Paul B. Bloom, DVM, DACVD, DABVP (Canine and Feline Specialty)
Allergy, Skin and Ear Clinic for Pets, Livonia, MI; Department of Dermatology, Michigan State University, East Lansing, MI, USA
Frequently, when a dog is presented with a skin problem the owner also complains about the smell. Although many times the odor does emanate from the skin, including the ears or anal glands, it is important to rule out dental disease, gastrointestinal (flatulence) or an unfortunate encounter with a skunk. Sometimes, dogs enjoy rolling in things that can produce a fairly strong odor. Assuming that the odor is dermatologic in origin, in addition to a thorough dermatologic examination, we should closely examine the ears, anal glands, lip folds and other intertriginous areas for disease. If it is a generalized odor, seborrhea oleosa, bacterial pyoderma/overgrowth or Malassezia dermatitis should be considered. Remember that none of these are a primary disease. Hypersensitivities and endocrinopathy are the most common primary causes for these secondary problems.
When an owner presents a dog with dermatologic disease, skin cytologies and skin scrapings should be included in your minimum data base (Note: if ear disease is present, ear cytology should be performed). Since bacterial pyoderma and Malassezia dermatitis is a frequent problem in dogs with skin disease/odor, these will be addressed in more detail. Discussion of the other causes of odors is beyond the scope of this lecture.
Superficial bacterial folliculitis (SBF) is more common in the dog than any other mammalian species. As opposed to humans with Staphylococcus aureus infections, Staphylococcus virulence factors, such as protein A, leukocidin, hemolysins, epidermolytic toxin have not been shown to play a role in canine pyoderma. Numerous studies have been unable to identify any differences in toxin profiles between Staphylococcus pseudintermedius from normal dogs and dogs with pyoderma. Because Staphylococcus pseudintermedius is a normal commensal of the dog and there are no virulence differences between strains, it appears that abnormal 'host factors' are an important cause of pyoderma in dogs. These include hypersensitivities, endocrinopathies and cornification abnormalities.
Remember that there is no such thing as a primary bacterial pyoderma in the dog - there is always a "due to." A dog presented for the first time with a SBF may only need to have a limited number of diagnostic tests performed. A recurrent or chronic case of SBF or any dog with a deep bacterial pyoderma will need to have the underlying cause aggressively pursued.
Causes of SBF (deep pyoderma causes are marked with an *) include:
1. Hypersensitivities (atopy, cutaneous adverse food reactions, FAD)
2. * Spontaneous hyperadrenocorticism or exogenous steroid exposure
3. * Demodicosis
4. * Hypothyroidism
5. Follicular dysplasias (e.g., color dilution alopecia, Chinese crested dogs)
6. Cornification abnormalities (sebaceous adenitis, ichthyosis)
Systemic therapy for canine pyoderma is becoming more problematic because of the increasing incidence of methicillin-resistant Staphylococcus. Topical therapy, either as a monotherapy or as part of polypharmacy, is becoming even more important. Topical therapy may not only decrease or eliminate the need for systemic antibiotics but, since many of the dogs with SBF have atopic dermatitis, bathing is also useful because it removes antigens from the skin. The limitations of using topical therapy include time constraints of the owner and the cost of the medication, which may be significant if the treatment involves a large area.
Shampoo ingredients that are effective for treating bacterial pyoderma include chlorhexidine, benzoyl peroxide, ethyl lactate, acetic acid/boric acid and triclosan with chlorhexidine being the most effective.
Topical therapy with mupirocin is very useful. Not only is it a very effective antimicrobial agent against gram-positive bacteria, including Staphylococcus aureus, but due to its unique mode of action, cross-resistance with other antibiotics is very uncommon. Note: if the Staphylococcus is reported as resistant to clindamycin, there is a nine-fold increase in the incidence of resistance to mupirocin.
Silver sulfadiazine has traditionally been used for its effectiveness against gram-negative bacteria, especially Pseudomonas. More importantly in regards to SBF, it is also effective against some gram-positive bacteria, including Staphylococcus aureus.
