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My male cat attacks his sick sister. How do I stop the aggression?

Updated On September 23rd, 2025

Pet's info: Cat | Domestic Shorthair | Male | neutered | 8 years and 1 month old | 12 lbs

we have two sibling cats. they are 8. the female is being treated for lymphoma. the boy cat has suddenly become very aggressive to her. he'll often hear her jump off the bed in the other room and tear after her full speed. she runs under the bed and he'll attack her. there is much hissing growling, screaming. he'll stand guard and keep her from leaving there. how can i protect our poor girl kitty and get him to stop attacking her

2 Answers

Most Helpful Answer

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Answered By Dr. Melanie, BVSc MS

Veterinarian

Published on August 14th, 2020

If the female is being treated for lymphoma, she will be smelling differently when she returns home from treatment, and Mochi will see her as a stranger, leading to the aggression. I would separate the cats, and slowly reintroduce them to each other. Take the cat who is aggressive, and put him in his own room with his own food, water, and litter box. Keep the door closed, and for a few days, feed them both treats on other side of door so they associate each other's smell with a good experience. Then let the other cat in the aggressive cat's room while the aggressive cat is in a cat carrier. They can smell each other, but not be in contact. If either becomes upset or show signs of aggression, remove the second cat from the room, and try again later. Praise them when they interact appropriately. When they are interacting well with each other through the carrier, you can let the aggressive cat out of his room, and see how he interacts with the other cat. Praise them when they interact appropriately, and separate them with any sign of aggression or upset. I would get a cat tree with lots of shelves to give the cats vertical space to get away from each other. I would keep the cat in his room when you go out until they are comfortable with each other. I hope this helps!

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    Answered By Dr. Melanie, BVSc MS

    Veterinarian

    Published on March 17th, 2019

    It sounds like Moose really did not like this kitten, and is now showing redirected aggression to your other cat. I would separate the cats, and slowly reintroduce them to each other. Take Moose, and put him in his own room with his own food, water, and litter box. Keep the door closed, and for a few days, feed them both treats on other side of door so they associate each other's smell with a good experience. Then let the other cat in Moose's room while he is in a cat carrier. They can smell each other, but not be in contact. If either becomes upset or show signs of aggression, remove the second cat from the room, and try again later. Praise them when they interact appropriately. When they are interacting well with each other through the carrier, you can let Moose out of his room, and see how he interacts with the other cat. Praise them when they interact appropriately, and separate them with any sign of aggression or upset. I would get a cat tree with lots of shelves to give the cats vertical space to get away from each other. I would keep Moose in his room when you go out until they are comfortable with each other. I hope this helps!

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    3Pet Parents found this answer helpful

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