Malassezia and Seborrhoeic Dermatitis
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 in 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. Pachydermatitis.
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.
The clinical signs may mimic or complicate allergic skin diseases. The most obvious signs will include pruritis, 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 left), interdigital areas (figure 2 below right) and skin folds are frequently affected, probably in part owing to a favourable microclimate.
Hyperpigmentation, lichenification and malodour may be prominent in chronic, generalised cases. Some dogs with pedal pruritis have Malassezia infections of the nail fold, characterised by brown discolouration of the claws.
The cytological examination of smears or tape-strip preparations are rapid and inexpensive methods for determining skin populations of M. pachydermatitis 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 3 below, with thanks to Virbac).
There are conflicting data on what should be considered as abnormal numbers of yeasts, and 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 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.
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). 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. These are all good ways to increase the chances of success.
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.
This article was kindly provided by Dechra Veterinary Products, makers of Malaseb. This article was previously published in 2010.