Can a dog go YEARS without showing symptoms of Heartworm?

My dog got diagnosed with heartworm (microfilaria in her blood) about a week ago and we started the first stage of treatment. Pred to weaken immune system to allow the steroid, Doxy, to kill the worms infected in her blood. On top of Benadryl to keep her sleepy/calm.

I was told from the vet - heartworm is not common in the area I’m living in, and because my now 5 year old dog came from Mississippi 5 years ago, she may have had it since then. There’s a 6 month period where it may not have shown up on tests and I’ve never had her tested for it considering she was tested negative when I got her at a shelter and I’m in an area where it’s rare.

I’m in confusion because my dog has never shown any type of symptoms, EVER, in 5 YEARS with anything. I assumed she was as healthy as can be and I was doing a good job as an owner

Would it be a good idea to get a second test from another vet to get a second opinion? In my head, to make sure I’m not getting scammed for large amounts of money for the second part of treatment, I should. Am I overreacting? Idk if I should worry about this at all lol.

I’m 100% going through with her treatment if needed but I want to know if a second test from another vet is necessary.

Anything would be great to know about this situation/disease as well!