Hi, and welcome!
I'm in South Louisiana, and I rescue lots and lots of dogs who test heartworm-positive. I've spent hours reading peer-reviewed literature on the disease. We're in the center of a HW epidemic -- it's all around us, year round. There are break-through infections reportedly happening here (with dogs on prevention testing positive). I say that because I want you to recognize that you live in a special place where what works for northerners who have a real winter, low HW incidence in the natural population, and no ivermectin-resistant mosquitoes isn't what is likely to be best for your
dog. You are in one of the most dangerous places in the United States for HW -- our infection rates top even Florida's.
The ONLY product I personally like for our area is Advantage Multi -- NOTHING ELSE. We beg adopters to keep our dogs on it when they adopt, and not switch to something else. We'd rather they use something else than nothing, but this is the gold standard product we ask them to please ask their vet to prescribe. I switched to it after I moved here because what was safe to use out West isn't the best protection for this area.
The reason I believe A-M is the best protection is that an Auburn University study on HW resistance a few years ago showed A-M is the only product that is still 100% effective against the new ivermectin-resistant strain that's emerging. Every other product had some incidence of failure. This is incredibly scary. The strain is still rare (they think), but no one really knows how fast or slowly it's spreading. Socialize with dog people down here, though, and you'll find some pretty easily who've experienced a break-through infection with a dog kept on prevention.
A-M works very differently from other products because it builds to a steady state in the blood serum after 3-4 months. This means it's killing the larvae constantly at that point, all month long. The other products work by killing whatever larvae built up for the past 30 days when you give the dose, and then the liver filters the drug out, to let the larvae build up again for the next 30 days -- so the drug's only in the dog for a day or so. If it fails to mop up all the larvae in that day or so, whatever survives can create the HW infection.
As a bonus, A-M's flea prevention is effective in our area. That makes it very cost-effective. From KV Supply, it runs about $14/month.
The key is applying it correctly: separate the fur so it all goes on the skin (not the fur). The only major down-side is it stinks pretty badly and you have to keep them dry for a few days.
This interview helps explain the resistance thing:
Interview with a Heartworm Specialist: Part 2 | petMD
This may help too: