Yeasts of the genus Malassezia constitute lipophilic fungal agents that have evolved as cutaneous commensals and opportunistic pathogens in various mammalian and avian species. Their involvement in canine and feline dermatological and otological conditions represents a daily clinical challenge for veterinary practitioners worldwide.
At the recent NAVDF congress in Orlando, our colleague Ross Bond, a world specialist on the subject, had the opportunity to provide a comprehensive overview, covering pathogenic, diagnostic, and therapeutic aspects.
History and Taxonomy of Malassezia Yeasts
The association between Malassezia yeasts and canine external otitis dates back to the pioneering work of Bengt Gustafson, who published his thesis in Stockholm in 1955 covering 201 cases of primarily acute external otitis in dogs. The Cocker Spaniel constituted the most represented breed in this study, and it remains remarkable that this breed predisposition is still recognizable in contemporary clinical practice. Gustafson isolated yeasts alone in 108 cases and yeasts associated with staphylococci in the majority of remaining cases, an observation that practitioners performing daily cytology will immediately recognize as familiar.
He observed that yeast frequency decreased in chronic cases, giving way to infections with Proteus and Pseudomonas, a finding with which the contemporary veterinary community would unanimously agree. Of 97 healthy ears examined, he detected only low yeast growth in eight cases only. The morphological characteristics of these yeasts—oval shape, polar budding, absence of mycelium, slow growth, and absence of fermentation—led Gustafson to classify them under the genus Pityrosporum, now designated Malassezia. After consulting the Central Bureau for Fungal Cultures in the Netherlands, and facing the absence of an available reference strain (the rhinoceros isolate from the 1930s having disappeared), he temporarily proposed the name Pityrosporum canis.
His experiments demonstrated that inoculation of this yeast in healthy dogs induced a mild transient otitis, and that application of malt agar supplemented with olive oil in the auditory canal promoted Malassezia proliferation and the development of otitis. These observations enabled Gustafson to conclude that Pityrosporum canis represented a cause of otitis and that this condition could occur through activation of yeasts normally residing in the external auditory canal.
Seventy years later, the scientific consensus fully confirms that M. pachydermatis constitutes a secondary opportunistic otic pathogen requiring some form of predisposition or preexisting auricular abnormality to generate clinically significant otitis. Dr. Gustafson had therefore correctly identified the fundamental mechanisms of this pathology as early as the mid-twentieth century.
Classic appearance of Malassezia pachydermatis
History of Malassezia Dermatitis
Regarding Malassezia dermatitis in dogs, the first reports are attributed to the Belgian practitioner Dufait, who published several articles in the late 1970s and early 1980s describing this condition. Professor Larson in Brazil also contributed through publication of case series documenting the clinical manifestations of this emerging pathology. Ken Mason then took up the torch by presenting these observations at various scientific meetings and professional conferences. The first mention of Malassezia dermatitis at an AAVD/NAVDF congress probably dates back to 1987 in Phoenix, Arizona, where Ken Mason presented an abstract describing three canine cases.
Clinical Recognition and Acceptance
This initial presentation elicited a mixed reception within the scientific community, with some practitioners remaining skeptical about the pathogenic relevance of this yeast in the dermatological context. Progressive acceptance of the clinical relevance of this yeast followed, and Malassezia dermatitis is now an integral part of daily veterinary practice in small animal medicine worldwide.
Taxonomic Evolution and Genus Complexity
Origins of Nomenclature
The genus Malassezia was first observed in 1846 and initially designated Cryptococcus. Malassez subsequently described these spores in cells from human dandruff, thus establishing the link between these organisms and human scalp desquamative conditions. The genus name Malassezia was proposed in recognition of this researcher’s work, then modified by Sabouraud to Pityrosporum, before being restored in the 1980s to its original designation.
Lipid and Morphological Classification
The initial taxonomy was relatively simple and understandable: all Malassezia yeasts were recognized as lipophilic, and those strictly lipid-dependent, requiring special culture media supplemented with fatty substances, were grouped under the single species Malassezia furfur. The eminent mycologists of the time, such as Evelyn Guého and Gillian Midgley, nevertheless emphasized the existence of great morphological diversity within this group, distinguishing different oval forms designated as oval form 1, oval form 2, oval form 3, and so forth. The morphological diversity strongly suggested the existence of multiple distinct species within this taxonomic grouping initially considered monospecific.
