Heartworm Positive:
He has heartworms, and badly enough that he actually has baby heartworms swimming in his blood stream
When you give him a heartworm pill, he has a 1:300 (one in three hundred) chance of embolizing dead worms to his brain, lung or kidney.
He has a 299:1 chance of doing fine.
So some people would give him a heartworm pill and cross their fingers.
Then, they'd keep giving him heartworm pills without fail for 12 months.
At that point, (12-18m) any worms over 6 months old right now - are dead from "old age" and they've had no more BABIES to replenish them. He'll be practically heartworm negative.
6 months later (18-24m), the last of the old worms will have died out and he will be TOTALLY heartworm negative.
This can be accelerated with the treatment recommendations at the end of this short paragraph.
PROBLEM: And the reason why people 'treat" heartworms quickly with the Immiticide (a tame cyanide compound) is so that the worms in the heart don't do more damage or embolize.
If he embolizes - it could potentially kill him. I've only seen that ONCE in 25 years of practice. The dog had been actually treated but the owner let the dog run up and down the fence, at high speeds, and it died suddenly. Under that exercise loading, it was guaranteed to embolize and it did.
I've seen a couple embolisms, including my brother David's dog, (who was actually treated) and it threatened him, but didn't die. Anoither embolism just gacked up red foam for a couple days.
Any of these guys, you keep them "pretty calm" while we're doing all this whether you're "treating" or "preventing the worms away". You don't want a flotilla of those bastitches breaking free to the lung or brain.
I put an article on my website about this decision - and some other articles on heartworms.
RECOMMENDATIONS FOR NON-IMMITICIDE THERAPY:
Every 2 weeks: Sentinel dose
On the opposing 2 week: Heartguard.
Once a month: 1 week of Doxycycline.
THIS IS OVERKILL without hurting the dog but it is the FASTEST clearance of damaging adult worms WITHOUT the arsenic shots.
Keep Calm Stress Support
In canines, the three best "happy" compounds you can get over the counter do NOT include CBD, how about that!? Valerian Root, Bacopa and St. John's Wort when found together are DYNAMITE and even help with epilepsy. This is the closest thing I've found and I sometimes have owners give this to anxious pets.