Dermatology and the aging dog

Geriatric dermatology – An introduction

Skin is an amazing organ, continuously battling the elements and protecting vital structures, but as dogs age, their skin and hair coat age as well. Callus formation, loss of elasticity, skin wrinkling, and changes in hair color and texture can be seen. The immune system ages with the dog, and older animals are more susceptible to skin infections and may have difficulty keeping their resident Demodex mite populations in check. Additionally, some systemic diseases are more common in the geriatric dog, many of which have cutaneous manifestations – for example, hypothyroidism and hyperadrenocorticism can be most evident at times through their skin changes. Metabolic conditions, such as superficial necrolytic dermatitis, also occur more often in the aging population. Finally, geriatric canines are more likely to develop cutaneous masses – both benign and malignant tumors – than their younger counterparts. This paper offers an overview of dermatology in the geriatric dog.

General changes to the skin and hair coat

Senile changes occur at various ages in our pet population – typically later in life for small breed dogs, and earlier for larger ones. Genetic factors, breed differences, nutritional support and many environmental factors can all influence the onset of aging changes. Senescence is a normal process that cannot be avoided; as dogs get older, their ability to rejuvenate their cellular population wanes, there is increased cellular atrophy in most structures, and a variety of changes at the microscopic and ultrastructural levels can be observed.

The epidermis may thin, especially with extreme age, poor nutrition, and certain endocrinopathies that are more common in older animals. Some dogs develop skin hyperpigmentation with age, which cannot be attributed to more common causes (sun exposure, trauma, endocrinopathies) 1. Epidermal collagen fibers show increased cross-linkage, and their bundles appear more fragmented. Elastin fibers lose elasticity and show increased calcium and pseudoelastin in their fibers; fragmentation may also be seen 2. Arrector pili muscles can appear fragmented, vacuolated, and sometimes hard to find. Some older literature describes cystic dilation of epitrichial sweat glands, and even atrophy of sebaceous glands 1. Blood vessels and lymphatics do not typically change in appearance with age. Perifollicular mineralization in the dermis is a reported change in older poodles 3,4.

Commonly visible changes to aging skin and hair coat, along with potential causes, include 2,5,6:

• Graying of the hairs (Figure 1): atrophy of pigment cells and reduced tyrosinase activity
• Dull coat appearance: changes in sebum production
• Thinner hair coat: decreased follicular activity
• Skin wrinkling: tissue dehydration, loss of skin elasticity, possible epidermal atrophy
• Callus formation: prolonged pressure exposure and hyperkeratosis
• Paw pad hyperkeratosis (Figure 2): aging hyperkeratosis as well as hyperkeratosis secondary to abnormal wear of the paw pads, and also gait changes secondary to osteoarthritis

In terms of intervention, there is no prevention for aging changes to the skin. However, as exposure to the sun can hasten these changes, reducing exposure and utilizing sunscreens and physical sun barriers, especially in sparsely haired and lightly pigmented dogs, may be of benefit. Moisturizing the coat, nasal planum and paw pads can combat dryness, whilst padded bedding can minimize pressure on bony prominences and may decrease callus formation.

Endocrine and metabolic conditions

Common endocrinopathies, such as hypothyroidism and hyperadrenocorticism, significantly affect the skin and hair coat and tend to occur in middle-aged and older animals. Owners often notice the cutaneous changes, which may be the only reason the animal presents to the veterinarian, although systemic signs are also likely to develop. Another metabolic condition, hepatocutaneous syndrome (HCS) or superficial necrolytic dermatitis (SND), also occurs more commonly in older animals.

