From canis major:
Some dogs experience loss of pigment on the nose leather or blistering of the nose. Collies and related breeds can get a weepy, crusty dermatitis on the nose if they spend too much time in the sun. Treatment for Collie nose involves keeping the dog out of the sun and treating the ulcerated area with a steroid preparation. Once the nose is healed, it can be blackened with tattoo ink to protect it from sunlight.
Other causes of loss of nose color are vitaligo, snow nose, and plastic dish nasal dermatitis
Vitaligo causes black pigment in the nose and sometimes the lips to fade to brown.
Snow nose causes the nose to fade to brown in winter; normal color returns as summer approaches. Snow nose occurs mainly in white-coated breeds; the color change can become permanent in older dogs. It is not associated with disease.
Plastic dish dermatitis can occur if the dog eats or drinks out of plastic or rubber bowls. It is triggered by a reaction to an antioxidant found in the dishes.
And from a site on OES:
Some dogs and cats will occasionally lose pigment from their noses, causing this feature to turn white or red. In most cases, this isn't a health problem, but it looks unusual, to say the least. And for folks who show their pets, a fading nose can keep a pet out of competition until the normal black color returns -- if, in fact, it ever does.
The most common cause of a fading nose is "snow nose," says Grant Nisson, D.V.M., a veterinarian in private practice in West River, Maryland. Vets aren't sure why, but many breeds of dogs will lose pigment from their noses during the cold months. (This rarely occurs in cats.) People once thought that snow nose was caused by bright sunlight reflecting off snow and bleaching the nose white -- or by a combination of cold and trauma, since dogs often use their noses as miniature snow shovels. Vets have found, however, that even dogs living in warm, southern climates may get snow nose, so weather doesn't appear to be a factor.
There is no proven way to prevent snow nose, although some breeders swear that giving pets vitamin E and kelp will help restore the color. (Your vet can recommend safe amounts.) Vets sometimes advise getting rid of plastic food bowls and replacing them with metal or ceramic bowls since some pets may be allergic to plastic. Finally, your vet may suggest a thyroid test be done. There is no evidence to prove that it is true, but some vets believe that low thyroid levels can cause the nose to lose its color.
Snow nose isn't the only condition that can cause the nose to fade, although it is the only one in which the color eventually comes back. When a nose goes pale and stays that way, your pet may have vitiligo, a condition in which skin cells lose some of their melanin, or pigment. Pets with vitiligo may turn white on the paws, lips, and fur as well. Vitiligo appears to be a hereditary problem, affecting Doberman pinschers and Rottweilers more than other breeds. It will keep your pet out of the show ring but is otherwise harmless, says Dr. Nisson.
A more serious condition that can cause the nose to fade is Vogt-Koyanagi-Harada (VKH), or Harada's, syndrome, which occurs only in dogs. Vets suspect that it is caused by an immune system disorder that damages the eyes and the pigment in the skin. It can turn any part of your dog's body white, and without treatment, it can lead to blindness, says Dr. Nisson. Vets often prescribe steroids to pets with this condition, which help keep the immune system from going out of control.
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