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PathogenesisPopulations at Risk for Infectionor Disease

Prof. dr. Bhisma Murti, MPH, MSc, PhD

Department of Public Health, Faculty of Medicine,Universitas Sebelas Maret

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Infectious disease epidemiology Involves two populations

1. Animal

2. Infectious agentHas its own life cycle, immunology, and evolution

Involves additional considerations

Reservoir

Transmission

Contact and mixing patterns of infective and

susceptible hosts

Degree and duration of infectiousness

1. Measurement of disease events

2. Infectious disease epidemiology

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Infectiousness Time lines of infection

Time line of infectiousness

Time line of disease

1. Measurement of disease events

2. Infectious disease epidemiology

Incubation period

Time from infection to development of symptomatic

disease

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Infectiousness Time lines of infection

Time line of infectiousness

Time line of disease Incubation period

Symptomatic period

1. Measurement of disease events

2. Infectious disease epidemiology

Infection without symptoms = inapparent case

Can be infectious

Pathogenicity = probability of developing

symptoms after being infected

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Infectiousness Time lines of infection

Time line of infectiousness

Time line of disease Incubation period

Symptomatic period

Nondiseased

1. Measurement of disease events

2. Infectious disease epidemiology

Recovery from symptoms

Recovered

Immune

Carrier recovers from symptoms but remainsinfectious

Death

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Time lines of infection

Time

Nondiseased Dead

Recovered

Immune

CarrierIncubation

PeriodSymptomatic

Period

Time of

infection

Susceptible

Development

of Symptoms

End of

Symptoms

Time Line of Disease

Latent

Period

Infectious

Period

Noninfectious Removed

Dead

Recovered

Susceptible

Time Line of Infectiousness

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Time lines of infection

Time

Latent

Period

Infectious

Period

Noninfectious

Removed Dead

RecoveredSusceptible

Time Line of Infectiousness

Nondiseased

Dead Recovered

Immune

Carrier

IncubationPeriod

SymptomaticPeriod

Time of

infection

Susceptible

Development

of Symptoms

End of

Symptoms

Time Line of Disease

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Time lines of infection

Time

Latent

Period

Infectious

Period

Noninfectious

Removed Dead

RecoveredSusceptible

Time Line of Infectiousness

Important to Infectious Agent and

Production Medicine Vet

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Transmission probability The probability that, given contact

between an infective source and asusceptible host, successful transfer ofthe parasite will occur so that thesusceptible host becomes infected

Depends on:

The characteristics of the infective sourceThe parasite

The susceptible host

The type and definition of contact

1. Measurement of disease events

2. Infectious disease epidemiology

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Transmission probability The probability that, given contact

between an infective source and asusceptible host, successful transfer ofthe parasite will occur so that thesusceptible host becomes infected

Depends on:

The characteristics of the infective source

1. Measurement of disease events

2. Infectious disease epidemiology

Another animal

Insect vector

Inanimate object: water, feed, fomites

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Transmission probability The probability that, given contact

between an infective source and asusceptible host, successful transfer ofthe parasite will occur so that thesusceptible host becomes infected

Depends on:

The type and definition of contact

1. Measurement of disease events

2. Infectious disease epidemiology

The mode of transmission determineswhat types of contact are potentiallyinfectious

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Contact pattern

contagious disease Some diseases contact pattern is determined

by density

When confined in smaller areas, contacts are more

frequent

Important for diseases spread by casual contact

Other diseases contact pattern is determined

by pathogen or animal characteristics Sexually transmitted diseases will only affect

sexually active adults, not calves, pregnant animals,

or anestrous animals

1. Measurement of disease events

2. Infectious disease epidemiology

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Contact pattern

non-contagious disease Vertically transmitted diseases

Dam to offspring in utero

Vector borne diseases Snail

Biting insects / arthropods

1. Measurement of disease events

2. Infectious disease epidemiology

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Basic reproductive number (R0

)

1. Measurement of disease events

2. Infectious disease epidemiology

R0 =Number of

contacts per

unit time

Transmission

probability per

contact

Duration of

infectiousness= cpd

R0 is a complex parameter that summarizes

many of the important aspects of aninfectious agent in a host population

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1. Measurement of disease events

2. Infectious disease epidemiology

Time

Numberofindividua

ls

Epidemic, low R0

SusceptibleImmune

Infected

New Cases

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1. Measurement of disease events

2. Infectious disease epidemiology

Time

Numberofindividua

ls

Epidemic, high R0

Susceptible Immune

Infected

New Cases

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1. Measurement of disease events

2. Infectious disease epidemiology

Time

Numberofindividua

ls

Endemic, low R0

Susceptible

Immune

Infected

New Cases

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1. Measurement of disease events

2. Infectious disease epidemiology

Time

Numberofindividua

ls

Endemic, high R0

Susceptible

Immune

Infected

New Cases

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1. Measurement of disease events

2. Infectious disease epidemiology

Epidemic, low R0 Epidemic, high R0

Endemic, low R0 Endemic, high R0