Chronic itching in dogs is commonly caused by fleas or dermatitis, according to WebMD. In dogs, dermatitis may also appear as the consequence of a flea allergy.
Signs of flea infestation in dogs include flea droppings or eggs in a dog's coat, excessive scratching and biting of the skin, explains WebMD. It can also cause hair loss, scabs, hot spots or pale gums. Flea infestation is more common in areas with a humid climate.
Signs of dermatitis in dogs include excessive scratching, hair loss, scabs and scaly patches, notes WebMD. The condition can also be accompanied by an ear canal infection or bacterial skin infection. Early signs include sneezing, face-rubbing, a runny nose, watery eyes and licking of the paws.