That looks like wonderful and tenacious RINGWORM. Get the dog to the vet for a few baths in an antifungal shampoo or treat the spots(and the entire body) with Clotrimizole or Lotrimin cream(found at WalMart).

You will probably get it too if you sleep with the dog(or just come in contact with her or the places she is... Ok, so you're going to get it...), so look out for spots that are similar looking. Get the Lotrimin or Tonalftate cream, that will take care of it. It spreads fairly rapidly if not treated soon enough and can become a systemic infection.

Clean everything and I cannot stress this enough. The fungal spores are wherever the dog has been. So if you have a backyard, I would make up a 5-10% bleach solution and put the bleach in a hose sprayer and douse the yard.

I would treat the cat as well. If you have chlorhexidine shampoo or the solution(for cleaning cages), make up a gallon and bathe them in it. Pour it on and let it dry.

Hope that helps!

PS: What are you treating the animals with for fleas and ticks? Frontline? It could also be a tick bite that has turned into Lyme disease. Has the dog had all of her shots including the Lyme Disease vaccination? Do you see ticks at all?