Specificities of Malassezia pachydermatis
Malassezia pachydermatis, the predominant species in carnivores and undoubtedly the most relevant for veterinary practice, was classified as non-lipid-dependent because it was capable of growing on Sabouraud Dextrose agar, the routine mycological culture medium. Human lipophilic species demonstrate their clinical importance in various dermatological conditions such as Pityriasis versicolor, characterized cytologically by the classic “spaghetti and meatballs” appearance of the pseudohyphal phase of what was formerly designated M. orbiculare, as well as in seborrheic dermatitis of the scalp.
Contribution of Molecular Mycology
The advent of molecular mycology techniques has considerably complicated the taxonomy of the genus. Complete genome sequencing of M. pachydermatis revealed that it actually belongs to the strictly lipid-dependent group, as it shares with other species the absence of the gene encoding fatty acid synthase. Its apparently paradoxical ability to grow on Sabouraud agar is explained by the sufficient presence of palmitic acid in the peptone component of this medium, an adequate quantity to support the growth of this species but insufficient for other more demanding species. The other species require marked and substantial lipid supplementation for laboratory culture.
Feline Species and Animal Diversity
In cats, various strictly lipophilic species have been isolated in culture with subsequent molecular confirmation of colony identification, notably M. sympodialis, M. globosa, M. furfur, M. nana (particularly in the auditory canal) and M. slooffiae (notably at the level of nail folds). Rui Kano was the first to describe M. nana in cats and cattle, thus establishing the importance of this species in these animal populations. Other species are primarily associated with human skin in culture, while some are linked to specific animal hosts: M. caprae in goats, M. equina in horses, M. gallinae in chickens, and M. botryllophilus in bats. Distinction between these species cannot be accomplished solely by culture; a molecular sequencing test remains necessary for precise and definitive identification. This increasing taxonomic complexity raises important questions concerning the clinical relevance of this species diversity. The ability to precisely identify the species involved in a given clinical case could potentially influence therapeutic decisions, particularly in the context of the emergence of antifungal resistance.
Cutaneous Ecology and Anatomical Distribution
Mechanism of Pathogenicity
The transition from commensal to pathogenic status is frequently observed when the homeostatic balance between host immunity and fungal virulence is disrupted, requiring a targeted therapeutic approach associated with identification and correction of underlying predisposing factors.
Commensal Colonization in Dogs
Malassezia pachydermatis constitutes a normal inhabitant of the skin and healthy mucous membranes of dogs, forming an integral part of the commensal cutaneous microbiome. A study of 40 healthy dogs conducted many years ago revealed a culture isolation frequency greater than 30% in the auditory canal, confirming that this anatomical location represents a major reservoir of commensal colonization. In contrast, the axilla, groin, and back showed an isolation rate of 10% or less, suggesting much more sporadic and limited colonization of these sites. To detect this yeast on the skin of a healthy animal by culture method, the preferred sites are the interdigital space and the lip region, where culture recovery rates are significantly higher. Among mucosal sites, the anus represents the preferred location, with more than 50% of animals showing anal colonization detectable by culture. A temporal study with weekly sampling confirmed that the anus remains the site where the yeast persists most consistently over time, with other sites showing more intermittent and variable colonization.
Presence in the Gastrointestinal Tract
Unexpected and initially surprising observations have emerged concerning the presence of Malassezia in the gastrointestinal tract. A subsequent study conducted in collaboration with Arti Kathrani, a specialist internist, and Bart Theelen, working at the time at the Westerdijk Fungal Biodiversity Institute in the Netherlands (currently in Minnesota), involving 45 dogs with enteropathy undergoing endoscopy for various diagnostic investigations, enabled the culture of Malassezia in eight of them, confirming preliminary observations and establishing the authentic presence of these yeasts in the canine small intestine.
Cutaneous Microbiome and Species Diversity
Microbiome studies using molecular techniques have revealed an unexpected diversity of Malassezia species on canine skin, particularly through DNA detection. Richard Harvey, a renowned clinician and researcher, recently published a microbiome study conducted in collaboration with certain exhibitors present at veterinary conference trade shows. A study examining the umbilical region of 20 healthy dogs identified Cladosporium in all individuals and M. pachydermatis in only two of them, confirming the results of previous classical culture studies. In contrast, DNA of M. sympodialis, M. restricta, M. slooffiae, and M. arunalii was also detected by molecular methods. These observations contrast strikingly with culture results, constituting a particularly intriguing and scientifically troubling observation. Over 31 years of systematic culture of Malassezia from canine skin on lipid-supplemented media theoretically designed to support the growth of all Malassezia species, only on one occasion was it possible to isolate something other than M. pachydermatis. Similarly, in the intestinal study mentioned previously, M. sympodialis was cultured only once from all samples.
