Why is my mellow cat growling and hissing? Could it be fleas?

I'm sorry you are facing this frustrating problem. I would recommend that you take the one who is suddenly being "grumpy" to the vet. Sometimes if they are painful or not feeling well, they will become more irritable. You can try temporarily separating them again, or separating them for at least part of the day so that the "victim" has a "safe zone" or some time where he or she feels safe. But the sudden onset of the change in behavior does concern me that something may be going on with the one who is acting more aggressive. Best of luck, thank you for asking Petco Pet Education Center, formerly Petcoach.

I would recommend a full vet check to identify any medical causes for his altered behavior, discomfort and possibly over grooming. I would be concerned about a UTI or cystitis causing pain and a urine sample may be required to investigate further

Okie is gorgeous! Having a physical exam and screening bloodwork and a fecal exam and urinalysis to rule out a medical cause for his crankiness such as an ear infection, diabetes, kidney/liver issues, hyperthyroidism etc. would he best. Using a Feliway mulicat calming pheromone plug -in several rooms in your home along with daily VetriScience Composure chews or oral liquid may help significantly with stress related aggression /inappetance. Both of these products are available without a prescription on chewy.com and PetCo.com. Good luck!

I would have Nurmal (love her name!) checked out by a vet. Sudden changes in behavior in a senior cat can point to medical problems such as hyperthyroidism and arthritis (pain may make her cranky). I recommend getting a physical exam, geriatric blood panel, and urinalysis. These tests will help the vet identify if she is suffering from a medical illness. Should one be found, the vet can discuss treatment options with you. I would take her into the vet on Monday if possible. Good luck, I hope this helps!

Vague behavior changes like you are describing can be the first sign of illness in an older cat, so if you feel something is wrong I would encourage you to see your veterinarian again. Blood work might be recommended to rule out kidney or liver disease. The vomiting could be a sign of inflammatory bowel disease or even an intestinal mass and if the blood work is normal an abdominal ultrasound might be recommended. Finally, the vocalizing and hissing could be a sign of pain from something like arthritis and there are some supplements and now pain medications that can be used in cats.

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.

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.