Number of cases |
Endemic |
Ascending branch |
Descending branch |
Secondary Peak |
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Figure 9 . Stages of an epidemic curve
Time 4
What influences the appearance of the epidemic curve ?
Rapid rise in the ascending branch :
• Where the transmission is fast and effective , for example a highly infectious disease with a short incubation period coming onto the farm in the water .
• Exposure of a group of animals to a toxin , for example a contaminant affecting a number of fish in the wild within a short period of time ( see point source epidemic on following page ).
Gradual rise in the ascending branch :
• A disease with a long incubation period
• Low level of infectiousness of the disease , for example – a virus that requires direct contact in a dispersed , sessile ( attached ) animal .
• Disease requires an intermediate host , for example whirling disease ( Myxobolus cerebralis ) requires the mud worm to complete the life cycle .
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