Yeast dermatitis is a fairly common inflammatory skin condition caused by overgrowth of Malassezia spp. of yeast, which are normal inhabitants of the skin, ears and mucocutaneous areas of dogs. Malassezia pachydermatitis, the most common type of yeast causing skin infections in domestic dogs, is lipophilic, meaning that it has a particular affinity for fatty tissues and cells. Yeast infections occur when the microorganism reproduces uncontrollably, over-populating and invading the areas where it normally resides.
Causes of Yeast Infection in Dogs
"Yeast" is an often-used but poorly-understood word. Yeast is a general term for unicellular, nucleated, usually round-shaped fungi that reproduce by budding. Budding is a form of asexual reproduction where a portion of the cell body is pinched off and becomes a new individual yeast cell. Because Malassezia is a normal resident of canine skin, it only becomes problematic when it changes from a harmless to a pathogenic form. The precise causes of this transformation are poorly understood.
Some factors that may contribute to yeast infections include allergies to fleas or other external parasites (which disrupt the skin barrier), food allergies, nutritional deficiencies, prolonged use of corticosteroids (which suppress normal immune function), endocrine disorders, cancer, chemotherapeutic treatments, immunosuppressive illness and concurrent bacterial skin irritation or infection. Yeast overgrowth causes increased oil production from the dog's skin, accompanied by severe itching (pruritis) and continual scratching that can create sores and further weaken the outer layers of skin, providing an environment where more yeast can flourish.
Prevention of Yeast Infection
Malassezia infections are common in dogs, but thankfully are somewhat preventable and largely treatable. Dogs with conformational predispositions to developing these infections should be kept especially clean and dry in and between their ears, facial folds, toes, armpits and other skin wrinkles. Dogs with predisposing causes to yeast overgrowth should be treated for those conditions.
Only a veterinarian can properly assess a yeast infection and prescribe appropriately tailored management protocols to eliminate yeast overgrowth, reduce the dog's discomfort, manage or eliminate secondary bacterial infections and reduce the chances of reinfection. Malassezia yeasts are reported to have been transmitted from the hands of healthcare workers (who owned infected dogs) to patients they handled in a human intensive care nursery, causing systemic fungal infection in those infants. Accordingly, canine yeast infections should be considered zoonotic, meaning that they have the potential to be transmitted from dogs to people – especially people with compromised immune status, such as the very young, the very old or the very ill.
Cutaneous (skin) yeast infections are common in domestic dogs and are most frequently caused by an overgrowth of Malassezia species of yeast. These infections cause a number of annoying symptoms, both for affected dogs and for their owners.
Symptoms of Canine Yeast Infection
When the normal reproduction cycle of yeast becomes out-of-control, the organisms colonize, invade and damage the dog's skin, which sets up a cycle for accelerated yeast overgrowth. The most obvious observable symptoms of yeast overgrowth include one or more of the following:
Intense itchiness (pruritus)
Skin irritation and inflammation, especially in and around the ears, between the paw pads and digits (toes), on the nasal folds, anal area, armpits (axillae) and neck
Scratching (frenzied scratching and chewing can lead to weeping sores around the neck, ears, tail base, armpits and elsewhere)
Skin redness (erythema)
Hair loss (alopecia)
Scaly skin ("dandruff")
Greasy, oily skin
Greasy haircoat (sometimes, so much oil is produced that the dog will leave greasy patches on its bedding)
Foul-smelling, rancid skin (often overwhelmingly offensive to owners)
Coarse, thickened skin
Ear infection (severe; usually with a smelly, yellowish-green discharge)
Raised, red crusty areas on the skin
Weeping skin sores, usually from self-trauma
Behavioral changes associated with pruritis and pain, such as: Depression, Loss of appetite (inappetence; anorexia), Anxiety, Aggression
Dogs at Increased Risk
Dogs of any age, gender or breed can develop yeast infections. Certain breeds are more prone to developing yeast infections, either because of genetics or because of breed characteristics such as thick, wrinkled skin that provides a moist environment for yeast overgrowth. Those breeds include the West Highland WhiteTerrier, Poodle, Basset Hound, Cocker Spaniel, German Shepherd Dog and Dachshund. Yeast infections are more common in hot, humid environments.
Yeast infections in companion dogs usually first show up as intense skin itchiness (called pruritis), with accompanying scratching and biting at affected areas. The most common causative organism is Malassezia pachydermatis. It can be rather difficult to diagnose this fungal/yeast infection.
How Yeast Infection Is Diagnosed in Dogs
The initial database for a dog presenting with severe itchiness and other signs of a cutaneous yeast infection includes a complete history and physical examination, together with sampling of affected areas of skin. Direct impressions can be taken by pressing a glass slide onto the skin sores, or by pressing acetate tape to the lesions and then applying the tape to the surface of a glass microscope slide. These samples are examined microscopically, with or without specific staining processes, using high power or oil immersion to identify round-to-oval budding forms of yeast that indicate the presence of Malassezia. Samples of waxy debris from infected ears can be taken with a cotton swab, rolled on a glass slide and examined in the same way to identify yeast, bacteria, mites or other possible causes of the dog's condition. Skin scrapings and bacterial or fungal culture on specific growth media are used to rule out other bacterial or fungal organisms and certain external parasites, such as Demodex mites.
Skin biopsies can be taken and examined by a process called histopathology. Unfortunately, while histopathologic findings may suggest the presence of Malassezia, they are not always diagnostic. Various types of allergy (hypersensitivity) tests and food allergy trials can be used. Systemic diseases and overall organ health can be assessed by routine blood work (complete blood count and serum biochemistry profile) and urinalyses. More advanced tests are available to assess endocrine function, among other things.
If a dog's ears are infected, it is extremely important to assess whether the eardrums (tympanic membranes) are intact before any liquids, gels, cleansers or other medications are applied, to prevent potentially serious damage to structures of the middle and inner ears.
Yeast infections of the skin of dogs are common, but thankfully they are largely treatable. The most frequent culprit is Malassezia pachydermatis. Only a veterinarian can assess the infection and prescribe appropriately tailored treatments designed to eliminate yeast overgrowth, reduce the dog's itchiness and other symptoms of discomfort, manage or eliminate secondary bacterial infections, identify and treat any predisposing factors, resolve the accompanying scaling, exudation and foul odor and reduce the chances of recurrence.
Treatment Options for Canine Yeast Infection
Current treatments for canine skin infection by Malassezia organisms include administration of systemic oral drugs, application of topical medications and resolution of the underlying cause of the condition.
In most cases, topical anti-seborrheic, antibiotic and/or anti-fungal/anti-yeast medications are recommended. Depending upon the formulation (medicated shampoos, solutions, lotions, rinses, sprays, wipes or powders), these are applied daily or several times per week.
Systemic prescription anti-fungal drugs may be warranted in severe cases of yeast infection, or in those cases that do not respond appropriately to other therapies. Currently, ketoconazole, itraconazole, fluconazole and tervinafine have been used with some success as multi-week, long-term oral anti-fungal treatments for canine Malassezia infection. In severe cases, secondary Staphylococcal bacterial skin infections often develop. If they do, oral antibiotics at an appropriate dose and for an appropriate duration may be added to the treatment regimen.
Of course, addressing and resolving the underlying cause of a yeast infection is the best way to manage its course. As Malassezia is a normal inhabitant of canine skin, the signs caused by "infection" with this yeast occur only when the organism overgrows abnormally.
The prognosis for dogs with cutaneous yeast infections is quite good. However, many dogs will require periodic lifelong treatments to manage future outbreaks. A veterinarian is in the best position to provide advice as to the best treatment options for dogs.