PathogenesisPopulations at Risk for Infection

or Disease

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

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

Infectious disease epidemiology• Involves two populations

1. Animal2. Infectious agent

Has 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 events2. Infectious disease epidemiology

Infectiousness• Time lines of infection

Time line of infectiousness Latent period

Time from infection to development of infectiousness Period of infectiousness

Time from development of infectiousness to end of time when the host can infect another host

Noninfectious periodRemoved from population - harvestedDeadRecovered

1. Measurement of disease events2. Infectious disease epidemiology

Infectiousness• Time lines of infection

Time line of infectiousness Time line of disease

1. Measurement of disease events2. Infectious disease epidemiology

Incubation periodTime from infection to development of symptomatic

disease

Infectiousness• Time lines of infection

Time line of infectiousness Time line of disease

Incubation period Symptomatic period

1. Measurement of disease events2. Infectious disease epidemiology

Infection without symptoms = inapparent case

Can be infectious

Pathogenicity = probability of developing symptoms after being infected

Infectiousness• Time lines of infection

Time line of infectiousness Time line of disease

Incubation period Symptomatic period Nondiseased

1. Measurement of disease events2. Infectious disease epidemiology

Recovery from symptomsRecoveredImmuneCarrier – recovers from symptoms but remains

infectiousDeath

• Time lines of infection

Time

Nondiseased• Dead• Recovered• Immune• Carrier

IncubationPeriod

Symptomatic Period

Time of infection

Susceptible

Development of Symptoms

End of Symptoms

Time Line of Disease

Latent Period

InfectiousPeriod

Noninfectious• Removed• Dead• RecoveredSusceptible

Time Line of Infectiousness

• Time lines of infection

Time

Latent Period

InfectiousPeriod

Noninfectious• Removed• Dead• RecoveredSusceptible

Time Line of Infectiousness

Nondiseased• Dead• Recovered• Immune• Carrier

IncubationPeriod

Symptomatic Period

Time of infection

Susceptible

Development of Symptoms

End of Symptoms

Time Line of Disease

• Time lines of infection

Time

Nondiseased• Dead• Recovered• Immune• Carrier

IncubationPeriod

Symptomatic Period

Time of infection

Susceptible

Development of Symptoms

End of Symptoms

Time Line of Disease

Important to Patient and ClinicianImportant to Patient and Clinician

• Time lines of infection

Time

Latent Period

InfectiousPeriod

Noninfectious• Removed• Dead• RecoveredSusceptible

Time Line of Infectiousness

Important to Infectious Agent and Important to Infectious Agent and Production Medicine VetProduction Medicine Vet

Transmission probability• The probability that, given contact

between an infective source and a susceptible host, successful transfer of the parasite will occur so that the susceptible host becomes infected Depends on:

The characteristics of the infective sourceThe parasiteThe susceptible hostThe type and definition of contact

1. Measurement of disease events2. Infectious disease epidemiology

Transmission probability• The probability that, given contact

between an infective source and a susceptible host, successful transfer of the parasite will occur so that the susceptible host becomes infected Depends on:

The characteristics of the infective source

1. Measurement of disease events2. Infectious disease epidemiology

Another animal Insect vectorInanimate object: water, feed, fomites

Transmission probability• The probability that, given contact

between an infective source and a susceptible host, successful transfer of the parasite will occur so that the susceptible host becomes infected Depends on:

The type and definition of contact

1. Measurement of disease events2. Infectious disease epidemiology

The mode of transmission determines what types of contact are potentially infectious

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 events2. Infectious disease epidemiology

Contact pattern – non-contagious disease• Vertically transmitted diseases

Dam to offspring in utero Vector borne diseases

Snail Biting insects / arthropods

1. Measurement of disease events2. Infectious disease epidemiology

Basic reproductive number (R0)

1. Measurement of disease events2. 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 an infectious agent in a host population

1. Measurement of disease events2. Infectious disease epidemiology

Time

Num

ber o

f ind

ivid

uals

Epidemic, low R0

Susceptible

Immune

InfectedNew Cases

1. Measurement of disease events2. Infectious disease epidemiology

Time

Num

ber o

f ind

ivid

uals

Epidemic, high R0

Susceptible Immune

Infected

New Cases

1. Measurement of disease events2. Infectious disease epidemiology

Time

Num

ber o

f ind

ivid

uals

Endemic, low R0

Susceptible

Immune

InfectedNew Cases

1. Measurement of disease events2. Infectious disease epidemiology

Time

Num

ber o

f ind

ivid

uals

Endemic, high R0

Susceptible

Immune

Infected

New Cases

1. Measurement of disease events2. Infectious disease epidemiology

Epidemic, low R0 Epidemic, high R0

Endemic, low R0 Endemic, high R0