Are you seeing spots on your dog? Is your cat’s nose turning white? You may not be imagining it. Cats’ and dogs’ skin color can change, and it may get lighter or darker. Is it a problem? Fortunately, it usually isn’t. With luck, this discussion will shed some light on the subject, but first, we should start with a clarification of terms.
There are several terms that describe the loss or gain of pigment (see sidebar, page 50 ). We use the terms vitiligo and lentigo to define syndromes that are associated with pigment change. In terms of color, they are opposite, and many of us have heard of them, but have trouble remembering which is which. For the record, vitiligo describes a condition where pigment is lost (white or pink patches), and lentigo describes a condition where pigment is gained (black spots). Both cats and dogs can have either, but their origin and presentation differ slightly between species.
In both dogs and cats vitiligo results when the skin’s melanocytes (pigment-producing cells) are damaged. The precise mechanism isn’t completely understood, but we believe most of the time it results from the immune system targeting and destroying them. Interestingly, although the impact may be immune-mediated, it is not inflammatory, painful, or itchy. In both species it is likely an inherited genetic anomaly that becomes first apparent in young adulthood. In dogs (especially Belgian Tervurens, Rottweilers, and Dobermans), and some cats (most often Siamese) depigmentation begins around the eyes and nose, then expands over several months to cover much of the face and sometimes other parts of the body. Other symptoms are unlikely, but the cosmetic changes rarely go away. A less common, more diffuse form of vitiligo sometimes appears in cats that creates variable patterns over the body of leukotrichia (white hair). An internet search can reveal photos of some of these unusual looking cats.
Another curious form of vitiligo can develop in dogs, where pigment disappears from skin that has been traumatized. Weeks after recovering from a skin injury, for instance, people may notice that their dog’s hair has grown back white in patches. Some researchers have speculated that damage to nerves or nerve endings plays a role in decreasing the function of melanocytes in these cases, but the exact cause of the change remains unknown. Again, the effect is only cosmetic, but unfortunately is often permanent.
Lentigo is the form of pigment change more of us are likely to encounter. Unlike vitiligo, it does not seem to involve any type of immune activity or trauma. In dogs, lentigo probably has some genetic basis, since some breeds (e.g., Dachshunds) seem more prone to it, but the origin is not fully understood. Regardless of the breed, it tends to develop after maturity, in clusters or as individual black spots (lentigines) in the skin, often on the abdomen, but potentially anywhere. Vizslas and Weimeraners have a tendency to develop them around the nose. The spots are likely to remain indefinitely, but are not likely to grow, although other spots may appear nearby. As with vitiligo, the effect is only cosmetic.
Cats have a particularly interesting form of lentigo that appears only in individuals with orange coat color (see Sidebar). Orange cat lentigines begin to form in early adulthood in the skin of the lips and eyelids, and occasionally on the ears or foot pads—essentially anywhere haired skin joins non-haired skin. At first they appear to be tiny freckles that then grow and increase in number over the life of the cat. As an example, I once knew a 26 year-old orange tabby named Ross who had so many lentigines that his lips and eyelids were nearly completely black. It was a dramatic look.
No one knows for sure why this happens, but it is the only form of lentigo we see in cats, and it only occurs in orange ones, or in the orange haired areas of tortoiseshell or calico cats. One interesting theory suggests that the skin cells of the affected areas are prone to a mutation that causes a reversion back to the ancestral black color. It was mutation of the same gene that created the orange coat color in the first place. Because the change occurs at a cellular level, lentigines start microscopically small and only grow as the altered cell replicates itself. Fortunately, the change has no medical consequences, and it provides a helpful indicator when estimating the age of orange cats.
It’s natural for pet owners to become concerned about spots (light or dark) developing on the skin, since many of us have familiarity with melanoma and other types of skin cancer. It is important to remember that vitiligo and lentigo reflect only changes in skin color. The tissue structure and appearance are otherwise the same; therefore, any evidence of thickening, bleeding, or crusting may signal another condition that warrants investigation. Check with a veterinarian if you are uncertain. A biopsy of the skin is usually the simplest way to learn if an underlying problem exists.
VCA Rancho Mirage Animal Hospital located at 71-075 Highway 111, Rancho Mirage, CA. 760-346-6103. Visit www.vcaranchomirage.com