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Week 3 - Module 2 (Lec 9) (Pathogen heterogeneity (Between strains…
Week 3 - Module 2 (Lec 9)
Populations
Open = animals added or removed in a period of time
Closed = No animals added or removed
Host heterogeneity
Disease independent
Age, gender
behaviour
Disease dependent
Transmission route
Disease stage
clinical symptoms
Pathogen heterogeneity
Immunogenic variability
different location within host lead to different effects
Between strains
Virulence
Vulnerability to host immune response
comp via cross-immunity
Overall Population
Target Population
Study population
Population at risk
Number of individuals in the target/study
population biologically capable of
experiencing the disease
Measures of disease frequency
reasons for performing disease frequency
Types of Frequency Measures
(Morbidity and mortality)
• Prevalence
• Incidence
• Incidence Risk
• Incidence Rate
• Reproductive number (ratio)
• (Proportional morbidity rates and case
fatality rates)
Methods of measurement
“Static”, snap-shot of the population:
– e.g. did the animal/person have the condition
in the moment of measurement or not?
“Dynamic”, longitudinal view of the
population
– e.g. Did the animal/person get the condition
during the study period
Prevalence
Fraction of diseased in the population at
risk in a given point in time.
Characteristics
Requires knowledge of the population
size
• Useful for comparing prevalence of the
disease between populations of different
sizes
• May not reflect how quickly
animals/people are acquiring the disease
Calculating prevalence
Define the population
• Classify each animal in the population as
either affected or not affected at the point
of examination
Incidence
The number of new occurrences of
disease that occurred in a particular
observational time period
Characteristics
Requires knowledge of the date of each new occurrence
of the disease
• Not useful when comparing populations of different sizes
• Reflects how many animals were classified as acquiring
the disease
• The time period must be specified when calculating and
reporting
Calculating incidence
Select a population of animals unaffected by the disease of
interest at a particular point in time – study start date
• For diseases that can be experienced more than once by the same
animal/person (eg lameness; diarrhea), determine whether the aim
is to count occurrences of the disease or animals/people becoming
affected
• Monitor all animals/people for (new) occurrences of the disease
or animals/people becoming affected for a specified time period
• Calculate either:
– i) the total number of occurrences of the disease or
– ii) the total number of animals/people that became affected at least once during
the time period
Factors for the cycling of incidence
Changes in virulence of the agent
• Fraction of susceptible units in the
population
• Seasonallity of transmission
• Age-dependent mixing
• Stochastic effects
Incidence Risk
The probability of a given animal being a
new case in a particular observational
time period
Characteristics
Requires knowledge of the date of each new occurrence
of the disease
• Useful when comparing populations of different sizes
• Reflects the risk or probability that any one animal will
acquire the disease
• The time period must be specified when calculating and
reporting
• Useful for closed populations
Calculating
Select a population of animals unaffected by the disease of
interest at a particular point in time – study start date
• For diseases that can be experienced more than once by the same
animal (eg lameness; diarrhea), determine whether the aim is to
count occurrences of the disease or animals/people becoming
affected
• Monitor all animals/people for (new) occurrences of the disease
or animals/people becoming affected for a specified time period
• Calculate either:
– i) the fraction of new occurrences of the disease or
– ii) the fraction of animals that became affected at least once during the time
period