Why is my cat suddenly hissing at other cats after our trip?

Cats will often do this if they have been separated for a period of time, especially if one cat looks or smells different - your female cat may have strange smells on her from her time outside. Most of the time, this reaction is mild and the cats will get along again after a day or two. If there is any serious fighting or if his aggressive behavior towards her does not seem to be improving, then you may need to separate them and gradually reintroduce them to each other - feel free to ask for additional help on this if needed :)

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!

Sudden behavioral changes can sometimes indicate pain or an underlying illness, so if this came on suddenly a vet visit is always a good first step to rule out any health problems. If your kitty is in good health, looking for something that may have triggered it such as a new object, new people, possession over toys etc may help you understand what caused it. Stress can make cats act uncomfortable or unpredictable, so removing the stressors or making it so your cat has a place to escape and relax may help him feel better. Pheromone diffusers/collars can also be useful in dealing with stress and aggression, and placing one on your cat or plugged into the area where he is being territorial may help him calm down.

Thank you for submitting your question regarding your cats. I suspect that Stella is stressed by the return of her housemate. Callie likely smells and acts a bit different after being gone for so long. I would keep them separate to allow them time to readjust to each other. They should get along fine, but it will take some time. I hope this information helps!

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.

If your cats have spent most of their time outdoors and now are spending time inside together then this change in environment could be causing tensions. If they are still outside all the time then it is possible that a new cat has come into the neighborhood and this has disrupted the social structure of the cats, or potentially one of the cats could be feeling unwell and therefore being more defensive, or they could feel they are having to compete for food/shelter. I would be sure both cats have equal access to food and water (they may need their own separate areas to eat/drink so they don't feel threatened) and litter boxes indoors. When indoors I would recommend the use of the Feliway Multicat diffuser https://www.amazon.com/CEVA-Animal-Health-D89410B-MultiCat/dp/B01AVHPFDS/ref=sr_1_4?ie=UTF8&qid=1503838561&sr=8-4&keywords=feliway+multicat, which is a pheremone product. I would consider an examination by your vet to look for underlying health problems as well.

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.