Hypotheses on Molecular/Culture Divergence
This stunning divergence between molecular detection and culture raises several fundamental explanatory hypotheses: culture media might not be optimal for all demanding lipophilic species despite applied lipid supplementations, these species might be present in low numbers below the cultural detection threshold making their isolation unlikely, mixed microcolonies might exist with preferential persistence of M. pachydermatis during successive subcultures leading to loss of other more fragile species, or the detected DNA might not correspond to viable organisms but rather to genetic material from dead or damaged cells. Contamination of canine skin by human Malassezia DNA transferred by handling and contact also remains plausible, particularly in a context where human-animal interactions are frequent and intimate.
Environmental Observations
A particularly surprising recent discovery comes from a published study concerning vine leaves from Italian vineyards, where Cladosporium and Malassezia were identified as the most abundant fungi in this plant substrate. This observation is unusual because Malassezia is generally not considered a plant organism but rather as an obligate inhabitant of mammalian skin, raising intriguing questions concerning the global ecology of this fungal genus.
Microbiome and Atopic Dermatitis
Cody Meason Smith, an eminent mycologist working in collaboration with Dr. Hoffman, published a remarkable array of studies on the canine cutaneous microbiome. In healthy laboratory dogs, molecular analyses show that M. restricta and M. globosa predominate, exactly as in healthy human skin. In contrast, in laboratory dogs undergoing experimentally induced canine atopic dermatitis flares, M. pachydermatis and M. restricta become the most abundant in molecular analyses, with background detection of more strictly lipophilic species as well.
The Malassezia globosa Question
This observation concerning M. globosa raises important clinical questions. M. globosa presents a characteristic spherical morphology with thick and prominent budding, distinct from the narrow bud observed in certain Candida species. Although round yeasts are sometimes observed in cytology, particularly of auricular origin, they do not correspond to routine cytological observations in the majority of veterinary clinics. The question therefore remains: if M. globosa is important and present according to molecular studies, why is it not observed in routine cytology?
Culture Limitations and Contamination
Several explanations can be proposed for this apparent discordance. Current culture media, even those enriched with lipids like modified Dixon agar, might not be sufficiently optimal to culture all these diverse Malassezia species with strict and varied nutritional requirements. These species might be present in insufficient numbers, below the traditional culture detection threshold, not generating enough colonies to be noticed. The phenomenon of mixed microcolonies constitutes another plausible explanation. During successive subcultures of mixed colonies, M. pachydermatis, an easy-to-culture and fast-growing species, could persist while more demanding and slow-growing lipophilic species could progressively die and disappear. The detected molecular DNA indicates the presence of DNA but does not necessarily confirm the presence of viable organisms and metabolically active ones. Genetic material from dead or degraded cells could persist in the cutaneous environment and be detected by PCR amplification. Contamination of canine skin by human Malassezia DNA transferred by handling, petting, and close contact between owners and animals could also contribute to some of the observed molecular results. Technical methodological problems in microbiome studies, although outside the domain of dermatological expertise, could also influence results and their interpretation.
Climatic Influence
Malassezia yeasts reside in the stratum corneum, at the level of what Bart Theelen, an eminent mycologist, would call the transitional mantle zone, where they are influenced by climatic factors such as environmental heat and humidity. Dermatological practice in London, located 4,400 miles northeast of this conference location, occurs in a much cooler and much less humid climate than many North American regions. The mycological laboratory incubator functions as a humid incubator at 32 degrees Celsius, reproducing optimal conditions for yeast growth. Many dogs worldwide live in humid and hot environments, either year-round in tropical and subtropical regions, or during summer months in temperate regions. Dogs living in such climatic conditions, particularly those in the southeastern United States, show an increased frequency of Malassezia proliferation according to reports from practitioners working in different geographical and climatic zones.
Interactions with Host and Bacteria
Yeasts in the stratum corneum are influenced by host chemistry and immune factors in the broad sense, including innate and adaptive immunity. They metabolize sebaceous lipids produced by sebaceous glands and lipids derived from keratinocytes, abundant substrates in the stratum corneum due to their necessity for lipid-dependent yeast metabolism. Their interactions with commensal and pathogenic cutaneous bacteria remain poorly understood and constitute a domain requiring in-depth research. A recent publication indicated that certain lipophilic Malassezia species present on human skin interact with Staphylococcus aureus and reduce its tendency toward biofilm formation, suggesting a potentially beneficial role in regulating bacterial virulence and modulating cutaneous microbial ecology. These commensal yeasts could therefore exert protective effects by limiting the pathogenicity of other cutaneous microorganisms, adding additional complexity to our understanding of cutaneous microbial ecology.
