Disease Transmission
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Transcript of Disease Transmission
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Disease Transmission
January 21st, 2010
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Basic Terms• Pathogen
– Microorganism that causes disease• Commensal
– Microorganisms that live in the human or animal without affecting them
• Colonization– Living bacteria are present in the host– You can be colonized but not infected– Example: antibiotic resistant Staphylococcus aureus
• Infection– Invasion by microorganisms that starts the disease
process
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Basic Terms• Incubation period
– Time between infection and appearance of symptoms
• Prodromal period– Time when symptoms are mild
• Morbidity– Illness
• Mortality– Death
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Basic Terms• Fomite
– Inanimate object that transmits disease (your cell phone!)
• Vehicle– Medium that transmits disease– Water– Food– Air– Blood
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Basic Terms• Vector
– Arthropods that carry infectious disease– The microorganism can multiply in the vector
• Reservoir– Any site where an infectious agent survives– Animal– Natural environment (water, soil)– Diseases can be acquired from their reservoirs– A reservoir does not have to play a role in disease
transmission, but it provides a source of pathogens
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Basic Terms• Zoonotic disease
– Disease that can be transmitted from humans to animals
– Some very important environmentally transmitted diseases are zoonotic
– Transmission cycles of many diseases go on naturally among animals until we blunder into the cycle
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Epidemiology terms• Incidence
Number of new cases of diseasePerson-time at risk
• Incidence always incorporates time- the duration of observation
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Epidemiology terms• Prevalence
Number of new cases of diseaseNumber of people at risk
• Prevalence is at a single point in time
• A snapshot of the disease in the population
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Epidemiology terms• Attack rate
Number of new infectionsPerson-time at risk of infection
• Attack rate is usually used during an epidemic
• Incidence during a limited period of time• Example: people who ate at a certain
restaurant
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Epidemiology terms• Epidemic
– Large number of cases of a disease in a relatively short period
– An epidemic of measles in unvaccinated schoolchildren
• Endemic– Always present in a population– Malaria endemic regions of the world
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Epidemiology concepts
• R0: The reproductive number• the average number of secondary
infections produced when one infected individual is introduced into a host population where everyone is susceptible
• infection can get started in a fully susceptible population if and only if R0 > 1
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A model of disease transmission: The SIR model
• Common model for describing the transmission of infectious diseases
• Can be used for mathematical modeling of disease transmission, but is also a conceptual model
• Describes a population where individuals fall into three main categories:– Susceptible (S): can get disease– Infectious (I): Have disease, can spread to
others– Recovered (R): Can no longer spread disease
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A model of disease transmission: The SIR model
• Different individuals within this population can be in one of a few key states at any given time– Susceptible to disease (S)– infectious/asymptomatic (I): transmission risk
because they do not know they are ill– infectious/symptomatic (I)– non-infectious/asymptomatic; recovered (R)
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A model of disease transmission: The SIR model
• A dynamic model: individuals are moving from state to state over time
• This is key: the transmission of infectious diseases is a dynamic process– Within human populations– Within animal populations– Between humans, animals, and insect vectors– From humans to the environment and back to
humans
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The SIR model: key detailsThere are two sets of variables:• Variables describing the states people are in
– S=susceptible– I=infectious– R=non-infectious/asymptomatic
• Variables describing how many people are moving between these states (parameters)– Example: γ=Fraction of people in state R
who move to state S
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•S: Susceptible•I: Infectious (symptomatic+asymptomatic)•R: Non-infectious•W: Concentration of pathogens in the environment•β: Infection rate due to exposure to pathogen•δ: Fraction of people who move from state I to state R•γ: Fraction of people who move from state R to state S•Solid lines: Individuals moving from state to state•Dashed lines: Pathogen flows between individuals in different states
The SIR Model
ENVIRONMENTW
S R
I
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Epidemiology concepts: Epidemic curves
• An epidemic curve can provide information on the following characteristics of an outbreak:– Pattern of spread– Magnitude– Outliers– Time trend– Exposure and/or disease incubation
period
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Epidemic curves• The overall shape of the epi curve
can reveal the type of outbreak– Common source– Point source– Propagated
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Epidemic curve: a point source outbreak
Example: Legionnaire’s Disease
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Epidemic curve: a point source outbreak
A gastroenteritis outbreak in a nursing home
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Epidemic curve: a common source outbreak
a foodborne outbreak from a widely distributed food product
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Epidemic curve: a propagated outbreak
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SARS in Toronto: A propagated outbreak with two waves of cases