Download Fundamentals of Microbiology: Disease transmission

Disease Transmission
January 21st, 2010
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
Basic Terms
• Incubation period
– Time between infection and appearance of symptoms
• Prodromal period
– Time when symptoms are mild
• Morbidity
– Illness
• Mortality
– Death
Basic Terms
• Fomite
– Inanimate object that transmits disease (your
cell phone!)
• Vehicle
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Medium that transmits disease
Water
Food
Air
Blood
Basic Terms
• Vector
– Arthropods that carry infectious disease
– The microorganism can multiply in the vector
• Reservoir
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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
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
Epidemiology terms
• Incidence
Number of new cases of disease
Person-time at risk
• Incidence always incorporates time- the
duration of observation
Epidemiology terms
• Prevalence
Number of new cases of disease
Number of people at risk
• Prevalence is at a single point in time
• A snapshot of the disease in the
population
Epidemiology terms
• Attack rate
Number of new infections
Person-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
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
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
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
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)
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
The SIR model: key details
There 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
The SIR Model
g
S

I
W
d
R
ENVIRONMENT
•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
Epidemiology concepts: Epidemic
curves
• An epidemic curve can provide
information on the following
characteristics of an outbreak:
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Pattern of spread
Magnitude
Outliers
Time trend
Exposure and/or disease incubation period
Epidemic curves
• The overall shape of the epi curve can
reveal the type of outbreak
– Common source
– Point source
– Propagated
Epidemic curve: a point source
outbreak
Example: Legionnaire’s Disease
Epidemic curve: a point source
outbreak
A gastroenteritis outbreak in a nursing home
Epidemic curve: a common source
outbreak
a foodborne outbreak from a widely distributed food product
Epidemic curve: a propagated
outbreak
SARS in Toronto: A propagated outbreak with two waves of cases