ABSTRACT: During the last 20 years, the yeast Malassezia pachydermatis has been acknowledged as a relatively common cause of dermatitis in dogs. The organism occurs naturally on the skin, mucosal surfaces and in the ear canals of dogs, but it can proliferate in response to abnormalities, for example, associated with allergic diseases, keratinisation defects and hormone disturbances.

Certain breeds, such as West Highland white terriers, Basset hounds and Cocker spaniels are thought to be more susceptible and it is now recognised that certain presentations, historically considered to represent ‘idiopathic’ seborrhoeic dermatitis, actually reflect infection with M. pachydermatis.

Approximately one third of dogs with atopic dermatitis have concurrent Malassezia dermatitis, so treatment of secondary infection and underlying disease is, therefore, fundamental to the successful management of these cases.

Clinical signs

The clinical signs may mimic or complicate allergic skin diseases. The most obvious signs will include pruritus, erythema, greasy exudation and traumatic alopecia. Dogs with skin folds, may be at particular risk of superficial bacterial or Malassezia infection; concurrent overgrowth of both bacteria and yeasts is common.

Lesions may be localised or generalised, but the external ear canals (Figure 1), interdigital areas (Figure 2) and skin folds are frequently affected (Figure 3), probably in part owing to a favourable microclimate.

Figure 1: Pruritic area and greasy exudate within exterior ear canal

Figure 2: Interdigital pedal erythema with greasy exudate

Figure 3: Basset hound with intertriginous pruritic areas in the skin folds

Hyperpigmentation, lichenification and malodour may be prominent in chronic, generalised cases (Figure 4). Some dogs with pedal pruritus have Malassezia infections of the nail fold, characterised by brown discolouration of the claws. 

Figure 4: Lichenification and hyperpigmentation typical in chronic M. pachydermatitis

Diagnosis

The cytological examination of smears or tape-strip preparations are rapid and inexpensive methods for determining skin populations of M. pachydermatis in dogs and they can be undertaken by the veterinary nurse, under the direction of the veterinary surgeon. The yeast is readily recognised in Diff-Quik-stained specimens owing to its characteristic peanut-shape (Figure 5).

Figure 5: Characteristic peanut shaped M.pachydermatitis yeasts

There are conflicting data on what should be considered as abnormal numbers of yeasts and the breed and site will influence ‘normal’ populations. Research conducted at the Royal Veterinary College (RVC) has shown that population sizes in the axillae of unaffected Basset hounds may overlap with those of hounds with seborrhoeic dermatitis associated with the yeast.

The outcome of colonisation and infection, and any resulting disease, will depend in part on the immune response of the host, rendering attempts to define ‘significant’ populations futile. One useful approach is to consider the yeast to be potentially significant if it is readily found in consistent skin lesions, and to then determine its importance by trial therapy. Repeat sampling of lesions that persist after treatment can help differentiate ongoing infection from another concurrent disease.

An evaluation by the veterinary surgeon for underlying diseases that may favour infection is also an important part of diagnosis.

Treatment

As the yeast is located within the superficial, cornified layers of the skin, topical therapy can be the sole treatment for Malassezia dermatitis, provided there is good compliance.

In a recent systematic, evidence-based review of interventions for Malassezia dermatitis in dogs, the authors concluded that there was good evidence for the use of one topical treatment (2% miconazole nitrate plus 2% chlorhexidine shampoo, twice weekly for three weeks) (Figure 6).1 Regular maintenance treatment is often needed at intervals determined by clinical observation. Veterinary nurse clinics are an excellent way to improve client compliance – by educating them on the importance of therapy, teaching shampooing methods, encouraging them to discuss their concerns and ensuring regular contact with the practice are all good ways to increase the chances of success.

