This document discusses measures of disease occurrence, specifically prevalence and incidence rate. Prevalence is defined as the number of animals with the disease of interest at one point in time divided by the total population at risk. Incidence rate measures the average speed at which the disease is spreading by dividing the total new cases during a time period by the average number of animals at risk multiplied by the time period. While related, prevalence and incidence rate can differ based on the duration of the disease - a short but highly incident disease will have low prevalence, while a long but less incident disease can have high prevalence.

1. WELCOME
TO
OUR PRESENTATION

2. Measures of disease occurrence:
Prevalence
Incidence rate

3. Prevalence
Prevalence (sometimes called point prevalence) is a measure
of the number of animals with the disease of interest at one
point in time, as a proportion of the total number of animals
in the population.

4. P𝑟𝑒𝑣𝑎𝑙𝑒𝑛𝑐𝑒 =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑠𝑒𝑠 𝑎𝑡 𝑜𝑛𝑒 𝑝𝑜𝑖𝑛𝑡 𝑖𝑛 𝑡𝑖𝑚𝑒
𝑃𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑎𝑡 𝑟𝑖𝑠𝑘 𝑎𝑡 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑝𝑜𝑖𝑛𝑡 𝑖𝑛 𝑡𝑖𝑚𝑒

5. Incidence rate
Incidence rate (specifically true incidence or incidence density rate) is a
measure of the average speed at which the disease is spreading.
Incidence rate is the total number of new cases of disease divided by the
total time that each animal in the population was at risk of getting the
disease.

6. For simplicity this can usually be calculated as:
𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑟𝑎𝑡𝑒 =
𝑇𝑜𝑡𝑎𝑙 𝑛𝑒𝑤 𝑐𝑎𝑠𝑒𝑠 𝑜𝑓 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑑𝑢𝑟𝑖𝑛𝑔 𝑎 𝑝𝑒𝑟𝑖𝑜𝑑 𝑜𝑓 𝑡𝑖𝑚𝑒
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑛𝑖𝑚𝑎𝑙𝑠 𝑎𝑡 𝑟𝑖𝑠𝑘 × 𝑇𝑖𝑚𝑒 𝑝𝑒𝑟𝑖𝑜𝑑

7. Prevalence versus incidence rate
 The two are related, according to the duration of disease.
 A disease with a high incidence rate but of very short duration will
have a relatively low prevalence.
 A disease of relatively low incidence rate with a long duration will
have a high prevalence.

8. THANK YOU ALL