Pathogenesis and Virulence Mechanisms
Enzymatic Production and Metabolic Activity
Fungi, nutritionally absorptive organisms, release a wide array of enzymes into their environment to create substrates assimilable by the fungal cell through the cell wall and cell membrane. These enzymes, particularly during marked and massive yeast proliferations, possess the potential to damage the host’s epidermal cells and activate innate and specific immune systems. The cell wall of Malassezia contains adhesion molecules facilitating adherence to corneal squames, a process that can be quantified by spending weeks microscopically counting Malassezia cells to determine whether they adhere to squames or not and examining the molecular factors involved in this process. These yeasts present IgE-binding epitopes responsible for immediate hypersensitivity reactions and pathogen-associated molecular patterns (PAMPs), recognized by immune cells via C-type lectin receptors, as well as by T helper 17 cells. The increasing importance of T helper 17 lymphocytes in fungal immunity, both innate and adaptive, is increasingly documented in contemporary scientific literature, although detailed immunological mechanisms exceed the domain of dermatological expertise and require consultation of specialized immunology literature for in-depth understanding.
Lipases and Cutaneous Biochemistry
In human medicine, fungal lipases play a recognized and well-established role in the development of dandruff and seborrheic dermatitis of the scalp. Aristea Velegraki and her collaborators have demonstrated that lipoperoxidation of squalene generates metabolites recognized as biochemical markers of dandruff skin, potentially involved in the pathogenesis of this condition. However, squalene does not represent a major component of canine sebum, unlike human sebum where it constitutes a substantial lipid fraction.
Enzymatic Characterization and Phospholipases
Commercial kits such as API Zym allow characterization of the enzymatic activities of culture supernatant and cell pellet from a broth culture. After centrifugation of the cell pellet from a broth culture, enzymatic activities can be detected in the test kit. Some enzymes appear associated with the cell pellet rather than the supernatant, while others are more abundant in the supernatant than in the cell, and some present equivalent levels in both fractions. M. pachydermatis produces a whole range of enzymes, including esterases and lipases. Phospholipase production is particularly important in M. pachydermatis, with higher levels in strains associated with lesional skin compared to healthy skin isolates, and in certain genotypes more associated with cutaneous conditions than with commensal colonization.
Virulence and Protein Expression
The late Claudia Cafarchia, unfortunately recently deceased, and her research group demonstrated the importance of phospholipase production, showing higher enzymatic levels in pathogenic strains. Administration of azoles, fungistatic drugs, disrupts yeast enzymatic production, as do essential oils according to certain publications. Significant proliferation of Malassezia therefore generates massive enzymatic production potentially involved in the pathogenesis of dermatitis and otitis. Recent work involving Thomas Dawson and various collaborators has emphasized the importance of studying fungal protein expression in physiologically relevant environments rather than under artificial laboratory conditions. A broth culture does not reproduce the physiological conditions of the stratum corneum, and the protein expression of yeasts in this natural environment differs substantially from that obtained in artificial broth. These authors have developed sophisticated scientific techniques to demonstrate protein expression of yeasts directly in the stratum corneum, revealing expression profiles different from those observed in liquid culture.
Animal Experiments and Commensal-Pathogen Transition
Experiments on laboratory beagles showed that daily application of Malassezia to skin under occlusion for one week allowed creation of a local dermatitis plaque with greasy brown exudate matting the hairs, similar to lesions observed in routine clinical practice. However, cessation of application led to complete healing in one week, demonstrating that normal skin therefore effectively controls its Malassezia population and does not develop infection from simple external application. This work was carried out under the Home Office Scientific Procedures Act, rigorous British legislation regulating the use of animals in experimental scientific environments and guaranteeing animal welfare. An underlying alteration is necessary to allow pathological yeast proliferation and development of clinical signs. As clinicians recognize and understand well, normal skin does not simply become infected through application of yeasts. Something must be fundamentally altered to allow this commensal yeast to proliferate opportunistically and generate clinically significant disease.
Predisposing Factors and Underlying Conditions
Diversity of Triggering Factors
Consensus guidelines have established a list of predisposing factors including breed, allergy, desquamation defects, endocrinopathies, skin folds, climate, and unidentified cases. This list has major clinical implications requiring in-depth understanding. In unidentified cases, representing the most frustrating category, the absence of understanding of the initial triggering factor prevents any correction and inevitably leads to chronic recurrent and relapsing disease, generating intense frustration in the owner and necessitating continuous and repeated treatments without definitive resolution.
