Malassezia Dermatitis

Chapter 41 Malassezia Dermatitis

Daniel O. Morris

Malassezia dermatitis (MD) is a superficial fungal (yeast) infection occurring on and within the stratum corneum of the epidermis of many mammalian species. There are several different species of Malassezia yeast recognized, and various animals may serve as the natural hosts for specific species of the yeast. For example, most domestic carnivores harbor Malassezia pachydermatis as part of their natural cutaneous microflora, while human beings primarily harbor Malassezia furfur. As commensal organisms, Malassezia yeast colonize the skin in very low numbers. Overt infection is defined by increased numbers of the yeast on the skin surface in conjunction with inflammation. In dogs with atopic dermatitis (AD), M. pachydermatis may be recognized by the immune system as an allergen, in which case a highly inflammatory and pruritic response can be mounted to relatively low numbers of yeast organisms, blurring the line between “colonization” and “infection.” The role of M. pachydermatis in feline dermatitis is less well defined, although it is a known commensal of feline skin as well.

ETIOLOGY

The genus Malassezia is now described to consist of nine species: M. furfur, M. sympodialis, M. globosa, M. obtusa, M. restricta, M. sloofiae, M. equi, M. japonica, and M. pachydermatis. Malassezia yeasts are characterized by a thick, multilayered cell wall and the production of blastoconidia by budding. With M. pachydermatis, the budding process imparts the appearance of unshelled peanuts when examined microscopically.

In dogs and cats, Malassezia yeast colonize the skin during the immediate perinatal period. Although M. pachydermatis is the species most commonly isolated from normal and inflamed skin of dogs and cats, other species are occasionally identified. M. globosa, which has a more spherical shape with tiny budding “heads,” is sometimes associated with feline ceruminous otitis, but rarely so in dogs. M. furfur, which is a commensal of human skin, has been isolated from normal and inflamed canine skin, although its role in true “infection” is unknown; dogs may serve as mechanical carriers only. Other species isolated from normal canine and feline skin include M. sympodialis and M. obtusa.

PATHOGENESIS

In dogs, cats, and human beings, Malassezia yeast contribute to the pathogenesis of several disease states, including seborrheic dermatitis, AD, endocrine or metabolic diseases, paraneoplastic diseases, and disorders of cornification.

Canine Malassezia Dermatitis

M. pachydermatis colonizes sparsely haired areas and moist areas of the skin and mucosa of normal dogs in higher numbers than in more densely haired and dry areas.

• In a diseased state, alterations of the skin’s surface microclimate contribute to increased susceptibility to yeast infection. Primary diseases that cause increased moisture, altered surface lipids, and/or disruption of stratum corneum barrier function encourage secondary overgrowth of the organism.
• Pruritic inflammatory diseases (allergic and parasitic) cause microclimate changes due to scratching (disruption of barrier function), licking (added moisture), and increased production of sebum.
• Endocrinopathies (especially hyperadrenocorticism) directly cause alterations in sebum characteristics and stratum corneum function.
• Metabolic diseases that result in parakeratotic hyperkeratosis (such as zinc-responsive dermatosis, generic dog food dermatosis, and hepatocutaneous syndrome or superficial necrolytic dermatitis) also appear to be risk factors.
• Secondary MD is also associated with primary (idiopathic) seborrhea of dogs, especially in basset hounds, cocker spaniels, dachshunds, and West Highland white terriers. Several other skin disorders can also be associated with MD (Table 41-1).
• A correlation between the onset of MD (or otitis) and the recent use of antibiotics has been documented. However, the opposite has also been noted where MD resolved following treatment of concurrent staphylococcal pyoderma with antibiotics alone.
• Similarly, glucocorticoids have also been suggested to be a risk factor for development of MD; but again, some cases will spontaneously resolve with steroid therapy of an underlying inflammatory disease and without specific antifungal therapy.

Table 41-1 COMMON DIFFERENTIAL DIAGNOSES FOR SKIN DISORDERS THAT MAY BE ASSOCIATED WITH SECONDARY MALASSEZIA DERMATITIS

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Key Point

The most common primary disease association with canine MD is AD.

