) but the
bacteria DOES NOT spread from person to person
Rates of disease in population
Epidemiologist LESS concerned with absolute number of cases than rate
(consider small vs. large city)
Attack rate percentage of people who become ill in population after
exposure
o Reflects infectious dose, immune status of population
Incidence rate number of new cases/time/population
o Measure of risk of an individual contracting a disease
Prevalence is total number (died & alive) of cases at any time or for a
specific period in a given population
o Reflects overall impact of disease on society (includes old and new
cases as well as duration of disease)
o Both expressed as cases per 100,000 people
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Morbidity/Incidnetthe burden of disease in population at risk
o Contagious disease (influenza) often have high morbidity
rateinfected individual may transmit to several
Mortality overall death rate in population
o In developed countries, most often associated with non-
communicable disease (cancer/heart attack)
o Infection is a major cause of death in developing countries
Case-fatality rate % of population that dies from a specific disease
o Plague feared because of very high rate
o Rate of AIDS has decreased from improved treatmentprevalence
has increased as more with disease survive
Endemic disease constantly present in population (common cold)
Outbreak a higher than expected cluster od disease cases over a specific
time in a population
Epidemic usually large number of cases (over a larger region), either
introduced or endemic disease
Pandemic global (AIDS)
Descriptive Studies
Collect data that characterize occurrence (time/place/individuals affected) of an
outbreak used to compile risk factors for spread
The persondetermining profile of people who become ill
o Age/gender.ethinicity/occupation/etc
The place Geographic location helps pinpoint source, yield clued about
potential reservoirs, vectors, or boundaries that may affect transmission
The time season important which also influence rate of spread
Outbreak Epidemiology
Common-source epidemic/outbreak
Rapid rise in cases suggests exposure to single source of pathogen
i.e food poisoning from a contaminated product
Requires tracing to find what all victims have in common
Propagated epidemicslow rise in cases suggests contagious disease spreading
in population
1st case is call the index case or patient zero
requires contact tracing to find potential new victims
Some have feature of both common-source and propagated epidemic
Analytical Studies
Determine relevancy of risk factors from descriptive study
Cross-Section studies survey range of people at a single point in time for
presence/absence of disease
May suggest association between risk factors & disease but DOES NOT
establish cause of disease
Outbreak & Epidemic ten
to be use simultaneously
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