Is this really an outbreak ?
An outbreak is a series of disease events clustered in time and space . The disease events are usually new cases of a disease occurring at higher frequency than what is normally expected .
There are three ways to measure and describe disease frequency :
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1 . Count the number of cases 2 . Observe the pattern of cases
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3 . Calculate measures of disease frequency
1 . Count the number of cases
Sometimes knowing what is “ normal ” is a challenge in aquatic animals . For example , when the disease of interest occurs endemically , or we don ’ t have much information about the expected level in the population of interest , confirming we have an outbreak can be problematic . It may come down to a judgement call . Other factors such as increased awareness and reporting of a disease may influence the number of new cases being notified .
Knowing what is normal is particularly difficult in wild populations . For example , the parasite Bonamia exitiosa which can cause mortalities on native oyster farms occurs endemically and without symptoms in wild native oysters in Victoria . The background level of infection in the wild through routine surveillance may be somewhere between 5 per cent and 20 per cent varying with season and other factors . If we have a report of 25 per cent infection in the wild population , is that an outbreak ?
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Photo . Beaufort Inlet ( Photo courtesy of Marion Massam , DPIRD WA )
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