How soon will a cat show signs of heartworm?

That's a really hard question because, as we said, a lot of cats don't even show symptoms, so it can vary from cat to cat. Just keep in mind if you're showing any kind of respiratory signs symptoms, or not eating, lethargy, eating less, bring your cat in and we can examine them.

It takes about six months for a heartworm to fully develop from the larva to an adult stage. After that, the cat can start showing signs at any moment.