Dogs that chase after other dogs, animals and even people are a liability. Often this behavior is a playful 'practice-pursuit' due to a dog’s instinct to run after and catch anything that moves. You need to stop, or at least be able to control this behavior. Listed below are a few solutions.
Never encourage a dog to chase any animal, such as cats, out of the garden. Unchecked, dogs find chasing a rewarding pastime, but you need to re-train him to think that it is not, otherwise it can lead to unpleasant, even fatal, consequences.
It is important to enlist a professional and experienced trainer to help you remove your dog’s chase tendencies, since it is unlikely you will be able to resolve it alone.
This can be useful in discouraging a 'chaser', but you must be taught how to do this correctly. Find a trainer who is experienced in using discs and other chase-deterrent effective training aids, particularly in serious chase cases.
When this is done gradually it may prove useful, providing there is no risk or stress to other people or animals involved.
The most important factor in re-training a 'chaser' is to keep his attention firmly on you and be sure that he understands and instantly responds to the heel, stay, and recall commands. Maintain his attention with high-value (his favorite) toys and treats.
Do not let a 'chaser' oft the lead in public places, or where there's livestock, until retraining is firmly established and you are absolutely sure you have full control of your dog. Keep him on an extending lead or long line while re-training so that you have control at all times.
Warm Reminder and Advice
Disc-training employs the use of small metal discs on a string that rattle if shaken or thrown close to the dog to discourage undesirable behavior(s), but it should only be done by someone who fully understands how and when to use them and back them up with appropriate handling and training, otherwise it won't work and more harm than good can be achieved it used incorrectly.