In canine hypothyroidism, common cutaneous manifestations include a thinning hair coat, especially at frictional areas, but also including the tail (Figure 3) and bridge of the nose; a predisposition to bacterial folliculitis; hyperpigmentation; scale; and myxedema in more advanced cases. Hairs need thyroxine (T₄) to enter into anagen. Hypothyroid animals have more telogenized hairs that may bleach and lighten in color with repeated sun or chemical exposure as new hairs are no longer cycling in. Additionally, telogen hairs can epilate with time, and hypotrichosis and alopecia may be observed without new hairs cycling. Receptors for thyroid hormones are also found on sebocytes, so a lack of hormone could lead to sebaceous gland atrophy. Changes in T₄ levels can affect fatty acid concentrations in the skin, which in turn can lead to a keratinization defect, resulting in widespread scale and a dull and dry coat appearance. These changes also make the animal more susceptible to bacterial and Malassezia infections as the skin loses some protective barrier function. Hyperpigmentation is a non-specific sign resulting from chronicity and can be seen in various hormonal and inflammatory conditions. Hypothyroid dogs can accumulate hyaluronic acid in the skin, causing a thickness of the skin and the classic “tragic face” appearance of myxedema 7.

Non-cutaneous signs of hypothyroidism include weight gain, lethargy, mental dullness, and heat-seeking behavior 8. Diagnosis is confirmed via blood testing, but remember that many things can interfere with the total T₄ levels, so care should be taken in interpreting this test result in patients with other illnesses or those taking certain medications such as glucocorticoids, sulfonamide antibiotics, phenobarbital, clomipramine, and non-steroidal anti-inflammatory drugs (NSAIDs). Interpreting a complete thyroid panel, including total T₄, free T₄, and thyroid stimulating hormone (TSH) +/- antithyroid antibodies, may provide a more accurate diagnosis. Treatment of canine hypothyroidism is with oral levothyroxine and is generally well-tolerated.

Hyperadrenocorticism in the dog can lead to hair loss, especially symmetrically on the flank. The skin may be thinned and hypotonic. Comedones and milia are common, and phlebectasias may be present 9. Secondary skin infections are more common in animals with hyperadrenocorticism. Calcinosis cutis is an occasional sequela (Figure 4). In this condition, adrenal glands overproduce cortisol, either due to a primary adrenal tumor or, more commonly, as the result of adrenal hyperplasia influenced by a pituitary tumor. This excessive cortisol significantly affects the hair follicles and sebaceous glands, leading to atrophy – the consequence of which is hyperkeratosis (excessive scale) and hypotrichosis-to-alopecia. Hair follicles are also filled with hyperkeratosis, leading to the comedone and milia formation. Easy bruising occurs due to weakened blood vessels from the cortisol influence, and phlebectasias result from vessel dilatation. Striae or stretch marks can be seen secondary to the thinned skin and poor healing, which results in this scarred pattern 9,10.

Non-cutaneous signs of hyperadrenocorticism include polydipsia and polyuria, as well as polyphagia, excess panting, and exercise intolerance. Affected animals may have a classic “pot-bellied” appearance. Diagnosis is not always straightforward, and a combination of results from abdominal ultrasound, ACTH stimulation test, and low-dose dexamethasone suppression test (LDDS) may be needed. Treatment may involve surgical removal of the adrenal gland in some adrenal-dependent cases, although oral trilostane or mitotane (the most common option to treat pituitary-dependent cases) is also occasionally used.

Hepatocutaneous syndrome (HCS) or superficial necrolytic dermatitis (SND) is a metabolic condition that occurs in the dog, whereby a hepatopathy causes hypoaminoacidemia and aminoaciduria. As a result, affected dogs develop skin lesions, such as crusting, erosion and ulceration, and occasional vesicle formation. These skin changes most commonly affect areas exposed to wear or trauma, such as paw pads (Figure 5), elbows, hocks, genitalia, muzzle and the periocular area. Skin biopsy and abdominal ultrasound, along with blood and urine amino acid levels, can confirm the diagnosis. SND previously resulted in relatively short survival times (as low as 3-6 months after diagnosis) despite supplemental nutritional therapy. Recently, longer survival rates have been reported with this condition (and in dogs with aminoaciduric canine hypoaminoacidemic hepatopathy syndrome (ACHES) without skin lesions) through a combination of therapies 11,12. In terms of intervention, a mix of intravenous amino acid +/- lipid infusions, balanced diets, and enteral supplements may provide the best outcome 13.

