If Untreated, Can Fleas Harm My Dog?

Yes they can. My friend had a cat she wouldn't take it to the vet until the cat was vomiiting wouldn't eat and when she took him the vet told to use the fles stuff and the cat passed away cause she waited to long. Please take him/her to the vet. Good luck

Yes as your dog will continue to scratch and itch as the fleas will multiple and eventually your dog will have open, infected and oozing sores that will subject it to more disease and infections.