Canine sebaceous adenitis

Introduction

Sebaceous adenitis in dogs is a distinctive dermatological disorder characterized by an inflammatory pathogenesis that primarily affecting the sebaceous glands, resulting in their degeneration 1. Practicing veterinarians worldwide will encounter cases, and it is crucial to have a comprehensive understanding of this condition to provide optimal care. This article will explore sebaceous adenitis, including its clinical features, diagnosis, available treatments and prognosis, with the aim of assisting veterinarians in managing this challenging condition.

Etiology

The term “sebaceous adenitis” refers to the inflammation and subsequent destruction of the sebaceous glands; these are responsible for producing sebum, an oily substance that helps keep the skin and coat healthy 1,2. The exact etiology of canine sebaceous adenitis is not fully understood, but it is believed to have a multifactorial origin. Genetic predisposition is strongly suspected to play a significant role in the development of this condition 1. In some breeds, such as the Akita and Poodle, it is assumed that sebaceous adenitis follows an autosomal recessive mode, and ongoing studies are seeking to identify specific genetic markers and mutations that may contribute to susceptibility 3,4,5. 

In addition to genetic factors, immune system dysfunction is believed to be a major contributor to the etiology 1. Glandular destruction is linked to a cell-mediated immunologic response targeting the glands. Immunohistologic analysis of samples from affected individuals reveals the presence of dendritic antigen-presenting cells and T-cells concentrated in the middle part of the follicle, extending into the sebaceous duct 6. This observation strongly suggests an immune-mediated pathogenesis, and the positive response often noted to treatment with cyclosporine further supports this theory.

Further possible contributors to the etiology involve anomalies in lipid metabolism, defective lipid storage, or abnormalities in keratinization, which can lead to obstruction of sebaceous ducts and inflammation caused by lipid leakage 1. Moreover, various environmental factors, including stressful events like illness, general anesthesia, surgery and exposure to heat, may initiate or exacerbate the condition 1,3. Additionally, exposure to sunlight (photoaggravation) can further worsen the disease 1. A comprehensive understanding of these potential factors is essential in the diagnosis and management of sebaceous adenitis.

Signalment

Canine sebaceous adenitis is a rare and complex dermatological condition that primarily affects certain breeds such as the Standard Poodle, Akita, Samoyed, Havanese and Vizsla, although it can occur in mixed breeds as well 1,7. The disease tends to appear in young adult to middle aged dogs, although it can be seen at any age. No sex predilection has been noted 1,7.

Clinical appearance

The clinical signs of sebaceous adenitis and their severity can vary significantly among individual dogs, and are also influenced by the breed 8. In short-haired breeds, such as Vizslas, Miniature Pinschers and Dachshunds, clinical skin lesions typically begin with annular patterns characterized by hair loss and erythema. These lesions often exhibit fine, white, and non-adherent scales (Figure 1). Over time they may enlarge peripherally, taking on a polycyclic appearance or coalescing into larger affected areas (Figure 2) 1,6.

Skin lesions associated with sebaceous adenitis often display a symmetrical pattern, and typically affect specific areas of the body. Commonly affected regions include the pinnae (Figure 3), face (Figure 4), head and dorsal trunk 1,9. In more severe cases, external ear inflammation (otitis externa) may be observed, and ulcerative lesions in the pinnae have been reported 10. Additionally, a recent report has indicated blepharitis and meibomian gland dysfunction in a dog affected by sebaceous adenitis 11.

One characteristic sign of sebaceous adenitis is the presence of follicular casts, which can vary in their visibility. These casts are characterized by groups of hairs matting together due to a sheath of keratin debris (Figures 5 and 6). This debris remains attached to the hair above the follicular ostia. While this feature is not entirely specific, it can be an invaluable aid in diagnosing the condition 3,12.

While varying degrees of pruritus may be observed in affected dogs, it is typically not regarded as a defining characteristic of the condition unless secondary pyoderma is also present 2,7. Nonetheless, it is noteworthy that the author has seen instances of canine sebaceous adenitis with pruritus even in the absence of secondary pyoderma. Moreover, a review of medical records for 24 dogs with sebaceous adenitis found that 19 of them exhibited pruritus, but only eight had concurrent superficial pyoderma 13.

