Diagnosing your cat for heartworms, regardless of the fact there is no recommended/successful treatment for it, may help depending on how early you may catch the infection and/or how severe the infection is. They may be able to treat the clinical signs and make your pet more comfortable instead of suffering (again, depending on severity). Felines with heartworms often have a hard time breathing, are lethargic, and possibly anemic with more severe infections, and I can't imagine that would be too comfortable for the animal. So instead of letting them suffer through the clinical signs, you can at least make them more comfortable, which could elongate the lifespan even with the disease.

Heartworm preventative is available through your vet, online, or at pet specialty stores with a prescription only. Heartworm treatment can kill a heartworm positive pet, since it may kill the adult heartworms and microfilaria (heartworm "babies) in circulating blood and they can "clog up" and accumulate in the lungs, which causes death. Therefore it is important to have your pet tested for heartworms prior to administering the preventative

Heartworm is transmitted by mosquitoes, so even indoor pets are susceptible.

EDIT: OMG I sound like a heartworm brochure... I'm to the point now where I'm just regurgitating information lol