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I have a large mixed breed intact male dog that will turn six months next week.

His father is a purebred American Staffy and his mother is a daniff (great dane x Neapolitan Mastiff). He is usually very calm and friendly.

He is currently 29 kg. He has been with us for 2.5 months and lived with our children, 10 yo desexed female dog and 2 cats.

The past couple of days he displayed very eccentric behaviour. He suddenly lashed out at our other dog. Full on fight, not just playing.

Then yesterday he chased my 7yo daughter, then chased one of the cats and bit her.

I put him aside then after a short while he went back to his normal friendly behaviour, receiving treats from my kids and playing around with the cats.

I understand that this can be a testosterone spike or similar due to him changing from a puppy to da dog, but are these signs of concern?

No changes to his diet or exercise or circumstances that I can relate to, that might have cause this.

Any thoughts on how to deal with these sudden behavioural changes and aggression?

Thanks in advance.

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    Welcome to pets.SE! Please call your vet and ask for advice. Sudden changes in behavior most times have some health issue as cause. If your dog sae a vet and the vet stated the dog healthy, then we can try to find answers here. For example could the dog feel pain in certain petted points, or in special movements, so it reacts with surprise and agression "because something hurted it". Commented Apr 11, 2022 at 8:08

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Sudden aggression most often has one of two reasons:

  1. There's a female in heat nearby or
  2. He's in pain.

Less common is

  1. He developed a guarding behavior you didn't notice yet and that was triggered by your daughter and cat.

First you should get him checked by a vet. Please be responsible and put a muzzle on him the whole time you're at the vet. Dogs are predators, so they evolved to hide pain and illnesses, but pain does make them more aggressive. The vet should also look for signs of impaired vision. My own dog has gone blind and he got more aggressive because he couldn't see body language and signs clearly anymore.

If the vet doesn't find anything, you should keep an eye on his behavior. Does he growl and protect his food bowl or a certain toy? Does he chase anyone who enters a certain area or comes too close to anyone? That would be a certain sign of guarding behavior. It's very hard to diagnose and treat over the internet, so I suggest you get help from a professional dog trainer. Not the puppy school type, but someone who trains problematic dogs and comes to your home to analyze the dogs behavior.

If there's a female in heat nearby (and it's exactly the right time for that), this kind of behavior can last several weeks longer. A word of caution, though: neutering a dog is scientifically proven to increase aggressive behavior. I'd rather have a lust-crazy dog than an aggressive one...

For more information about aggression in neutered dogs, please read:

In the meantime I urge you to always put a muzzle on him when he's outside. There are special muzzles designed to be worn all day that allow the dog to eat, drink and pant. As long as you don't know what caused his behavior, you have a gigantic ticking bomb living with small children who tend to run and play and trigger all but the most patient dogs.

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