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
Incidence rate conditions
Measures the speed of disease spread
Period of time must be specified
Used to study factors associated with an animal becoming ill
Expressed in terms of:
Animal-year
Animal-month
Animal-week
Animal-day
incidence rate exact calculation requires
date of disease known, exact amount of animal-time contributed by each animal in the population is known
incidence rate exact calculation example
Study involving 25 cows over a 6 month period
• 1 cow followed for 1 month
• 2 cow followed for 2 months
• 2 cow followed for 4 months
• 20 cow followed for 6 months
What is the person time at risk?
Person-time
= (1 x 1) + (2 x 2) + (2 x 4) + (20 x 6)
= 133 cow-months
Person-time at risk approximate calculation
(cases)
/
(animals at risk at start + ½ animals added – ½ animals lost – ½ animals that got sick) x study period
Person-time at risk approximate calculation example
Study involving 25 animals over a 6-month period
25 were all healthy at the start of the study
5 are lost during the study (jumped the fence)
6 got sick during the study
What is animal-time at risk?
= (25 + 0 – 2.5 - 3) x 6 = 19.5 x 6
= 117 animal-months
Prevalence
Numerator
/
Denominator
Existing cases at point in time
/
All at-risk animals including cases and non-cases
Prevalence
Time
How measured
Interpretation
Single point in time
Cross-sectional study
Risk of having disease
Incidence risk
Numerator
/
Denominator
New cases occurring during time period in group free of disease
/
All at-risk animals present at start of period
Incidence risk
Time
How measured
Interpretation
Duration of period of observation
Prospective cohort study
Risk of developing disease in period
