We just found out yesterday when we took him in for a heartworm test and he is positive

He is only 18 months old and possibly had the heartworm from when we rescued him. I don't know what stage yet, the vet is sending out another blood test to determine that. Good thing we can get care credit.com to help pay this $1,200 bill without interest for one year. btw he never showed any symptoms except that he sleeps alot during the day.
We adopted our dog knowing he had heartworms and it was hard and expensive complicated by the fact that he is a Jack Russell that loves to (and has to) run every day. He was about 2 when we adopted him. Mine never showed symptoms either.
His heartworms were quite extensive so he stayed at the vet for 2 days and received injections in his spine (if I am remembering correctly). He tolerated that fine but was lethargic for a few days after. He was crated 24/7 (except for bathroom breaks) for 4 weeks. Had another xray at that point and still many, many heartworms so crated for 4 more weeks. He was going stir-crazy at this point so towards the end of the second four weeks, I blocked him in the family room and let him out of the crate so he could move a little. I wouldn't recommend doing this but he was truly going crazy.
After 8 weeks, had another xray, still had heartworms but not nearly as bad. At that point, the vet and I made a decision to not crate him any more. I really think he would have gone mad or died if we kept him in it any longer. We decided it was time to be a dog and I would rather him have some fun and if it was his time to go, then it was his time.
Good news is, he's been great ever since. He doesn't have as much stamina as other Jacks and he may not live as long as other Jacks but he has had NO problems related to the heartworms. His heartworm tests have been clear since.
I'm affectionately (or maybe not

) known as the "heartworm nazi" at the dog park. I cannot stress enough for everyone to give their dog heartworm pills every month.
I hope that your pup has a very mild case and it's a quick fix.
Good luck and let us know how it goes!