What happens to a dog bitten by a brown recluse spider?

I'm sure that my dog was bitten by a spider while living in a basement and has been having skin sores for years now that won't heal and in places that she can't reach to lick. she also smells like burning hair. and has other sores that heal up but then come back. I wish I could help her. I have taken her to a dozen vets

and changed her food a lot of times. sometimes it helps for a while sometimes not.

Any ideas?