3 . Calculate measures of disease frequency
Now let ’ s look at the frequency of disease compared with what would be expected in a similar group of animals under similar conditions .
There are two main measures of disease frequency and the difference between them relates to the time period involved .
The prevalence of disease is the proportion of individuals within the population at risk that have the disease at a particular point in time . This measure reflects the number of existing cases . Where we are using a mass screening diagnostic test , only one test is used to give us the prevalence of disease .
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Prevalence percentage number of animals with a disease at a particular point in time
= x 100 number of animals in the population at risk at that point in time
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Note . Animals that are vaccinated or previously exposed ( and immune ) are not included in the population at risk
Warning danger zone
Disease incidence is a more challenging area as we are measuring new cases of disease over a period of time . This is different to prevalence that is measuring existing cases at a single point in time . Incidence can be considered a dynamic measure whereas prevalence is a static measure . Incidence can help estimate the risk of disease in the future . For example , if we know that 1 in 10 batches of native oysters will be infected with bonamiasis ( a parasite ), when growing out a crop , the farmer can expect that there will be an approximate 10 % risk of bonamiasis affecting crops in the future .
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