What is a case ?
Defining a case is an essential step in any outbreak investigation .
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Case = the unit of interest 4 that meets a set of standard criteria for having the disease or outcome of interest .
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The case definition can be broad or more specific depending on what is known about the disease under investigation .
Most importantly the case definition should differentiate the animals affected by the disease under investigation from unaffected animals and those that may be suffering from other conditions ( particularly endemic diseases ) that occur in the population of interest .
The case definition may include the species and age of affected animals , presenting signs , response to treatment or other characteristics . If a laboratory diagnosis is available this should be included in the case definition with as much detail as possible ( including the type of test undertaken ).
Remember the aim is to find the source of the problem whether we have a diagnosis or not .
As more is known about the characteristics of cases , the definition can be tightened and made more specific .
Perfecting the case definition is an art . It is important to balance a broader case definition , which may include animals that are not affected by the disease of interest but may have a similar condition , with becoming more specific and risk not including legitimate cases . The case definition may apply at an individual animal , tank , farm or catchment / water body level .
Photo . Spiny sea urchins ( Photo courtesy of Mooncheese Studio )
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4 Refer to page 43 for the definition .
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