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 disease
• Person-time at risk
• Incidence always incorporates time- the duration
  of observation

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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

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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

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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 gt 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 details

• There are two sets of variables
• Variables describing the states people are in
• Ssusceptible
• Iinfectious
• Rnon-infectious/asymptomatic
• Variables describing how many people are moving
  between these states (parameters)
• Example ?Fraction of people in state R who move
  to state S

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The SIR Model
g
?
d
I

• S Susceptible
• I Infectious (symptomaticasymptomatic)
• R Non-infectious
• W Concentration of pathogens in the environment
• ß Infection rate due to exposure to pathogen
• d Fraction of people who move from state I to
  state R
• ? 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

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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 Legionnaires 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