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 • 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

S R W ENVIRONMENT The SIR Model g  d I • 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

Disease Transmission