Skin and aging immunity

The skin’s immune system is complicated. Innate components include the physical barrier of the epidermis, which can recruit phagocytic cells, activate complement, and produce cytokines. Adaptive responses include antigen recognition and subsequent responses by lymphocytes 14. Aging may not be a disease, but an aged immune system makes fighting infection more challenging. Additionally, our older pet population will be more likely to have other underlying conditions (such as endocrine disease, chronic allergies, immune-mediated skin disease, and cutaneous neoplasia) which can predispose them to secondary skin and ear infections.

Like their younger counterparts, senior dogs are susceptible to ectoparasites such as fleas or scabies mites. Demodicosis in the older dog (Figure 6) is often a warning sign of internal disease or immunosuppression, although adult-onset demodicosis can also be idiopathic. Only hyperadrenocorticism, hypothyroidism, and leishmaniasis were associated with adult-onset demodicosis in one study 15, but interestingly underlying neoplasia was not. The clinician should remember to look for causes of immunosuppression in older animals with all cases of demodicosis. Many parasiticides have been used to treat the condition, including avermectins, milbemycins, amitraz and others, but it has recently become much easier to manage with the advent of newer parasiticide therapies, such as the isoxazolines.

Bullous impetigo (Figure 7) is a superficial pustular-to-bullous condition most commonly reported secondarily to staphylococcal infections in aged animals with endocrinopathies, such as hypothyroidism, diabetes mellitus, or hyperadrenocorticism 16. Superficial and deep bacterial folliculitis/furunculosis cases, often caused by Staphylococcus spp., are also seen in older dogs with endocrinopathies, allergic dermatitis, or any barrier-disrupting condition. Occasionally, these bacterial infections occur more frequently in geriatric animals with no apparent underlying condition; this could be due to the fact that as the skin’s immune system ages, it is slower to respond to infection. Antimicrobials (topical and systemic) may be required for extended periods to clear the infection properly. Malassezia dermatitis typically occurs secondarily to allergic dermatitis, conformational issues in the skin and ears, in areas with increased cutaneous humidity, and secondary to excessive moisture as seen in swimmers. The skin may show extreme lichenification and hyperpigmentation in older animals with chronic Malassezia issues.

Topical therapy is ideal for superficial skin infections when possible; 3% or greater concentrations of chlorhexidine should be effective against Staphylococcus spp. and Malassezia spp. infections. Systemic antibiotics should be chosen based on bacterial culture when possible.

Cutaneous tumors

Skin tumors can occur throughout a dog’s life, but they are seen more frequently as animals age. Skin masses can arise from epithelial or mesenchymal origins; neural, vascular, adipocytic and fibroblastic tumors can all occur. Various aspects, such as genetic predisposition and immunologic factors, can generally affect the growth of skin tumors, whilst other contributors, such as environmental factors (including ionizing radiation and ultraviolet light) can have cumulative influences. More exposure to these factors comes with prolonged age, which may play a role in certain tumor development.

Melanomas and mast cell tumors are common in older dogs 17, but since abundant information about these tumors and their treatment is already available to the practitioner, these will not be reviewed in detail here, and this discussion will focus on tumors seen frequently in the specialty dermatology and general practice. Briefly however, melanomas are most commonly seen as solitary lesions on the head, limbs or digits, and can appear gray or black. They may be well- or ill-defined and are often raised. Dogs are usually older than nine years of age at onset 18. Mast cell tumors can have a varied clinical presentation, often appearing as solitary masses, ranging from dermal-to-subcutaneous, soft-to-firm, and haired-to-alopecic. Masses may ulcerate or be pruritic, and the average age of onset is eight years 19.

