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Cats can have difficulty with thier breathing for several different reasons. It is a general symptom and a "red flag" that your cat needs veterinary attention soon. Since you are dealing with a cat that is aging, you need to locate another vet that will address your cats issues and dietary needs as he ages.
Since you have already seen one vet, I would suggest locating another vet to get another opinon regarding the breathing.
It is sometimes normal for cats to have labored, heavy breathing - but only after exercise. Many cats have this happen to them - they will play for a short period quite intensely and then have trouble catching their breath afterward. If your cat does this, don't work them so hard. You should take breaks once your cat starts to seem like it's running out of breath. One of my cats used to have this problem - he would get way too into the fun and games, and he wouldn't ever stop doing it on his own. You have to be responsible as an owner in this situation, and force them to stop for at least 5-10 minutes.
There are a ton of different diseases, parasites, or problems that can cause cats to have breathing problems - these can include wheezing, heavy breathing, shallow breathing, breathing through the mouth instead of the nose, rapid breathing, or lethargic breathing.
Your vet is going to need to tell you what is causing this. Any sudden changes in the way the cat breathes are an obvious sign of something wrong. But here are a few of the big causes of it:
Cat Asthma - This is a chronic, allergic condition - rarely fatal, but you'll have to make changes to your cat's lifestyle.
Cat Upper Respiratory Infections - A number of diseases cause this, but they all affect the respiratory system, and all can cause breathing problems.
Parasites such as Cat Heartworms and Cat Roundworms - Worms in cats can cause them to cough and have trouble breathing as the worms pass through their system. The life cycle of worms often includes a stop through the lungs of the cat.
Allergies. Cats can have them too, so something in your house might have started causing this.
Injury of some kind. Broken ribs or bones, wounds, etc. can all cause problems with the cat that could lead to breathing issues.
I hope this helps. If you need further information, please feel free to contact me.
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