There are many reasons why a dog or puppy would lose hair on its snout, or nose. Unless the dog is of a hairless breed, such as the Chinese Crested, sudden or even gradual hair loss on the nose of a dog is a bad sign. The dog should be taken to a vet to determine if the hair loss is part of a more serious medical condition.
One of the most common causes of facial-hair loss in dogs and puppies is ringworm. Despite its name, ringworm is a treatable fungal infection of the dog's skin. According to Mar Vista Animal Medical Center, ringworm begins as small, round bald patches on the paws, nose and tail area, then spreads across the body. Along with hair loss is crusty or scaly skin.
Several genetic diseases can cause hair loss in a dog, but only a few specifically target the snout. These include sebaceous adenitis (which affects mostly poodles and akitas); vitiligo (which affects mostly rottweilers and Belgian teruvens) and zinc-responsive dermatosis (where the dog has trouble processing zinc). This is usually seen in Siberian huskies and Alaskan malamutes. There is also black-hair-follicle dysplasia, usually seen in papillons, where all of the black hair falls out and never grows back.
Although rare, some kinds of dog parasites attack the face. They cause extreme itchiness so that the dog scratches or rubs off all of the hair on its snout. These parasites include mites that cause demodectic mange (seen more often in puppies than adult dogs) that especially like to live along the mouth, where it is moist. Other parasites that target the mouth and snout include lice and chiggers (another type of mite), according to "Dog Owner's Home Veterinary Handbook."
If the dog has a very short nose or brachycephalic face like a Pekingese, then it cannot clean the wrinkles around its snout. Bacteria set in and cause skin fold pyoderma (hair loss, itchiness, constant wetness and inflamed skin). If a dog is covered in filth constantly because of being confined in a very small cage or in a boarding situation, then the dog may lose hair on the belly, feet and nose because of being constantly exposed to ammonia fumes from urine.
Some dogs seem to lose their hair when they are exposed to sunlight. Although not fully understood, nasal solar dermatitis is seen most often in dogs with collie breeds in them. The hair loss happens right where the wet nose meets the rest of the face. Some dogs not only suffer hair loss, but bleed, according to Race Foster, D.V.M.