Canine Malassezia dermatitis

Malassezia pachydermatis is a commensal yeast that is normally present in low numbers in the external ear canals and superficial muco-cutaneous sites in dogs. Malassezia pachydermatis is characterized by its round to oval or classical peanut shape with monopolar budding. This lipophilic, non-lipid dependent, non-mycelial saprophytic yeast organism is most often associated with Malassezia dermatitis (malasseziasis or Malassezia overgrowth) in dogs. Other Malassezia may uncommonly be noted as a cause of Malassezia dermatitis such as M. sympodialis, which is smaller than M. pachydermatis and has a more rounded bulbous shape and narrower-based monopolar budding (1).

Malassezia dermatitis in dogs is usually a secondary problem due to an underlying skin disease such as allergic disease (including canine atopic dermatitis and flea allergy dermatitis), recurrent bacterial pyoderma, and endocrine diseases (especially hypothyroidism) (2). Many predisposing factors may result in the commensal M. pachydermatis becoming a pathogen. These factors include increased humidity, presence of skin folds, altered cutaneous pH levels, previous antibiotic therapy, and prolonged corticosteroid therapy (2–4). In addition to being a secondary disease, a significant number of dogs affected with Malassezia dermatitis are affected by concurrent staphylococcal pyoderma. Malassezia pachydermatis is thought to have a symbiotic relationship with commensal staphylococci, which produce mutually beneficial growth factors and micro-environmental alterations (1,2).

Clinical findings

Malassezia dermatitis (Figure 1) is common in dogs and affected sites include lip margins, ear canals, axillae, groin, ventral neck, interdigital skin, facial folds or tail folds, perivulvar skin, and perianal skin (5). Lesions may be localized or generalized. Pruritus, a major sign, is usually severe and is accompanied by unpleasant odor. Skin lesions may present in various forms, which can be affected by chronicity of disease, primary underlying disease, previous therapy, and concurrent bacterial infection. Some common presentations of Malassezia dermatitis include:

Figure 1.

Malassezia dermatitis.

1. Regional or generalized alopecia with erythema (exfoliative erythroderma);

2. Scaly, waxy, or greasy seborrhea (yellow or slate gray);

3. Crusts or papulocrustous lesions resembling superficial staphylococcal infection;

4. Lichenification and/or hyperpigmentation (leathery or elephant-like skin);

5. Paronychia with dark brown nail bed discoloration, with or without obsessive paw chewing;

6. Lip margin hypotrichosis and/or crusting; and

7. Intertrigo.

Several breeds are predisposed with the West Highland white terrier, basset hound, American cocker spaniel, shih tzu, poodle, boxer, Cavalier King Charles spaniel, German shepherd dog, and dachshund showing an increased risk for Malassezia dermatitis.

Diagnosis

The most useful and practical method of diagnosis of Malassezia dermatitis is cytologic examination (1,2,4). Samples collected using glass slide impression, acetate tape impression, superficial skin scraping, or cotton swab method are evaluated under the microscope to ascertain the numbers of Malassezia yeast, bacteria, and inflammatory cells present on superficial skin. If present, yeast organisms are often observed in clusters or adhered to keratinocytes (1). Each collection method has its own benefits based on patient temperament, clinician preference, and sampling site.

Cutaneous cytology is not always successful in finding Malassezia organisms. While some literature suggests that the clinician should rely on clinical lesion patterns to make a tentative diagnosis of Malassezia dermatitis in such a situation (4), in the author’s experience trial therapy is of minimal benefit in the absence of cytological evidence of Malassezia organisms. Although certain lesion patterns, presence of an offensive yeasty odor, and lack of response to previous appropriate therapy may be suggestive of Malassezia dermatitis, these should not be relied upon as diagnostic criteria without pursuing cutaneous cytology to confirm Malassezia organisms.

A further challenge in diagnosis of Malassezia dermatitis is the lack of agreement with regard to the significance of numbers of yeast present on cutaneous cytology (1,6). Varying numbers of yeast are present in different body sites, and normal numbers vary among breeds resulting in overlaps in yeast population densities in samples from clinically normal and diseased dogs (7). Ultimately the diagnosis of Malassezia dermatitis should rely on the combination of clinical presentation and cutaneous cytology. Even low numbers of Malassezia organisms noted on cytology may indicate Malassezia dermatitis if samples are collected from inflamed, pruritic skin. The findings of cutaneous cytology can vary between visits, and presence of a previous normal cytological analysis should not be regarded as current, if the patient exhibits new lesions or clinical symptoms.

