The risk is not very significant. Heartworms aren't typically found in cats. Cats are not the normal host for heartworms, but they can get them. However, it can be fatal for them.

Indoor cats typically have less likelihood of coming in contact with mosquitoes. In places like Florida, there can still be a few mosquitoes indoors sometimes, but the risk is greater outdoors due to increased contact with mosquitoes.

Heartworms are transmitted through mosquitoes. A mosquito will take a blood meal from a heartworm positive dog who's shedding their baby heartworms, this is called microfilaria. The mosquito will then ingest that, and later take a blood meal from your cat, injecting some of that blood with the microfilaria.

An internal parasite is a parasite that resides inside the body.

Feline heartworms are caused by a heartworm parasite.

There's not really a treatment for heartworms in cats.