Sonya V. Bettenay, BVSc (Hons), MACVSc, DED, FACVSc, DECVD
Inflammatory alopecia is hair loss associated with clinical lesions, which typically include erythema, papules, pustules and crusts. This is frequently accompanied by pruritus and associated licking, rubbing and/ or biting. Pruritus is a hallmark of many different diseases (see Table 1) but pruritus without significant clinical lesions in the dog and cat is associated with only a few: atopic dermatitis and food adverse reaction, and (less typically) infections with bacteria, yeasts or ectoparasites (scabies incognito, cheyletiellosis). These diseases may present with only subtle clinical lesions such as alopecia or alopecia and erythema; a careful physical examination with good lighting is indicated with each presentation of pruritus and / or alopecia.
Table 1. Differential diagnoses list of pruritic patients (not complete).
Flea bite hypersensitivity
Systemic lupus erythematosus
Epitheliotropic T-cell lymphoma
Important Diagnostic Steps
A detailed clinical history and a complete physical examination are important in all dermatology cases, but the detail of the history taking may--for practical reasons--be minimized with a first time occurrence of inflammatory alopecia.
Diagnostic tests aim to look for and to rule out the most common causes of inflammatory alopecia, namely the ectoparasites and secondarily infected allergies. Surface cytology and superficial as well as deep skin scrapings should be performed in all cases of inflammatory alopecia and trials (ectoparasite treatment trial, elimination diet) are indicated depending on the clinical lesion distribution, degree of pruritus and history clues.
Client questionnaires are completed either in my waiting room or, preferably, at home by the client prior to the appointment. This saves time in the consult room, allows a more thorough history with every client every day and gives the client time to think about their answers, discuss with their partner and possibly also to find old medicine bottles / shampoo bottles / food packets etc to bring to the appointment.
There are some clues which can be gleaned from the history:
Age of onset of disease: In young animals, dermatophytosis, demodicosis or ectoparasites (fleas/Sarcoptes) are common causes of inflammatory alopecia. Allergies typically onset between one and three years of age, although this is a rule of thumb timeline only. Dogs as young as 12 weeks have been diagnosed with both atopy, flea bite hypersensitivity and food allergy! Inflammatory alopecia which onsets in older dogs suggests either an endocrinopathy with secondary seborrhea or infection or a cutaneous neoplasia such as epitheliotropic lymphoma.
Seasonality: Is strongly suggestive of atopic dermatitis.
Otitis externa: A history of recurrent otitis externa suggests an underlying disease such as allergy (food or atopy) or endocrinopathy (hyperadrenocorticism, hypothyroidism, sex hormone imbalance.)
The presence of concurrent companion animal or human dermatitis: Is suggestive of dermatophytosis, cheyletiellosis, Sarcoptes, Otodectes or simply fleas.
Response to therapy:
A response to antibiotics does not provide a diagnosis of idiopathic pyoderma. Both allergic and hypothyroid dogs can exhibit a 100% clinical resolution of pruritus and lesions while on antibiotic therapy only to relapse following the discontinuation of therapy.
A response to antihistamines as the sole therapeutic agent is strongly suggestive of allergies.
A response to insecticidal/ ascaricidal treatment obviously supports an ectoparasitosis.
Type of Lesion
When alopecia is the presenting complaint, the physical examination aims at identifying the presence (or absence) of primary or secondary lesions. If erythema, scale, papules, pustules, crusts or other lesions (such as erosions / ulcers) are present then by definition, this is an inflammatory alopecia.
The type of lesion can give clues as to the possibility of an infectious component and also as to the diagnostic test needed. For example any lesion with a follicular orientation (follicular pustule, follicular papule, comedone, follicular cast) leads one to a differential diagnosis of the 'big 3' causes of folliculitis. The 'big 3' include staphylococcal folliculitis, dermatophytosis and demodicosis. As such, the presence of these follicular lesions requires the following tests to be conducted: Woods light (for the 50% of dermatophytes which are M. canis); deep skin scraping for Demodex (with a good squeeze both prior to and during the scraping wherever possible); trichogram (for the possibility of dermatophytes, but also for the possible diagnosis of Demodex) and impression smears with pressure.