Allergy Management
In the presence of allergy as a predisposing factor, complete elimination of yeasts by an antifungal, however effective and even the best antifungal drug known to humans, does not eliminate persistent residual allergic signs. If the sole criterion of therapeutic success for the owner is the complete disappearance of erythema and pruritus, short-term failure is inevitable and predictable, unless the owner is correctly and transparently informed of the partial response expected in these particular circumstances. The classic West Highland White Terrier out of control with conventional medications perfectly illustrates this situation: treatment of Malassezia and staphylococci reveals underlying atopic dermatitis which then becomes controllable with appropriate therapies. Primary cornification defects constitute a permanent and irreversible condition. The dog intrinsically possesses this keratinization disorder and remains affected by it continuously. Skin folds persist except for dietary intervention allowing their reduction through substantial weight loss or definitive surgical resection. If animals have anatomical folds, then these folds persist indefinitely unless made smaller by weight modification or surgically removed. Brachycephalics also constitute a particularly at-risk category, especially in the United Kingdom, where the prevalence of the French Bulldog has greatly increased.
The Basset Hound is frequently affected by Malassezia dermatitis
Atopic Dermatitis and Keratinization Disorders
Allergy, particularly atopic dermatitis, constitutes the predominant triggering factor in many veterinary hospitals. In some specialized hospitals, 50% of Malassezia dermatitis cases present atopic dermatitis as the probable underlying trigger. A study using contact plates as a culture method conducted by students as part of a research project demonstrated that healthy animals showed low isolation frequency and very low Malassezia populations, while atopics showed statistically significantly higher isolation frequency and populations. The green-bluish column representing healthy animals on the graph shows low frequency and very low populations, while atopics in red demonstrate much more frequent isolations and considerably higher populations as well.
Atopic Westie presenting Malassezia dermatitis
Complications of Atopic Dermatitis
Complicated atopic dogs do not simply present pure and simple atopic dermatitis. In many cases, there is also a secondary Malassezia dermatitis that considerably worsens the overall clinical severity. Approximately two-thirds of atopic dogs develop superficial pyoderma problems and one-third develop a Malassezia dermatitis problem. Elizabeth Molterdin and her colleagues published a very beautiful remarkable study in Veterinary Pathology a few years ago, referring to autosomal recessive congenital ichthyosis observed in American Bulldogs. This study elegantly demonstrates that scaly puppies affected by this genetic keratinization disorder become erythematous and pruritic when colonized by Malassezia, unlike healthy unaffected puppies who do not develop clinical signs despite similar exposure. The desquamation disorder therefore affects the epidermis in a way that promotes opportunistic proliferation of Malassezia and the appearance of clinical signs of inflammation and pruritus.
Breed and Anatomical Predisposition
Certain breeds show a marked particular predisposition to Malassezia dermatitis. The seborrheic Basset Hound constitutes the paradigmatic example, showing a median Malassezia population density in the axilla of the order of 10^5, which is 100,000 times higher than that of healthy mixed breed dogs used as a control group. Healthy Basset Hounds present intermediate populations, with substantial overlap between healthy and sick individuals on distribution graphs. A substantial quantity of Malassezia can therefore be present on clinically normal-appearing skin, depending on the enzymes produced by the yeasts and the cutaneous immunological reactivity of the host.
Ears are frequently affected by Malassezia dermatitis
Feline Predisposition (Devon Rex, Sphynx)
In cats, the Devon Rex behaves like the Basset Hound in the feline world, representing the feline equivalent of this predisposed canine breed. These cats show marked susceptibility to Malassezia. A contact plate study in the axilla of cats demonstrates that alley cats do not present much Malassezia, Cornish Rex do not present Malassezia, while seborrheic Devon Rex with black seborrheic deposit show growth on the contact plate. Healthy Devon Rex constitute an intermediate group between these extremes. Sphynx cats, closely genetically related to Devon Rex but lacking hair, frequently develop Malassezia otitis early in their life, often from a young age.
High-Risk Anatomical Areas
Skin folds constitute preferred anatomical areas for Malassezia proliferation. The umbilical fold of intact female Basset Hounds represents a frequently affected location, often showing material adhering to hair shafts. The facial folds of brachycephalics, notably French Bulldogs whose prevalence has exploded in the United Kingdom and North America, represent frequent locations of Malassezia dermatitis. Although this situation constitutes a disaster for animal welfare due to multiple health problems affecting these extreme brachycephalic breeds, it paradoxically ensures the financial viability of many veterinary clinics by generating a significant volume of consultations and treatments.
