The nose is a specialized skin formation specific to carnivores. In the dog, any discoloration problem must be diagnosed precisely because it can be a simple aesthetic defect that is not very problematic, but it can also be a precursor of a potentially serious cutaneous or generalized disease.
Anatomy of the Nose
The nose is located at the tip of the muzzle, at the edge of the hairy skin. It is a very special area of skin anatomy, characterized by a very thick epidermis (the outermost layer of the skin), resting on a dermis that contains no hair follicles, hence the absence of hair. The sebaceous and sweat glands are absent at this level. Its roles are multiple. In particular, the nose plays a major role in protecting the entrance to the respiratory tract and allows privileged contact with the outside environment due to the relative mobility of the wings of the nasal planum and its rich innervation.
Notes on Discoloration
Discoloration is due to the presence of particular cells called melanocytes, which are cells of nervous origin and which are distributed in various tissues of the body, including the skin. These cells are capable, through complex enzymatic pathways, of synthesizing pigments, which confer color and allow a role in recognition and protection against ultraviolet radiation. Two main types of pigments are found in carnivores: eumelanins (black color) and phaeomelanins (red to brown color). These pigments are transferred to the epidermal cells, the keratinocytes, which allows for a homogeneous pigmentation of the organ. It is therefore understandable that any damage to the melanocytes will have an impact on pigmentation. In addition, any inflammatory damage to the keratinocytes can also cause an abnormality in pigmentation.
By definition, hypopigmentation or depigmentation is an area that is lighter in color than normal, as opposed to hyperpigmentation, which is an area where the color is accentuated.
Primary causes of nose discoloration
There are many causes of nose discoloration. It is, however, important to know them well because, on the one hand, some of them have a genetic origin and are therefore potentially transmissible to the offspring, which will imply a withdrawal of the reproduction of the affected animal, and on the other hand, because it can be a call symptom of a general disease potentially dangerous for the animal.
However, the diagnosis of nose discoloration is sometimes difficult, as different entities may look alike. The most interesting complementary examination for the veterinarian in this location will be the skin biopsy, which requires general anesthesia because of the important innervation and vascularization of this region.
Genetic nose discoloration
These are caused by a genetic anomaly of the pigmentation system. It is possible to distinguish between depigmentation due to an absence of melanocytes and that due to melanocytes being unable to function normally.
In piébaldism, melanocytes are lacking in the nose, as well as specific parts of the coat and eye. As a result, there is a lot of discoloration. Dogs with white coats and pale blue eyes are the most common. Dalmatians and bull terriers are prone, but boxer and collie breeds are also affected. Deafness has been linked to it.
Vitiligo is a circumscribed depigmentation linked to a local destruction of melanocytes. It is a hereditary but not congenital defect, which means that the anomaly is transmitted by the ascendants but is not present at birth. Symptoms usually appear between the ages of 1 and 3 years.
Depigmentation is most often progressive. It appears as a spot, more or less symmetrical, more or less well circumscribed. The nose is often affected, but the mucocutaneous junctions (lips, oral cavity) or skin can also be affected. No inflammation is present: the skin is depigmented but normal. Breeds like Doberman, Rottweiler, Belgian Shepherd, and German Shepherd would be predisposed.
No treatment is available at this time. Due to the lack of pigment, these areas are very prone to developing actinic erythema ("sunburn"), which may require the use of protective sunscreens in sunny weather.
Oculocutaneous albinism is rare in canine species. They associate depigmented lesions of the skin and hair, sometimes of the nose, with ocular lesions.
In the Collie breed, a particular entity called cyclic neutropenia is described, characterized by an associated abnormality of pigmentation and certain blood cells. The coat is depigmented and appears gray. The puppies also present respiratory and digestive symptoms, are stunted and die rapidly within a few months. No treatment is available.
Acquired discoloration
In contrast to genetic nose discoloration, acquired depigmentation is secondary to a disease that causes the destruction of melanocytes and thus an abnormality of pigmentation. Many dermatoses can cause depigmentation, but some are more specific to the nose.