The presence of comedones suggests a possible endocrinopathy, depending on the age and the location of the lesions. Comedones on the chin, for example, are not generally suggestive for a systemic disease.
The Distribution Pattern of Lesions
The actual areas affected on the body often give the leading clue in the underlying etiology of inflammatory skin disease. A summary is given in Table 2.
Table 2. Localization of lesions and the common underlying diseases.
Localization of lesions
Possible underlying disease
Atopy, food adverse reaction, demodicosis, 2° pyoderma, pemphigus, DLE, 'other' immune, scabies*/Notoedres++
Scabies*, contact allergy (inner pinna), atopy, food adverse reaction, pemphigus
Paws and distal legs
Atopy, food adverse reaction, demodicosis, 2° pyoderma, pemphigus, 2° Malassezia dermatitis
Atopy, food adverse reaction, scabies* (in conjunction with elbows)
Allergies, scabies*, 2° pyoderma, demodicosis
Flea bite hypersensitivity
Atopy, food adverse reaction, flea bite hypersensitivity (esp. under the tail folds *) 2° Malassezia dermatitis, tape worms, anal glands*
Skin Surface Cytology
Surface cytology is typically performed to look for bacteria and yeast organisms. 'Dry skin scrapings' for cell cytology, so called 'sticky tape' preps, impression smears with regular and with 'adhesive' slides, impression smears with pressure, pustule rupture and impression and impressions from the underside of crusts are all options. The techniques to be used will vary depending on the individual clinician's experience and preference and of course on the site and type of lesions and skin present.
Cytological Preparations for Organisms
When organisms are identified one assumes that yeast are Malassezia and that the bacteria are Staphylococcus sp., although this is only an assumption as a culture is necessary to specifically identify the type of organism. The presence of large numbers of rod shaped bacteria on cytology from the skin surface is uncommon, but should not always be interpreted as a serious infection, it may represent only an overgrowth. The presence of rod shaped bacteria within certain anatomic sites however, for example ear canals or interdigital dermatitis of multiple feet, is almost always of concern and an indication to recommend a bacterial culture and sensitivity. The presence of intracellular bacteria (rod or coccal shaped) on any site indicates the need for a systemic antibiotic and is also reason enough alone to recommend a bacterial culture.
Cytological Preparations for Cells
Granulocytes (eosinophils, neutrophils) are the inflammatory cells which are most commonly seen with surface cytology. The presence of eosinophils is suggestive of an ectoparasite or flea bite hypersensitivity. However in cats, eosinophils are commonly seen with any or all of the hypersensitivity reactions. Neutrophils are called whenever there is keratinocyte injury, so they should be anticipated to be present with pustules, erosions, ulcers and crusted papules.
Superficial and Deep Skin Scrapings
If an infection is present, it needs to be identified and treated and the animal needs to be reevaluated following the therapy. If cytology and skin scrapings do not reveal any pathological changes, then trial therapies (ectoparasites, elimination diets) may be indicated.
Trial therapies are typically conducted to investigate for the possibility of food hypersensitivity and/or for flea bite hypersensitivity and ectoparasites such as scabies, Otodectes and Cheyletiella.
The topic of how one conducts a successful elimination diet is covered elsewhere in this conference. If more than one diagnostic trial is performed concurrently and the patient goes into remission, then a food rechallenge will differentiate food allergy from the ectoparasite(s) as the underlying predisposing cause. If there is not much improvement on combination trial therapy (for example food and ectoparasite), then atopic disease is a likely cause of the pruritus and secondary infections.
Skin biopsies can be an extremely useful diagnostic tool, but they cannot be expected to replace a good clinical work-up. A 'non-diagnostic' biopsy can nevertheless rule out a number of conditions which were on the list of possible diagnoses (for example dermatomyositis, other immune mediated disease, ectoparasites).
The finding of 'luminal folliculitis' even with no agents identified, can be interpreted as follows: it can lead one to follow the probability of a staphylococcal folliculitis; it can be interpreted (if enough follicles were sampled) as having ruled out Demodex and it can still suggest the possibility of dermatophytosis and the need for a fungal culture (by McKenzie Toothbrush technique).