Your best bet is to contact the vet and have a scraping done. If Marley has ringworm, say, or some other skin condition other than demodex, ivermectin will not work and may put her at risk (some dogs react really badly to ivermectin). Best to find out exactly what you're dealing with and go from there--and that means another trip to the vet...
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."