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Download Fundamentals of Microbiology: Disease transmission
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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 – – – – – Medium that transmits disease Water Food Air Blood 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 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: – – – – – 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