2: Measures of Disease Frequency Flashcards
4. Define, calculate and interpret of measures of disease frequency including: a. Prevalence b. Incidence risk c. Incidence rate d. Case fatality rate 5. Select appropriate measures for different contexts 6. Understand the difference between crude and specific disease frequency measures
How can we measure disease occurrence?
Counts
Proportions or Percentages
Ratio
Count Example
- Number of clinical cases
- Number of subclinical cases
- Number of non-cases (healthy animals)
- Number of animals with certain characteristics
- Combinations of 1 to 4
When are counts used
Do they reference the total population
Normally for rare diseases
No
Why are frequencies used to measure disease
to describe how common an illness (or other outcome event) is with reference to the size of the population (population at risk)
to quantify cases in relation to a population and measure of time
3 Main types of disease frequency
Prevalence
Incidence risk
Incidence rate
Define Prevalence
Proportion of animals in a defined population
that have the outcome of interest at a defined
instant [specific point] in time
Prevalence conditions
Measures existing cases
No distinction between old and new cases
Measure of how common disease is
reported as a proportion or percentage
Prevalence examples
In December 2017, a survey was conducted
in Sanza village in Tanzania. 43.5% of children
were found with malaria parasites in their
blood.
Prevalence of chicken anaemia virus in China
is 10.2% (95% CI: 7.6; 13.4%)
134 cattle are tested for tuberculosis on the 1st March 2020
6 are positive
What is the prevalence
6 / 134
= 4.5%
Confidence interval, defined by the upper and
lower limits is generally interpreted as
the range of values within which we expect the true population prevalence to lie with a certain probability
Define incidence risk
(also called cumulative incidence)
Proportion of new cases of disease in the population at-risk over a specified period of time
Incidence risk conditions
Measures the risk or likelihood of an individual animal
contracting the disease
Animals must be examined at least twice during the
study (at beginning and end)
What is incidence risk expressed as
between 0% and 100%
PROPORTION - between 0 and 1
Define incidence rate
New cases of disease in the population at-risk during the study period (person-time)
Why does incidence rate consider that some animals that starts at risk do not remain at risk
What other factors are considered
Not at risk
Develop the disease
Die
Leave the herd (sold or slaughtered)
Join at risk
Birth
Purchase
