Cat breathing with open mouth, no change in behaviour, has been to the vet 3 tim

Hello, About 3 weeks ago my cat started breathing with his mouth open. He's not panting or wheezing, but it is very audible, almost like the way dogs breathe. It gets worse during the evening and is not there while he sleeps. It's also the most noticeable when he is smelling something and sounds very hoarse. We took him to a vet 3 times so far - the first time they administered some pain killers which we gave him for 3 days after which he was fine for about a week. Then he started doing it again, and we took him to an emergency vet as his breathing was very loud and we got worried. They confirmed his chest and heart sounded fine and gave him a pain killer for the night until we had our scheduled appointment the next morning. When we took him then, they admitted him for a day, looked at his mouth and throat with an endoscope and did xrays. They confirmed that there was nothing obstructing the airways, that his lungs were clear and there were no apparent issues with his heart. We have him back now and he continues to breathe that way. There's no change in his behaviour, he eats normally, drinks plenty of water, goes to the toilet normally and is active and wants to play. We're worried because it's not something he's done before, and all websites are saying that this is concerning and that he should be checked by a vet. Has anyone experienced this with their cat or observed this behaviour in cats and is there anything we can do to investigate further? Do you know what might be causing it and what further signs should we monitor for to know if he's in danger? Thank you!

Hello - I agree that open mouth breathing is concerning and needs to be checked by a vet.  It sounds that in your case, this has been investigated to such an extent that your vet feels that it isn't an emergency - and it is not progressive (getting worse).  Is this true?  To my mind as a stranger who knows nothing about the case, dyspnoea is an emergency even if it reflects pain, which (although perhaps less distressing than being unable to  breathe in Oxygen), is still a serious welfare concern.  An emergency vet might simply administer fast-acting pain releif if that seems likely to work.  Of course, if this is pain, then within or outside of the emergency situation, the cause may need to be looked into. Perhaps pancreatitis could be an issue, or cystitis (struggling to pass urine is always an emergency).  Some cats mouth-breathe simply when they are stressed.  Another cause is potentially anaemia - ie a lack of Oxygen carrying capacity within the blood, which can make animals pant.  This would not show on an x-ray, but might on a blood test (PCV etc).  The trouble with struggling to breathe is that there is no further sign - the further sign tends to be death.  Therefore, we reccommend taking it seriously every time.