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Backward ( source ) tracing will provide information regarding the potential introduction of disease agents onto the farm . All movements onto a farm do not represent the same risk of disease transfer . Live animals are generally considered the highest risk for transfer of infectious diseases and should be prioritised in the tracing process . Figure 7 illustrates a range of sources of infection both indirect and direct .
Forward ( spread ) tracing focuses on the potential spread of disease to other parts of the farm , other farms and the wild ( where the disease is not already present in the wild ).
In the wild Undertaking tracing in the wild is much more difficult than on a farm . Many different stakeholders may need to be contacted – researchers , government , industry bodies , etc . The source of an infectious agent in a wild aquatic animal population may include movement of stock into a fishery for rebuilding purposes , live / dead bait use and infected ballast water from cargo ships . Forward spread is often unpredictable and will be influenced by factors such as ocean currents , supply chain arrangements , movements of infected animals and equipment .
When did the disease enter the population ?
If the incubation period is known for a disease agent , a tracing window can be constructed and movements during this time intensively investigated .
Incubation period is the period of time from exposure to the infectious agent through to when clinical signs are seen .
A tracing window refers to the most likely period of time during which the disease could have been introduced to an area ( tracing window for source ), or the most likely period of time during which the disease may have spread to another area ( tracing window for spread ). Figure 8 illustrates the tracing window for source and spread relative to the incubation period of a disease .
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