Figure 6: Post treatment, showing mild residual pruritic areas

In cats, generalised Malassezia infections are uncommon and have been mainly associated with severe systemic illness. More recently, yeast overgrowth has also been shown to occur in allergic cats and in the Devon Rex breed. Infection frequently affects the ears or claw folds, but generalised erythematous and greasy seborrhoea may also be seen in some cases.!?

Case study

A five-year-old, neutered, male Basset hound was referred to the Queen Mother Hospital at the Royal Veterinary College, with a long history of severe pedal pruritus. Gradually increasing dosages of glucocorticoids had been required to control the pruritus. Skin biopsies taken from new erythematous, plaque-like lesions on the dorsum indicated calcinosis cutis (Figure 7). Glucocorticoid treatment had subsequently been discontinued and replaced with ciclosporin, but pedal pruritus had persisted.

Figure 7: Erythematous plaque-like skin lesion of calcinosis cutis

On examination, the dog was in good general health, but alopecic, erythematous plaques affected the dorsum from the shoulder to the lumbar area. There was moderate erythema of the interdigital skin of all feet with greasy exudation, and the pinnae were mildly erythemat
ous.

No ectoparasites could be demonstrated on hair plucks and skin scrapings, but tape-strip cytology from interdigital skin showed large numbers of Malassezia yeasts. Malassezia dermatitis was, therefore, suspected as a cause of the pedal pruritus and twice-weekly shampooing with a two per cent miconazole and two per cent chlorhexidine shampoo (Malaseb, Dechra Veterinary Products) was introduced.

Yeast counts and pedal pruritus were markedly reduced after three weeks of treatment. At this point, a short course of oral itraconazole was added to the regimen during a period when bathing proved impractical.

Six weeks after presentation, yeasts were no longer seen on cytology and ciclosporin was discontinued. Mild residual pedal and pinnal erythema and pruritus was not influenced by dietary restriction and underlying low-grade atopic dermatitis was diagnosed.

Miconazole and chlorhexidine shampoo treatment of the paws maintained control of the pruritus without need of additional anti-inflammatory therapy. The calcinosis cutis slowly resolved over a three-month period. 

Author(s)

Ross Bond

BVMS PhD DVD DipECVD FHEA MRCVS

Ross Bond graduated from Glasgow University in 1985 and worked in large and small animal practice for five years. He first joined the Royal Veterinary College in 1990 and worked as a resident, research student and lecturer in dermatology until 1998. After a brief spell in a referral dermatology practice, he returned to the RVC where he is now a senior lecturer in veterinary dermatology.

Katarina Varjonen MRCVS

Katarina Varjonen graduated from the Swedish University of Agricultural Sciences in 2000. In September 2007, having worked in small animal hospitals in Sweden, she started her senior clinical training scholarship at the Royal Veterinary College.

Reference

1. NEGRE, A., BENSIGNOIR, E., GUILLOT, J. (2009) Evidence-based veterinary dermatology; a systematic review of interventions for Malassezia dermatitis in dogs. Veterinary Dermatology 20 (1): 1-12.

Additional reading

BOND, R. (2005) Malassezia d ermatitis. In: Infectious Diseases of the Dog and Cat, Third Edition.

Ed: C. E. Greene. Saunders Elsevier, St Louis. pp 565-569.

GUADUERE, E., PRELAUD, P. (1996) Etude retrospective de 54 cas de dermites a logiques, cliniques, cytologiques et histopathologiques, PractMed, ChirAnimComp. 31 (4): 309-323.

LLOYD, D. H. (1993) Bacterial and fungal skin disease in animals, In: Noble, W. C. Ed, The Skin Microflora and Microbial Skin Disease, Cambridge, Cambridge University Press, pp. 264, 271-272, 281.

MASON, I., MASON, K. V., LLOYD, D. H. (1996) A review of the biology of canine skin with respect to the commensals Staphyloccus mtermedius, Demodex cams and Malassezia pachydermatis, Veterinary Dermatology, 7 (3): 119-132.

• VOL 25 • No4 • April 2010 • Veterinary Nursing Journal