Fungal cultures are not helpful because M. pachydermatis are commensal organisms, making their isolation in culture of little or no diagnostic value (1,2).

Clinical management

Multiple treatment options for Malassezia dermatitis are available. Treatment is usually tailored according to factors such as localized versus generalized disease, overall patient health, underlying primary disease, and client preference or compliance. It is preferable to use a combination of topical and systemic therapy in order to achieve rapid and complete remission of clinical signs. Prescribing topical or systemic therapy alone may be adequate for some patients. Extensive topical therapy can be challenging in dogs with a thick hair coat, for large or non-compliant dogs, or if the pet owners are unable to meet the physical requirements that are sometimes time-consuming, which is the nature of the treatment. Often, treatment of Malassezia dermatitis is accompanied by other recommendations such as a dietary elimination trial, antibiotic therapy, and antipruritic therapy. The overall workload for the pet owner should be evaluated in order to ensure compliance with sometimes numerous treatment recommendations made. Diagnostic workup and treatment for underlying primary disease should be pursued while treating Malassezia overgrowth, otitis, or dermatitis. Therapy should be continued for 7 to 10 d beyond clinical cure (1).

Follow-up examination is usually recommended 3 to 4 wk after initiation of treatment in order to evaluate clinical response and re-evaluate cytological Malassezia numbers. Clinical improvement, though welcome, is not adequate evidence to confirm treatment success. On the contrary, lack of follow-up cytology may lead to further confusion about significance of Malassezia yeast in the disease process, if the clinical symptoms or skin lesions were to recur.

For mild cases or for localized lesions, frequent topical therapy with antifungal products containing ingredients such as 2% ketoconazole, 1% ketoconazole–2% chlorhexidine, 2% miconazole, 2% climbazole, 2% chlorhexidine, 3% chlorhexidine, 2% miconazole-2% chlorhexidine, 2% lime sulfur, 0.2% enilconazole, or 1% selenium sulfide is usually effective (1,2,4,8–10). Shampoos containing 2 active ingredients may provide better efficacy (4). Medicated antifungal wipes or pads such as those containing 0.3% chlorhexidine, 0.5% climbazole, and Tris-EDTA solution are effective against M. pachydermatis (11).

For patients with generalized or multifocal lesions, oral antifungal therapy in combination with topical therapy is most effective. Oral antifungal drugs effective against Malassezia organisms include ketoconazole, fluconazole, terbinafine, and itraconazole (5,11–14). Griseofulvin is not effective in the treatment of Malassezia infection (1). Patient factors such as age, clinical history, underlying or concomitant disease, and breed predisposition should be considered before use of systemic drugs; baseline and monitoring blood testing is encouraged.

If underlying disease or predisposing factors are not controlled, or are inadequately managed, regular antifungal therapy may be indicated. Pulse therapy protocols using oral antifungal drugs such as ketoconazole, itraconazole, and terbinafine can be prescribed (12,13). Similarly, application of 2% climbazole shampoo for the control of Malassezia overgrowth and prevention of recurrence has been described (9).

Malassezia hypersensitivity

One should not reach a diagnosis of Malassezia hypersensitivity without a complete and thorough dermatologic workup. Any dog with a classical history and cytological findings consistent with recurrent or persistent Malassezia dermatitis should be suspected of being affected by hypersensitivity to Malassezia organisms as long as underlying primary disease is well-managed or ruled out. Higher levels of Malassezia-specific immunoglobulin E have been found in atopic dogs compared with healthy dogs, suggesting that Malassezia may participate as an allergen in patients with atopic dermatitis (15). Hypersensitivity to Malassezia antigens is also thought to be important in atopic humans (16). If Malassezia hypersensitivity is suspected, intra-dermal allergy testing followed by observation for immediate and delayed reactions may be used to document hypersensitivity reactions (1). In dogs diagnosed with Malassezia hypersensitivity, immunotherapy with M. pachydermatis antigens may be a useful therapeutic measure (17), but it may not be effective in some dogs suspected to be hypersensitive to M. pachydermatis, and pulse therapy may be needed to prevent recurrence.

Prognosis

Malassezia dermatitis carries a good prognosis. Thorough efforts should be made to identify causative factors such as underlying allergies, endocrine disease, neoplasia, or skin folds, to help prevent recurrent infection. Once concurrent infections and primary disease are adequately treated, management of M. pachydermatis induced dermatitis is usually straightforward.