Malassezia Otitis: Pathogenesis and Microbial Ecology
Transition from Commensal to Pathogenic Flora
In the context of otitis, Malassezia acts as a secondary opportunistic pathogen, and the conceptual schema of otitis distinguishing predisposing, primary, secondary, and perpetuating factors proves very useful for delineating all elements explaining problematic and refractory otological cases.
Quantitative Analysis of Auricular Flora
Data from VP Hwang, one of Peter Hill’s former doctoral students, show the isolation frequency of various microbial species in otitis by quantitative culture. Coryneforms, coagulase-negative staphylococci, and micrococci present in healthy animals gradually disappear in otitis cases, while simultaneously a transition toward coagulase-positive staphylococcal and Gram-negative bacillus infections such as Proteus and Pseudomonas and similar organisms is observed. In Hwee Peng Hwang’s study, there was a fairly high frequency of Malassezia isolation in normal animals, about 25%, although sometimes lower rates are reported in other studies. Gustafson in his original work reported frequencies lower than this percentage. A small increase in frequency is observed in otitis cases compared to healthy animals. Quantitative analysis however reveals a major and fundamental difference concerning yeast population size. Twenty healthy dogs examined by semi-quantitative swab washing method showed two dogs with a single Malassezia colony and 18 dogs without detectable auricular Malassezia. Sick dogs from studies conducted recently with previous and current residents present a median population size of the order of 10^5, representing a 100,000-fold increase in population density in cases of otitis with Malassezia proliferation compared to healthy ears.
Iatrogenic Dysbiosis and Therapeutic Consequences
For many years, it has been observed that powerful antibacterial monotherapy, such as injectable enrofloxacin or piperacillin-tazobactam, effectively eliminates Pseudomonas in cases refractory to conventional treatments, but regularly creates Malassezia proliferation, or occasionally Candida. Piperacillin, a third-generation penicillin possessing extended activity against Gram-negative organisms, combined with tazobactam which blocks penicillinase like clavulanate in other formulations, allows resolution of desperate end-stage Pseudomonas otitis cases, but frequently generates subsequent yeast otitis.
Impact of Antibiotics on Flora
A study of 20 dogs currently under monitoring attempts to prevent this iatrogenic complication. Data concerning piperacillin-tazobactam demonstrate similar results although even more marked than with injectable enrofloxacin, the latter being less effective against Pseudomonas and less frequently generating subsequent yeast dysbiosis. These animals do extremely well clinically when administered piperacillin and tazobactam for their bacterial otitis, but regularly develop Malassezia proliferation requiring additional antifungal treatment.
Clinical Manifestations
Dermatological Signs in Dogs
The clinical manifestations of canine Malassezia dermatitis are varied and characteristic. A young Scottish Terrier may begin its existence with chronic dermatitis characterized by symmetrical lesions at the level of the medial thigh and groin, in the form of fairly well-demarcated alopecia plaques. A Jack Russell Terrier more advanced in the evolution of its disease presents marked excoriation and lichenification testifying to the chronicity and persistence of the inflammatory process.
Basset Hounds frequently show material adhering to hair shafts at the level of the umbilical fold. This brownish or blackish material is also found in interdigital spaces. This clinical characteristic proves diagnostically useful, because while atopic dogs, dogs infested by Trombicula (harvest mites), or affected by demodicosis present interdigital erythema, only those carrying yeasts or staphylococci develop this characteristic kerato-sebaceous debris. The neck fold of very malodorous and inflamed Basset Hounds shows similar deposits of kerato-sebaceous material.
Typical Symptomatology: Pruritus and Odor
Clinical signs include pruritus, erythema, scales and greasy seborrhea, pigmentation, lichenification, and of course, the characteristic unpleasant foul odor that accompanies some of these animals and permeates examination rooms. Paronychia with periungual crusts and brown nail discoloration may occur. Sometimes, the colored material may be superficial and can be mechanically removed, other times the stain seems to be somehow deeply impregnated into the claw keratin more permanently. An unusually frenzied nasal bridge pruritus constitutes another intriguing clinical manifestation.
Feline Particularities
Cats are not small dogs, and this fundamental statement remains true concerning Malassezia dermatitis. Allergic cats presenting intense cervicofacial pruritus may develop Malassezia dermatitis, although less frequently than dogs. The Rex cat represents the Basset Hound of the feline world in terms of predisposition to Malassezia. Devon Rex show marked susceptibility, with presence of brownish material on the abdomen and medial thigh, and blackish debris in interdigital spaces and nail folds. Sphynx cats, closely genetically related to Devon Rex but lacking hair, develop Malassezia otitis early.
