Why is one of my cats suddenly hissing and growling at the other?

If only pets could talk! Cats can be very reactive and can also get frustrated easily. It’s possible they saw another cat out the window and are reacting. You may try to separate them for a few hours. Keep the environment calm and check on them frequently to make sure they are not agitated. You want want to close the blinds to block out any views of the other cat. If the behavior continues, have them examined by the veterinarian. I hope this helps and please feel free to post any additional questions.

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.

Oh my, that is strange that Stan has suddenly become aggressive! A sudden change in behavior from the normal can signal a medial problem. Cats who are sick or are in pain can become aggressive to other animals and their humans. I would have Stan seen by a vet today or first thing tomorrow. The vet can give him a physical exam, blood tests, a urinalysis, and possibly other tests such as an x-ray to diagnose if there is a medical illness causing his aggression. If one is found, the vet can discuss treatment with you. If one is ruled out, I will give you a method to help them get along again. I would separate the cats, and slowly reintroduce them to each other. Take Stan, and put his 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 into Stan'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 Stan 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 Stan in his room when you go out until they are comfortable with each other.

Did Pernell go to the vet recently or leave the house for a period of time? When cats go to the vet or leave the house for a period of time (such as going to a kennel), they can come back smelling differently and that would cause the existing cat to view the returning cat as a stranger. Then that would lead to aggression. Or it could be that Gattino isn't feeling well. If he was in pain that would causing him to react aggressively to Pernell. I'd recommend a check up with the vet for Pernell to make sure everything is ok. I hope this helps!

Those are tough questions to answer without being there to visualize each cat. But I would say generally speaking whichever cat is acting abnormal. Abnormal behavior can mean anything from the cat being lethargic (lacking energy), vocalizing more, drinking more water, or even going to the litter box more frequently. As far as what illness could be causing this, there isn't enough information as of yet as to what symptoms either cat is displaying besides be increased aggression. Do some monitoring of both cats over the next few days to gather clues and then feel free to repost your question with specifics so that we can discuss some possible causes. However, in general we do worry about kidney and urinary infections in cats. Also a change in mood could indicate an endocrine disorder, brain malfunction or even diabetes. I hope this helps and look forward to more information about your fur babies. I do hope we can figure things out soon and get them to being friends again.

Something is probably going on with her, she is possibly ill or in pain and that may be causing her to be aggressive. i would recommend taking her to the vet for a checkup and blood test to make sure she is in good health first.

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.