One of my cats (male tabby short-haired, 14 months) has been suffering from quick shallow breathing for about six weeks. (I also noticed he used to breath quickly when he was a kitten but it passed). When he gets stressed or does even a small amount of exercise he starts panting like a dog. Yesterday we noticed that his legs had started twitching when he rests.
We have taken him to the vet numerous times. They have done a blood test, chest x-ray and ultrasound of his heart (their initial concern) and other organs which all looked OK. They gave him ventolin to try to improve his breathing but it didn't seem to help. They thought he might have allergic bronchitis/asthma and gave him some steroids, but that had no effect either; he seems to be getting worse. He is there now under observation and they're doing another chest x-ray.
His twin sister has also recently started coughing when she purrs. He doesn't, but did cough a bit a few months ago.
Any suggestions on what this could be?