When treating a dog with a SBF, an antibiotic should be administered for at least 21 days or 14 days past your clinical examination that has determined the infection has resolved, whichever is longer. For dogs with deep pyoderma, treat for at least 6 weeks or 21 days beyond clinical resolution, whichever is longer. In cases of SBF, do not use glucocorticoids (GC) when the pruritus is only at the lesions or when the pruritus is only mild at the nonlesional areas. If a dog with SBF has intense pruritus at nonlesional areas, then a tapering 21-day course of prednisone may be dispensed. Using GC in the presence of a pruritic pyoderma makes interpretation of response to therapy impossible (was it the steroid or the antibiotic/antifungal therapy that resolved the pruritus?). It also makes it more difficult to resolve the infection. Never use GC in cases of deep pyoderma!
Appropriate first-line antibiotics for a bacterial pyoderma are: cephalexin (or other first generation cephalosporin), a potentiated sulfa (trimethoprim/sulfonamide or sulfadimethoxine and ormetoprim), clindamycin, or a potentiated amoxicillin (amoxicillin/clavulanic acid).
Malassezia dermatitis is another frequent dermatologic cause for a 'smelly dog'. Malassezia is a genus of lipophilic yeast found as a commensal of the skin and mucosal surfaces that may cause skin disease in a variety of mammalian species. In normal dogs, these organisms are present in very small numbers on the skin (intertriginous regions), oral and anal mucosal surfaces, in the ear canals and the anal sacs. Since Malassezia is a commensal, why do some dogs develop Malassezia dermatitis (MD)? Disorders that affect the barrier function of the skin (e.g., pruritic skin disease) or the cutaneous lipid content (e.g., hypothyroidism) allow Malassezia overgrowth. In a genetically predisposed dog, a type I and/or type IV hypersensitivity reaction to Malassezia will occur.
Lesions associated with MD include lichenification, erythema, greasy exudate, dry scale (seborrhea), excoriations, papules, plaques, alopecia, or hyperpigmentation. A moist dermatitis with a musty odor is frequently present. Pruritus may vary from mild to intense and erythema may be present with minimal pruritus, especially interdigitally. The lesions may be focal or generalized and the distribution of the lesions overlaps with other pruritic diseases. Areas affected include the intertriginous regions, face, nail folds, perioral, pinna and flexor surface of the elbow.
Confirmation of MD occurs when on skin cytology either 1 organism is seen every 3–4 fields (1000x) or if any field has more than 1 organism. Malassezia organisms are budding yeast in the shape of a peanut or a footprint.
Treatment involves eliminating the overgrowth and, since MD is a secondary disease, identifying and managing the underlying disease. As previously mentioned, any disease that disrupts the barrier function, the lipid content of the skin surface, the cutaneous microclimate, or host defense mechanisms, may predispose the animal to MD. These include hypersensitivities (atopy, cutaneous adverse food reactions), ectoparasites (Demodex, Sarcoptes and fleas), endocrinopathies (hypothyroidism, hyperadrenocorticism), metabolic epidermal necrosis, cutaneous T-cell lymphoma and excessive skin folds. Bassett Hounds are genetically predisposed to having a higher number of Malassezia organisms on their skin, making them a greater risk for developing MD.
Unless the MD is very focal, the author prefers both topical and systemic therapy. This increases the likelihood that at the recheck examination the MD is resolved and any remaining pruritus is the result of the underlying hypersensitivity reaction. The author most commonly uses shampoo containing at least 3% chlorhexidine or 2% chlorhexidine combined with an azole. This is followed by a leave-on conditioner containing 2% miconazole. Depending on the severity and extensiveness of the lesions, the frequency of application varies from daily to 3 times weekly.
Ketoconazole (200 mg tabs) 5–10 mg/kg SID is the systemic drug of choice. If the dog does not tolerate ketoconazole, itraconazole 5 mg/kg given 2 consecutive days/week is also effective, albeit more expensive. If the dog has liver disease or is very small, fluconazole (comes in 50 mg tabs) - 5–10 mg/kg daily would be a good choice since fluconazole is not metabolized or excreted by the liver. Treatment should be continued for 14 days beyond clinical resolution based on your examination (not a phone call) with a minimum treatment time of 21 days. Please note that griseofulvin is ineffective against Malassezia.
Be sure to evaluate the dog for concurrent superficial bacterial pyoderma, since MD and bacterial pyoderma frequently occur simultaneously in dogs. In cases of concurrent superficial bacterial pyoderma, antibiotic therapy should be used simultaneously.
References are available upon request.