The role of M. pachydermatis in AD has been well described clinically and immunologically. The inflamed skin and ear canals of dogs with AD often harbor increased numbers of yeast (compared with the skin of normal dogs), and specific antifungal therapy will ameliorate a large portion of the pruritus experienced by many of these dogs.

• Dogs have been shown to mount an immunoglobulin E (IgE)-mediated (immediate) hypersensitivity response to allergens produced by the yeast in a manner similar to other environmental allergens (such as pollens, dust mites, animal danders, and molds).

Key Point

Malassezia overgrowth can provoke an overwhelming pruritic response in atopic dogs, which can occur acutely and be misconstrued as increased exposure to aeroallergens. Resolution of the yeast infection can reduce the pruritic threshold of an atopic dog by 75% to 100% in some cases, depending on concurrent exposure to other allergens. Therefore, undiagnosed MD is one of the most common reasons for perceived failure in the management of atopic dogs.

Canine Malassezia Otitis

• M. pachydermatis is commonly associated with otitis externa. As with the skin, there appears to be an increased incidence of Malassezia over-growth in the ear canals of dogs with allergic and seborrheic otitis, endocrinopathies, and iatrogenic immunosuppression.
• It is also commonly seen in general practice occurring in otherwise normal dogs, but after swimming or bathing (i.e., water trapping or “swimmer’s ear”).
• The ear canals, which express a higher temperature and humidity than the surface skin, are a very favorable environment for yeast growth, and the lipid-rich cerumen is also supportive.
• I have noted a higher incidence of primary Malassezia otitis and pododermatitis (in otherwise normal dogs) living in a humid coastal climate than in dogs living in more arid and temperate climates.

Canine Malassezia Mucositis

While rare, it has been reported that M. pachydermatis can promote oral pathology (stomatitis, pharyngitis, tonsillitis).

Feline Malassezia Dermatitis and Otitis

• While a definitive (immunologic) relationship between Malassezia yeast and AD has not been described in cats, it does appear to be associated with pruritic or inflammatory dermatoses such as AD, adverse food reaction, and ectoparasitism.
• Cats with increased numbers of Malassezia spp. in the external canals often exhibit a highly pruritic ceruminous otitis.
• In addition to primary pruritic diseases, feline MD appears to occur in conjunction with paraneoplastic skin diseases; in a review of 550 feline skin biopsies, Malassezia spp. were most commonly associated with feline paraneoplastic alopecia and thymoma-associated dermatosis or erythema multiforme. All cats with histology consistent with these underlying systemic diseases died within 8 weeks of skin biopsy sampling.

CLINICAL SIGNS

Canine Malassezia Dermatitis

• Although MD is usually intensely pruritic, the only primary lesion produced is erythema.
• Secondary lesions including excoriations, seborrheic plaques, lichenification, maceration, and intertrigo are common and cannot be reliably distinguished from staphylococcal pyoderma without cytologic examination.
• In rare cases, M. pachydermatis can cause a folliculitis that mimics staphylococcal folliculitis, dermatophytosis, and demodicosis.
• The clinical appearance of the skin in cases of MD is highly variable. It may either be dry and flaky (seborrhea sicca) or tacky and greasy (seborrhea oleosa).
• The distribution pattern of canine MD is variable but most commonly affects some combination of the face (especially periocular and perioral skin), feet (interdigital spaces and claw folds), intertriginous areas (axillae, groin or inguinum, facial folds, and vulvar and mammary folds), and perineum.
• Generalized cases of MD may occur in chronic cases of allergic dermatitis.
• Malassezia pododermatitis may occur with or without more widespread MD. The feet are the most common single body area affected in allergic dogs. Patients with interdigital Malassezia pododermatitis will be presented for the complaint of paw licking or chewing.
• Paronychia (inflammation of the claw beds) may also occur as the sole presenting sign of MD and often causes claw biting (also see Chapter 63). Physical examination will usually reveal a reddish-brown staining of the proximal claw or a waxy exudate in the claw fold, with inflammation of the surrounding soft tissue.

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Malassezia Dermatitis

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