Many benign growths in dogs, such as lipomas, occur with increased frequency in senior dogs. Lipomas arise from mature adipocytes, with obesity and age being risk factors. These tumors are most often present in the subcutis, but can present in a more dermal aspect, or alternatively infiltrate deeper structures. Nodular sebaceous gland hyperplasia, sebaceous epithelioma, and sebaceous adenomas are benign growths seen on the trunk, limbs, and face in older animals. Less often, sebaceous gland adenocarcinoma can occur. Lesions are wart-like or cauliflower-like in appearance, with affected regions including the trunk and limbs, as well as around the eyes 20. While oral papillomas are more common in young animals, older dogs can get papillomas in non-oral areas, such as the paws, head, and eyelids, and lesions can be single or multiple. In the older pet, there is more concern for malignant transformation to squamous cell carcinoma 20. In terms of intervention, observation is allowed for most benign growths in the geriatric dog, but if the lesion bothers the pet or the owner, surgical removal via traditional methods, cryotherapy or CO₂ laser is appropriate.

Chronic sun exposure can predispose dogs to develop actinic keratosis, and sun exposure is believed to be a trigger in some dogs that develop skin cancer. Squamous cell carcinomas (SCC) and hemangiomas/hemangiosarcomas are occasional sequelae. SCC in the dog has also been associated with papillomaviruses and other infectious and inflammatory dermatoses. It can occur in various locations, but is commonly seen on the digits 21. When UV light is implicated, lesions are more likely in non-haired or glabrous areas (Figure 8) such as the ventrum. Hemangiomas also occur on the ventrum of dogs secondary to prolonged sun exposure; these are usually blue or red in appearance and dermal-to-subcutaneous. Dermal hemangiosarcomas can also develop in this area, although they are usually less well-circumscribed than their benign counterparts. Multiple masses are often present on the ventrum 22. Actinic-induced lesions such as SCC, cutaneous hemangiomas, and hemangiosarcomas can be treated with surgical resection, cryosurgery, and CO₂ laser ablation. Topical imiquimod may also help to improve lesions.

Numerous follicular masses are seen in older dogs, including various follicular cysts, trichoepitheliomas and trichoblastomas, and can be solitary or multifocal. Trichoepitheliomas favor the trunk and limbs, while trichoblastomas (Figure 9) have a predilection for the neck and head. Follicular cysts are named after the area of the hair follicle that is affected and the type of keratin produced. They commonly present as solitary nodules, and can become inflamed and infected if traumatized. Complete surgical excision is the treatment of choice when follicular lesions are problematic, although observation is another option if lesions are few, inconspicuous, and not prone to trauma and secondary infection.

Epitheliotropic lymphoma (Figures 10-12) involves infiltrative T-lymphocytes that favor the epidermis and adnexal epithelium. It has a varied clinical presentation, with early lesions commonly mistaken for allergic dermatitis and bacterial or yeast infections. Previous inflammatory skin disease may predispose animals. The skin may appear erythematous with or without scale and exfoliation. Depigmentation around mucocutaneous junctions can occur, and these areas may develop a “puffy” or thickened appearance. Paw pads can show hyperkeratosis, erosion and ulceration, and may depigment. With time, erythematous plaques and nodules may develop, and ulceration can be seen 23,24,25. Pruritus can be variable. Diagnosis is via skin biopsy, with immunohistochemistry performed as needed. There is no one effective treatment for epitheliotropic lymphoma; various chemotherapeutics have been used with varying success. In some cases, especially with mucocutaneous disease, radiotherapy has been beneficial. Glucocorticoids, safflower oil, and retinoids have utility 23, and it is important to manage secondary infections and control pruritus in those animals with pruritus; lokivetmab may help with this in some patients.

Conclusion

In summary, a variety of changes to the skin and hair coat occur in geriatric dogs. A basic understanding of typical aging manifestations can help the clinician understand when abnormalities are present in an older canine. Aging immune system changes may predispose the skin to secondary infections. Geriatric animals have the propensity to develop endocrine and metabolic diseases as well as cutaneous neoplasia, and prompt recognition and intervention can help ensure a favorable outcome in some, but not all, cases.