I have what I think might be a very good controlled experiment since I have two dogs. They are both Great Pyrenees. One is "mine" a female 7 years old and has a thick impermeable healthy white coat.
The other is a rescue dog, about the same age. He has lived with the female for about 3-4 years. He's steadily lost hair and his flesh is a combination of pink and black skin underneath. He still has full hair around his face and chest/shoulders. But it's VERY thin back of that, and almost gone on his belly. He also has "tags" of flesh on his stomach area. And, he's skinny, and doesn't smell good even when he's bathed.
He's got access to as much food as he wants (dry) but has stayed skinny.
Obviously, it's not contagious as Wendy hasn't gotten this at all.
I am sorry I haven't put a lot of effort into him but have some money to at least try something. Is it possible this is worms? If so what type, and what type of medicine should I try? Honestly, I don't have the budget for the 150-300 that a vet would charge, but am willing to spend 30-40 on an OTC or internet-order treatment. Thanks.
Oh, one more thing, he's not extremely active, but no obvious sign of lethargy. Pretty quiet though.