Diseases of unknown origin
Some dogs present discoloration of the nose of unknown origin. Especially in the golden retriever and Labrador retriever breeds, animals may show a progressive decrease in the discoloration of the nose with age. The nose, initially black, becomes more and more chocolate-colored, even completely pink. The cause is unknown, although some link this entity to vitiligo.
In Nordic dogs, but also in other breeds, the nose becomes depigmented in the middle of the nasal planum in winter and spontaneously repigmented in summer. This entity is different from the one described above, where no spontaneous repigmentation is observed. This condition ("snow nose") is also of unknown origin, with no reported treatment. Some authors think that it could be due to hormonal variations, either in the sex hormones or in the pigment hormones.
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Parasitic diseases
Leishmaniasis is caused by the multiplication in the skin and other organs of the Leishmania infantum parasite. These parasites cause an inflammatory reaction and lead to a depigmentation of the nose, which can be an early warning sign of the disease. The depigmented lesions rapidly progress to erosions and ulcerations. Other skin areas are most often affected, and general symptoms are also frequent (depression, renal failure, etc.).
Diagnosis requires a blood test and/or the detection of parasites (by taking a lymph node, bone marrow or skin sample). Treatment is long and sometimes difficult, especially if renal insufficiency is associated.
Some mycoses can cause nasal depigmentation. In particular, aspergillosis is responsible for an initially unilateral depigmentation on the edge of a nostril, which can quickly become unilateral. The fungus (Aspergillus fumigatus) multiplies in the sinus cavities of breeds with long nasal bridges. They cause inflammation and pus discharge, which easily flows out of the nostrils. The pus contains toxic substances which are responsible for the damage of the melanocytes.
Most often, the condition is painful. Diagnosis is made by taking x-rays, blood samples, and, above all, by microscopic examination of the pus and its fungal culture. The treatment, which uses antifungal molecules, is difficult because the molecules used do not penetrate the sinuses well. It is therefore necessary to resort to local treatment under general anesthesia or even to surgical debridement in order to clean the nasal cavities. The prognosis must always be reserved, as relapses are possible.
Other fungi may, more rarely, be involved, such as cryptococci. The condition is then associated with other cutaneous or general signs.
Neoplastic conditions
Skin tumors may initially localize to the nose and appear as depigmentation, possibly associated with erosions. In particular, a tumor of certain cells, the lymphocytes, called mycosis fungoides, has a predilection for localization on the nose. It should therefore always be suspected if depigmentation occurs in an older animal. Usually, there are associated skin lesions, which implies looking carefully at the whole skin surface.
Autoimmune disease
These are rare diseases. They occur most often in adult individuals. The nose is sometimes the first area affected.
Discoid lupus is the most common autoimmune disease with dermatological manifestations in dogs. There is erythema, erosions, scabs, and ulcerations of the nose, which heal and leave depigmented areas. Sometimes depigmentation is the first sign to appear, but always on the basis of inflamed skin. The disease is very clearly photosensitive, which explains why the lesions are initially encountered during the summer months. Diagnosis is made by skin biopsy. Treatment varies according to the extent and severity of the lesions. Local therapies are preferred as they are potentially less toxic than systemic treatments.
Other autoimmune dermatoses often have nasal involvement, but rarely in the exclusive form of depigmentation. These include pemphigus and bullous pemphigoid. Erosive lesions are usually encountered, which may heal and leave hypopigmented areas.
Uveocutaneous syndrome is a rare condition that has been described in Akita inu and Siberian husky. The cutaneous symptoms are always preceded by potentially serious ocular symptoms, as they may be responsible for blindness. It is a bilateral uveitis, with rapid complications of cataract or glaucoma. Skin lesions are characterized by depigmentation of the skin-mucous junctions (lips, eyelids), hair, and nose. The diagnosis requires biopsies. Treatment is difficult because of ocular complications.
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Conclusion
This brief presentation does not aim to be exhaustive, but it allows one to understand that in dermatology things are not always simple, and that one must always keep in mind rare diseases. Depigmentation of the nose is a good example, with sometimes not very serious causes except on an aesthetic level, up to more rare but potentially fatal dermatoses.