As for the distribution of lesions, Demodex can be anywhere and everywhere. And it usually is!
have a pattern of distribution (ear tips, elbows, back of legs), but not always. I would never rule out a case based on where the lesions originated from.
In my patients, I have seen it be in places I would have never suspected. Some infections just don't read the book, I'm sorry to say!
Also worth noting, Demodex is usually treated with daily doses of oral ivermectin (it is an off labeled use and at a dose that is much
higher that that of Heartgard, so you absolutely do not want to use it unless directed by a veterinarian
) and Sarcoptes is usually treated with injections under the skin once every 2 weeks (also at a much higher dose than what is found in Heartgard). I had an owner who tried to treat Demodex topically, and the dog's infection ended up becoming resistant to ivermectin.
That only left medications like Interceptor, and that is ridiculously costly.
No matter what you decide to do, please do not start ivermectin without being sure that your dog is negative for heartworms.
What can happen is that if there is a mature heartworm infection that is unapparent (no outward signs like coughing, exercise intolerance, etc), and you start giving invermectin in any form, it can kill some of the worms and cause a clot to form. It can be deadly. It is rare, but it has happened.
Anyway, I would bathe the dog in the shampoo 3-4 times per week for 2-3 weeks. If you aren't seeing an improvement or at least a step in the right direction in about 10 days, I'd have that appointment made.
I hope everything works out!
Let us know how it goes!