For long-haired breeds such as the Akita, Poodle and Samoyed, sebaceous adenitis can present with similar signs to their short-haired counterparts. These breeds may experience severe matting and clumping of fur due to impaired sebum production. This is more pronounced than in short-haired breeds, and typically occurs in the head, pinnae, neck, back and tail regions. Hair loss with or without erythema is common, leading to noticeable bald spots, particularly on the back, tail, and neck (Figures 7, 8 and 9) 7,12.

In long-haired breeds color changes may also be observed, along with alterations in hair texture from curly to wavy or straight. These changes may progress to poor, dull, and brittle hairs. The accumulation of dead skin cells and matted hair in these regions can create an environment conducive to secondary bacterial skin infections, leading to discomfort and pruritus in affected dogs 1,7.

Diagnosis

When diagnosing sebaceous adenitis, it is crucial to differentiate it from other skin diseases that can present with similar clinical signs. These potential differentials include atopic dermatitis, leishmaniasis (in endemic areas), and infectious conditions such as dermatophytosis, demodicosis, and bacterial folliculitis. Immune-mediated skin disorders like pemphigus foliaceous and cutaneous lupus erythematosus should also be considered. Furthermore, hormonal imbalances, nutritional deficiencies (such as zinc or fatty acid deficiencies) and nutritionally responsive conditions (such as zinc-responsive dermatosis or vitamin A-responsive dermatosis) can also mimic sebaceous adenitis 1,7. 

Due to this array of potential differentials and the varied clinical signs associated with sebaceous adenitis, a comprehensive approach is imperative for confirming the diagnosis. A thorough clinical examination is the first step, looking for characteristic signs such as annular lesions, hair loss, erythema, and the presence of follicular casts. The distribution of skin lesions and their symmetry is also important and indicative of the condition.

A trichogram, which entails plucking hairs from the affected area, can be conducted to detect hair shaft abnormalities. In cases of sebaceous adenitis these hairs may exhibit a distinctive waxy appearance, with their roots possibly filled with keratin (follicular casts). Furthermore, when combined with skin scraping, this procedure can assist in ruling out follicular parasites such as Demodex mites.

Cytological examination of skin lesions can reveal any secondary infections or inflammatory cells present in the affected areas. This information can be valuable for assessing the extent of the condition and planning treatment. In some cases, blood samples may help rule out underlying medical conditions that can mimic the signs of sebaceous adenitis (e.g., testing for leishmaniasis in endemic areas, or hormonal imbalances). 

A skin biopsy is often recommended to confirm the diagnosis definitively. The histopathologic features of sebaceous adenitis are highly variable but typically consist of diffuse absence of sebaceous glands and isthmus perifollicular granulomatous to pyogranulomatous inflammation. Lymphocytes, mast cells, plasma cells and eosinophils can also compose the dermal infiltrate that targets the sebaceous gland. Additional features may include orthokeratotic hyperkeratosis, follicular keratosis, and acanthosis (Figure 10) 2,14,15.

Sebaceous adenitis may occur either as an idiopathic condition or as a secondary manifestation of other dermal inflammatory disorders, such as canine leishmaniasis, which is considered a reasonable differential diagnosis in endemic areas. Histologically, both canine leishmaniasis and idiopathic sebaceous adenitis can exhibit granulomatous or pyogranulomatous inflammation of the sebaceous glands; however, a recent report found that the two conditions can be differentiated histopathologically 14. According to the study, sebaceous adenitis in cases of leishmaniasis were histologically characterized by nodular to diffuse dermal infiltrate in addition to epidermal and subepidermal lesions and absence of marked hyperkeratosis and follicular keratosis, whereas in idiopathic sebaceous adenitis inflammation was generally restricted to the sebaceous glands, and hyperkeratosis and follicular keratosis were present.

Treatment options 

Managing sebaceous adenitis in dogs necessitates a comprehensive approach; the primary objectives of treatment are to reduce the accumulation of excess scales, enhance the quality of the hair coat, and mitigate inflammation and damage to the sebaceous glands. This multifaceted approach helps manage the condition effectively and improve the dog’s overall comfort and appearance. Treatment options encompass both systemic and topical approaches, and the choice depends on the severity of the condition and individual patient considerations. It is important to note that there is no one-size-fits-all approach, and that regular follow-ups and adjustments to the initial treatment plan are often necessary to achieve the best outcomes.

