Stray dog

Hello

Last Friday a stray dog turned up at our house very emaciated and its not an exaggeration to say she was close to death. We took her straight away to the vets in Baza and all of us were surprised that she was microchipped. The phone and address was Huesa a long way from us impossible for her to have walked it. They have been trying to call the owner for four days now with no answer so we are thinking maybe the owner died and the children abandoned her, we get that a lot in Fontanar. Anyway four days later with a lot of care she is like a new dog and we love her she is nine years old very calm called Luna. The vets have told us to go the the Guadia civil to denounce and change ownership, we are taking our passport and residencia, is there anything else we need to take and does anyone no the process ?