Is it a good idea to castrate a fear aggressive dog?

The first thing you MUST do, is get a muzzle for the dog whenever you take him out, to stop him biting another dog. THEN you can start training him & rebuilding his confidence. Start by taking him on walks without any other dogs around, to get him used to the muzzle & used to being on the lead, and walking nicely.

Get him to walk to HEEL nice & calmly. Gradually get him used to different distractions (ie, people, cars, bicycles, etc) passing close by so he learns to ignore them. The dog will slowly learn to trust you as "Pack Leader" and rely more on YOU to judge when it is necessary to become scared or nervous (so make sure you are always confident & calm).

Learn how to give a proper firm 'correction' to the dog (a quick firm tug or jerk on the leash to get his attention). Use a slip-leash (choke-chain) fitted correctly. Only after the dog has learned to trust & respect you 100% as the Pack Leader, should you even think about bringing another dog into the picture.

To use another dog - do NOT use another un-neutered male. Use an older calm placid female. If your dog obeys the SIT/STAY command, put him in the SIT/STAY (facing AWAY from the other dog) as the other dog passes by at a comfortable distance.

With your dog muzzled, find a nice big field and run along at a trot with your dog on the leash. Let the other person run along at a trot beside you with THEIR dog on THEIR leash - about 30ft away (or whatever 'safe' distance'). As you both run along with your dogs, gradually move closer until you are running along together beside each other.

With your dog muzzled - have your dog stand and face AWAY from the other (CALM FRIENDLY) dog and let the other dog sniff your dog's butt. Don't let them do a face-to-face stand-off. If your dog starts to bristle or snarl, give him a quick firm correction to teach him NO.

Use chew-treats to reward him if he behaves nicely and to teach him that "behaving nice with other dogs = a reward". Don't rush each of those steps. Make sure the dog is 100% ok with each stage before going on to the next step.

It may take days or it may take months. Slow & Sure is key.

Depends how the dog acts most of the time. Is he fearful of you/your family? Is he fearful of your other pets, if you have them?

Does he give warning before "freaking out"? Get an expert opinion. Personally, I'd stay away from other dogs until you get an opinion from a behaviourist/trainer.

You don't sound very old, so I'm going to assume it's not entirely safe you taking a large breed aggressive dog out for walks. Maybe take a family member with you on walks.

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