Feline Paraneoplastic Syndromes
Some practitioners have encountered pancreatic paraneoplastic alopecia in older cats, a dramatic and characteristic syndrome. Sudden weight loss in an elderly cat with sudden onset of dramatic, symmetrical, and complete alopecia, sometimes accompanied by blackish deposit. Another illustrative case shows the shiny skin associated with loss of stratum corneum, a fairly distinctive characteristic of this syndrome, and the brown deposit of secondary Malassezia dermatitis.
Exfoliative Dermatitis and Thymoma
Another cat was seen by several specialized residents a few years ago. Kathy Bortnick, a dermatology resident, collected some contact plates for mycological analysis. This cat presented exfoliative dermatitis caused by a thymoma, a thymic tumor. Contact plate cultures showed Malassezia growth. Rob, a soft tissue surgery resident and pragmatic visual surgeon, surgically removed the thymus. Kathy prescribed two baths with selenium sulfide shampoo. All Malassezia completely disappeared. Surgical resection of the thymus associated with simple topical therapy totally eliminated yeast proliferation. With the paraneoplastic problem and desquamation disorder controlled, Malassezia dermatitis disappeared. The major difference between cat and dog lies in the frequency of association with serious systemic conditions, notably visceral neoplasias and metabolic diseases, triggering Malassezia as a new manifestation in an elderly animal. This possibility requires particular vigilance and thorough investigation upon sudden appearance of Malassezia proliferation in a previously healthy cat.
Diagnostic Approaches
Quantification and Detection Methods
Malassezia quantification on skin and understanding its clinical relevance constitute complex diagnostic challenges.
Contact Plate vs Cup Brushing
Contact plates represent a simple but largely underutilized method. They are fabricated from bottle caps filled to the brim with culture medium, maintained in sterile Petri dishes to preserve sterility. Direct application on a cutaneous lesion for 10 seconds, followed by three-day incubation at appropriate temperature, allows appreciation of yeast abundance by observation of confluent or scattered growth. For cats, smaller caps of reduced dimensions apply easily in interdigital spaces and other restricted anatomical sites. Cup brushing definitely constitutes essentially a research tool rather than routine clinical practice. This technique requires flat skin and a cooperative patient, conditions not always met in practice. A sterile Teflon cup containing 2 ml of physiological saline with added detergent allows gentle rubbing of targeted skin. The aspirated liquid undergoes serial dilutions until obtaining a countable number of colonies after culture, allowing extrapolation of a population density expressed in colony forming units per square centimeter. Yeasts die rapidly in physiological saline with detergent, requiring immediate sample processing, incompatible with postal shipment or prolonged transport delay. Contact plate and cup brushing counts are not well correlated statistically. Adhesive tape counts are also not correlated with cup brushing. A methodological problem therefore exists. The reference method for Malassezia culture, one could argue, remains cup brushing as the gold standard, but daily clinical practice commonly uses adhesive tape or similar more practical techniques.
Cytology and Clinical Interpretation
The adhesive tape technique, popularized by the historical publication by Keddie and Libis in Sabouraudia, allows diagnosis of microbes residing in the stratum corneum. In the United Kingdom, Scotch or Sellotape Diamond Clear are commonly used, different commercial brands surviving or not the staining process according to their composition. A practical method presented by our colleague, consists of attaching a piece of adhesive tape to the end of a glass slide, creating an assembly manipulable with one hand. This hand can separate interdigital spaces or separate the labial fold or facial fold, allowing insertion of the slide, collection of squames, then rapid Diff-Quik staining after rolling and flattening the tape on the slide.
Factors Influencing Counts
The significance of counts depends on multiple factors. The sampling method fundamentally influences results. Dry scrapings and direct impression generally do not provide sufficient material transfer compared to ‘tape stripping‘, although other practitioners may have different perspectives. The anatomical site constitutes a major factor. Contact plate counts from lips and foot differ significantly. There are more yeasts in a labial fold in a healthy dog compared to the interdigital space. How to determine which population threshold constitutes pathological proliferation if the normal population varies substantially from one anatomical site to another?
Breed Influence on Populations
Breed considerably influences normal populations. Healthy mixed breed dogs show few yeasts, and when they possess them, populations remain very low. Seborrheic Basset Hounds show a median population density much higher than healthy mixed breed dogs.
Cytological Thresholds and Research
In cats, a contact plate study in the axilla demonstrates that alley cats do not possess much Malassezia. Cornish Rex do not present Malassezia. Seborrheic Devon Rex with blackish deposit show growth on the contact plate. Healthy Devon Rex constitute an intermediate group between these extremes.
