I will explain in detail how it is done. If I go into too much detail, let me know.
If there is a positive case on a primary farm, after being tested, the animals are destroyed, because the place has to be disinfected. If the animals that have been in contact with the other farms are not infected, we will lift the quarantine immediately. They can be sent to the abattoir if they are mature, or they will be left to continue to grow.
The same thing applies to the animals that are traced, either upstream or downstream. If the results are negative, the animals are released. If the results are positive, we investigate a second locus of infection. Otherwise, they are released immediately.