My cat is suddenly territorial, hissing in a corner. What to do?

Sudden behavioral changes are always concerning, and checking first for an underlying health issue is best. Cats can suddenly become aggressive if they're feeling unwell or are experiencing pain. If both cats check out healthy, looking for any changes that may be stressing them out such as new objects, new pets, new people, etc may help narrow down the causes of aggression. Making sure that both cats have access to their own food, water, litter, bedding, perches, etc can also help. A pheromone diffuser can also be placed in common/shared rooms to help decrease tensions and inter cat aggression.

It's possible the stress of your being gone caused the behavior change. I would give Hippy a few days to rest, relax, and make sure she can get to her food, litter, places to hide, etc without being bothered and see if that helps. Sometimes, pain or underlying illness can also cause a cat to suddenly lash out and hiss or swat at other cats, so if the behavior continues or you see visible signs of illness, a vet visit to rule out health issues is best.

A sudden behavioral change in cats that previously got along with no other changes to the environment can sometimes indicate issues such as underlying pain or illness. Bringing the cats in for an exam is a good first step to rule out any health issues that could be causing the aggression. If they check out healthy, adding in some pheromone diffusers can help with inter-cat stress. Making sure each cat has their own access to food, water, litter, places to hide, etc can also help decrease tensions.

There could be a reason for Aspyn's change in behavior. Sometimes a sudden change behavior can be a sign of a medical illness. I think it would be a good idea to have her seen by a vet for a health check.

Poor girl! There could be something that has spooked her. Or she could be not feeling well. Or there could even be an internal conflict with another cat in the household. The first thing I would do is make sure she is healthy. I would see if your vet can look at her and make sure she isn’t painful, no infection, no fever, no urinary tract infection, etc. if she isn’t feeling well, this definitely can cause a change in behavior. If everything is normal then I would make sure she is getting along with everyone in the house. Make sure there are plenty of litter boxes as this is something they commonly will become territorial over. You can even put out some feliway diffusers which are pheromones they help calm down cats. Unfortunately these don’t work on everyone. You make also look into vetriscience composure treats as a natural calming supplement. I hope this helps!

Hello Sudden behavior changes in cats can be due to changes in vision (cats can have issues with their retinas particularly), pain, neurological disease like seizures, tumors or infections, and other causes. A thorough exam of Schrodinger's eyes and all his systems could help locate the problem. I recommend having him evaluated by your veterinarian soon as some problems must be caught early for management to be successful.

A sudden change in behavior may have, at its root, injury or infection. Ideally, Rostom should be physically examined by your veterinarian to diagnose and treat any medical causes for his sudden aggression. Neutering him should be strongly considered as testosterone can be a driving force behind aggressive behaviors. Once medical causes have been ruled out, you can address behavioral concerns such as stress or anxiety. A pheromone product, such as Feliway Spray or plug in diffusers, may help decrease any aggression or anxiety. A gradual desensitization to loud noises can also be done to lessen Rostom's response to them.