Cyclosporine, an immunosuppressive medication, is a common choice for managing sebaceous adenitis. It plays a crucial role in modulating the abnormal immune response that triggers the condition, and it can also induce hypertrichosis, potentially reducing the plugging of the follicular infundibulum with keratinaceous material in affected dogs. A dosage of 5 mg/kg/day has been shown to effectively reduce the inflammation associated with sebaceous adenitis 16, and in most cases cyclosporine can be tapered gradually once clinical signs have resolved. However, it is crucial to emphasize that long-term treatment is essential for controlling the condition 16.

Notably, combining topical therapy with cyclosporine can produce a synergistic benefit, positively impacting both scaling and alopecia while also reducing inflammation of the sebaceous glands 17. Furthermore, cyclosporine may promote better regeneration of the glands compared to topical treatment alone 17.

Essential fatty acids (EFAs), whether administered systemically or applied topically, have demonstrated effectiveness in certain patients 18. Given their generally mild side effects, EFAs are frequently considered as an initial option, either independently or in combination with immunosuppressive therapies.

Vitamin A (retinol) and synthetic retinoids, which are derivatives of vitamin A, have been reported as a treatment of sebaceous adenitis, albeit with varying degrees of success 13,19. These compounds, in addition to their anti-inflammatory properties, play a crucial role in keratinocyte proliferation and differentiation, thereby normalizing the keratinization process and promoting skin health 20. Retinoids are less favored by clinicians due to the low availability of the drug and its adverse side effects, which include keratoconjunctivitis sicca, teratogenicity, gastrointestinal upset, and hepatotoxicity. Careful monitoring is essential to detect any of these side effects in treated patients 20.

Anecdotal reports suggest that a combination of oral tetracycline and niacinamide may yield positive results in some cases. Dogs weighing less than 25 kg typically receive both medications at a dosage of 250 mg q8h, while dogs above this weight are treated with 500 mg of each drug at a similar interval 7.

In addition to systemic therapy, sebaceous adenitis can benefit from a variety of topical treatments aimed at alleviating clinical signs and improving skin and coat health. Topical therapies play an essential role, particularly in mild cases or as part of a comprehensive treatment plan, and include shampoos, humectants and oil soaks. Specialized shampoos containing sulfur and salicylic acid are an integral part of managing sebaceous adenitis, typically used 2-3 times a week, with a 10-minute contact time before rinsing 1,18. During bathing, a soft brush can be employed to assist in the removal of scaling. After rinsing, a conditioner can be applied, or a 50-75% dilution of propylene glycol can be sprayed or rinsed onto the dog’s coat 9,18. These sprays are sometimes used daily and then reduced to 2 to 3 times per week for ongoing maintenance. Propylene glycol acts as a humectant, helping to retain moisture 9,18. Additionally, baby oil soaks, where neat oil or a 1:1 dilution with water is massaged into the coat and left for 1-6 hours, can also be used. Following this, the dog should be bathed using shampoo or dishwashing liquid to remove excess oil 9.

Prognosis 

The prognosis for dogs with sebaceous adenitis can vary depending on several factors, including the severity of the condition, the response to treatment, and the overall health of the individual animal. For mild cases, where the clinical signs are relatively limited and responsive to treatment, the prognosis is generally more favorable. With appropriate management, affected dogs can frequently enjoy a comfortable quality of life, with noticeable improvement in their skin and coat health. However, in more severe and advanced cases of sebaceous adenitis, the prognosis may be guarded. While treatment options are available to alleviate clinical signs and improve the quality of life, it is important to recognize that sebaceous adenitis is typically a chronic condition. Long-term management is often required, and relapses can occur even during treatment 6,8. It is, therefore, crucial for owners and veterinarians to work together to develop a tailored treatment plan for each case. Regular follow-up appointments and adjustments to the treatment regimen are common to ensure the best possible prognosis, and despite the chronic nature of the condition, with diligent care and appropriate therapy many dogs can enjoy a good quality of life.

Conclusion

Canine sebaceous adenitis is a complex dermatological disorder with diverse clinical manifestations. Its multifactorial etiology involves genetic predisposition, immune system dysfunction, and environmental factors. Diagnosis requires a comprehensive approach, and treatment options range from systemic immunosuppressive drugs to topical therapies. The prognosis varies, with long-term management often necessary, and a tailored treatment plan is crucial for each affected dog to optimize their quality of life.