Immune Status and Hypersensitivity
The host’s immune status plays a crucial role in count interpretation. Some animals present immediate hypersensitivity detectable by IgE test, by serology or intradermal tests. Others show delayed hypersensitivity during intradermal testing. In Basset Hounds, contact hypersensitivity correlates well with disease or its absence. Healthy Basset Hounds do not show contact hypersensitivity, unlike sick ones who develop it, if the effort of performing patch-tests on dogs is undertaken, a procedure not adapted to routine daily clinical practice. As for other allergens, the IgE test constitutes precisely that: an IgE test. If an intradermal test is performed, it is not a disease test but a immunological sensitivity test. A healthy Basset Hound presenting spectacular and dramatic immediate hypersensitivity to Malassezia during intradermal testing illustrates this dissociation between sensitivity and clinical disease.
Histopathology
Histopathology can be performed to document lesions of Malassezia dermatitis. It is probably not the best method to search for something residing in the stratum corneum due to the substantial disruption that occurs in normal formalin-fixed and paraffin-embedded sections according to standard histological protocols. On a comparative slide, the normal stratum corneum shows the common basket-weave orthokeratosis that pathologists would routinely observe. In contrast, in a cryostat section—technically difficult to perform, cutting skin in a cryostat representing a technical challenge—observation reveals how much the good barrier, the intact stratum corneum, is densely packed with a large stacking of squames constituting a compact barrier. It is a processing artifact, it is absolutely not like that in real life in vivo. In a scanning electron micrograph, the large stacking of squames forming the usual barrier is visible. The lipid seal is missing, removed and eliminated by sample preparation processing, but the large stacking of squames persists.
Artifacts and Histological Observations
When a dog biopsy is examined and shows all this absent stratum corneum or all this loose and disorganized material, it is a histological artifact and not a faithful representation of the in vivo structure. Some important characteristics can nevertheless be observed, including irregular epidermal hyperplasia of the interfollicular epidermis extending into the follicular infundibulum. Keratosis is certainly present. Some degree of edema and a superficial dermal perivascular or interstitial infiltrate characterize the lesions. Due to the substantial disruption of the surface stratum corneum during histological processing, a good place to search for Malassezia resides in the follicular ostia or infundibula, structures less disrupted than the superficial stratum corneum. In a standard hematoxylin-eosin (H&E) stained specimen, if this area is examined at higher magnification, the yeasts become visible. Although these organisms can be visualized with standard H&E staining, they can be cleared and made more evident with PAS (periodic acid-Schiff) staining and even better highlighted with silver staining if necessary for confirmation.
Key Histopathological Characteristics
Histopathological characteristics described by various authors and documented are quite concise. Keratosis, either orthokeratotic or parakeratotic, epidermal hyperplasia, spongiosis. Lymphocytic or neutrophilic exocytosis and a lymphocytic or mixed dermal infiltrate. Histopathology in cats remains less well defined and documented. Hyperkeratosis and hyperplasia constitute important characteristics, but characteristics of the underlying disease triggering Malassezia proliferation could also be observed in histological specimens.
Diagnostic Algorithm
Step-by-Step Approach
The approach to a suspected Malassezia dermatitis case logically begins with a detailed history and identification of clinical signs compatible with this condition. The next step consists of demonstrating whether the yeast is present or not. This will normally be done by cytology in daily clinical practice, although in research environment this may be performed by culture, quantitative culture allowing precise population assessment. Counts do not necessarily need to be high to justify treatment. If yeasts are detected in reasonable numbers, a trial therapy must be initiated and the response carefully observed. In their absence during initial sampling, resampling of additional sites or consideration of another diagnostic explanation is required.
Interpretation of Therapeutic Response
If yeasts completely disappear and clinical signs completely resolve, the diagnosis of Malassezia dermatitis is established with confidence and the search for an underlying cause becomes priority to prevent recurrences. If yeasts completely disappear with overall partial clinical improvement, the diagnosis of Malassezia dermatitis is confirmed and investigation then treatment of residual allergy, desquamation disorder, or anatomical fold problem must be undertaken. The classic West Highland White Terrier out of control with conventional medications perfectly illustrates this situation: treatment of Malassezia and staphylococci reveals underlying atopic dermatitis which then becomes controllable with appropriate therapies for allergic management. If yeasts disappear without any clinical benefit, their presence was incidental and not causal. Partial disappearance of yeasts with partial clinical improvement suggests Malassezia dermatitis, requiring revision of therapeutic compliance, extension and intensification of treatment to