We deal with this all.the.time with rescued dogs in the South. I've taken two dogs through fast kill this year, and helped adopters take several fosters through it. My last foster is starting fast kill this week, in fact. It's serious, but manageable.
Here's what I've learned from dealing with this with my foster dogs, and recently adopted dogs -- I'm just reporting what I hear from several different vets, and talking with lots of people in the rescue community, and my own experiences:
1. The dog needs to be limited in activity as long as there are worms -- if you go slow kill, that's a lot of months on exercise restriction. The harder the heart pumps, the more the disease will progress--exertion will make it worsen faster. ABSOLUTELY NO RUNNING OR "ZOOMIES" -- slow walks, not long ones, and easy-going play only.
2. 4 weeks on Doxycycline Hcl. (antibiotic) -- you'll likely need to do a month of this regardless whether you do slow or fast kill. Unfortunately, it's expensive. There's a shortage of this drug for dogs and people both right now, and you'll easily spend $200-$300 for a month's prescription of this generic antibiotic.
This drug cost less than $10 at Walmart Pharmacy just 6 months ago, now it's 20 times that. It's infuriating, but the cheap sources of it have all dried up due to mysterious manufacturing issues. The FDA keeps saying it's temporary, but they've been saying that since January. Brace yourself for sticker shock at the pharmacy -- and if your vet inflates it more, ask for a written RX and price shop it around town (you can check out GoodRX.com for local price listings--it varies a lot from pharmacy to pharmacy).
3. There's slow kill, and then there's ultra-slow kill. It used to be people sometimes used ivermectin (e.g., Heartgard) for slow kill, and it took 18-24 mo. to clear the worms, with a month of Doxycline every three months. I don't know anyone who still does that where I live.
In Louisiana, the vets I know are mostly shifting to using Advantage Multi for slow kill -- I'm hearing it's clearing the worms in just 6 months, with one cycle (month-long) of Doxycycline at the very beginning.
I'm hearing that the Advantage Multi is showing itself to be a way better form of slow kill than ivermectin. I can't point you to any websites or literature -- I can just say that here in the Deep South, vets who are on the "front lines" of heartworm battles are telling me this. If your vet doesn't know about it, have them call a friend who practices in the Gulf states -- or a vet school down here.
4. Fast Kill - if you can afford it, and the dog is Stage 1 (no symptoms) this is generally the preferred protocol for most vets. It's the only "approved" method of treating HW. (Slow kill is not "approved" even though it's quite common.)
The AHS recommends a 3-shot protocol, with Immiticide. There is a 2-shot alternative protocol. AHS claims the 2-shot protocol isn't as effective (I've done it with a dog with a very light load though, and my vet has done it on a lot of dogs, with good success...so I suspect there's a range of views on the 2-shot vs. 3-shot protocol....).
5. The cost of fast-kill varies widely. The wholesale cost of the immiticide is only about $40/vial (I'm told on good authority...). Many vets nevertheless charge over $1000-$1500 for the 3-shot treatment. I know one who does it for about $300 plus whatever the people-pharmacy gets for the Doxy. I know other vets who charge $1500, plus Doxy. Part of the difference is radiographs, and the length of overnight stays in the clinic. My own vet chooses not to do radiographs on asymptomatic dogs (stage 1) if the bloodwork is clean, since the radiographs don't affect the treatment protocol.
Immiticide treatment is hard on the dog (esp. with the severe exercise restriction post-treatment), but the benefit of fast kill is you get the treatment over with, and the worms will be dead and gone, and you can get on with life. This means they won't keep damaging the organs for another 2 years--very important. You also get the dog back off exercise restriction faster. It is not risk-free, though.
Here's how it works:
First, some vets want you to keep the dog on a HW preventative for 3 months before treatment (so that all life stages of HW will be dead after treatment). Since Advantage Multi is killing worms faster than Ivermectin, that's what I'm putting my HW+ foster dogs on for this initial three months.
Second, at least 4 weeks before treatment, you start the Doxycycline (2x day for 4 weeks, in my experience). This is not optional--it's a very important part of the treatment.
Third, they'll do the first immiticide shot. The dog will feel really lousy afterward for a few days and sleep a lot. You'll start the most severe part of the exercise restriction now -- on leash even in the backyard, just walk out to potty, and then back in the house; lots of crate rest and couch-snuggling, no playing at all. We used car rides during this time to break up the monotony of lying around--they can look at the window and feel like they're going somewhere, without getting the heart rate up.
If you do the three-shot protocol, you'll have to wait a month, then there are two more shots 24 hours apart. A few weeks later, the exercise restriction gets lifted, and you can start re-conditioning the dog.
The exercise restriction during fast kill is serious. This is the most dangerous time of the treatment. While the worms are dying, exertion is extremely dangerous for the dog. Clumps of worms can die at once if the heart pumps too hard, sending them into the capillary bed of the lungs. We want the worms to pass through slowly, a little at a time. Thus, no exertion. You must follow your vet's instructions closely if you go this route.
Also, after the first shot, expect to have the dog on prednisone after the treatment. It has a side effect of causing dogs to drink a lot, and pee a lot. Most dogs have accidents in the house, no matter how perfect their house training is -- they just can't help it. The pred helps protect them from complications as worms are dying, though.
I know this is a lot of info. I know many dogs who've gone through treatment successfully and gone on to be very athletic, active dogs. One of mine is now up to running 3 miles a day after his treatment last winter.
Good luck!!!