Which is what our calico, Molly, turned out to have. Heartworm is transmitted by mosquitos, but symptoms can take up to six months or more to manifest in cats. Our vet was flabbergasted because cats almost never get it in San Diego -- desert climate and very few mosquitos.
Cats, it turns out, can get heartworm and manifest no symptoms at all, but sometimes die very suddnly from it -- like running around the house one minute and dead the next. That's so unusual that it has been thought until recently that cats did not get heartworm.
Cats have fantastic immune systems, which kills most of the larvae injected by the mosquito. So where a dog will have up to 50 worms living in its heart, which will kill it, a cat will usually have only one or two. That causes the cat to tire easily, and the worm does emit a toxin that can cause breathing issues, but until the owrm dies of old age there is no real problem. Lifespan of heartworm is about one year, and it's the death of the worm that is problematic. It can be ejected from the heart into a lung and acts much like a blod clot.
There is neither preventive medication or treatment of heartworm for cats.
Molly was diagnosed three years ago and is is therefor out of the woods. She is a high speed kitty with no breathing problems at all. As expected, since the odds were highly in her favor. Still, it might be well to get your guy checked for heartworm.
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