The pruritic dog: Differential diagnoses (Proceedings)


A systematic approach to a dermatologic diagnosis begins with a good history followed by a thorough physical examination.


A systematic approach to a dermatologic diagnosis begins with a good history followed by a thorough physical examination. The information obtained by these two steps forms the basis to make a list of initial differential diagnoses. Often in a busy practice there is a conflict between the time needed to obtain a thorough history and physical examination and the time allotted for the client visit. By focusing on the most pertinent historical and physical findings that match specific disease "profiles" an initial diagnoses list may be formulated in a shorter period of time. This list of differential diagnoses is then used as the basis to formulate diagnostic and therapeutic plans based upon the rank order of the diagnoses.

Primary complaint:

The primary complaint provides the basis for identifying the pertinent information needed to compile the initial differential diagnoses. Often, more specific questions are asked based on the physical findings. When the primary complaint is pruritus, it is most often associated with other secondary issues including exfoliation (scales, crusts), alopecia or yeast infection. The etiologies of the majority of the primary complaints are relatively few. There are a limited number of "key" historical and gross findings needed to make the initial differential diagnoses list. It is important to include as many of the likely diagnoses in the initial list since both primary and secondary or other concurrent problems must be addressed at the same time.

Historical and clinical profile:


Pruritus is the most common complaint associated with canine dermatologic problems. Pruritus usually includes observation of licking, biting and/or scratching. It is known that almost any dermatoses may have some degree of pruritus. However, if pruritus is the primary complaint there are four diagnoses to consider in most cases. These include atopy or food allergies (ARF, adverse reactions to food)), fleas and Sarcoptic mange. The history and clinical findings associated with these diseases are predictable. There are reliable definitive diagnostic tests to help refine the differential diagnoses list by "ruling in" or "ruling out" specific diseases..

In the atopic dog approximately 75% will show some pruritic symptoms between 1and 3 years of age. A dog with onset of clinical signs over 6 years of age is much less likely to have atopy. Pruritus is the primary historical and clinical observation. Pruritic symptoms in a breed predisposed to atopic dermatitis increases the probability of atopy. A drug history of temporary response to anti-inflammatory doses of glucocorticoids, antihistamines or cyclosporine is supportive of atopy. If there is a concurrent chronic or relapsing otitis or superficial pyoderma associated with pruritus, allergy including atopy is likely. A history of seasonal symptoms in dogs with preventative flea control is most likely atopy. The face feet, extremities, axillae, and ears (medial pinnae and canals) are common distribution sites for atopic disease. In approximately 85-90% of the allergic dogs, atopy is the cause. The diagnosis of atopy is based on the history and clinical signs. The allergen specific tests (serum or intradermal) only identify the specific air-borne allergens that may be causing the pruritus.

A food allergy or food intolerance (ARF) is a potential cause of nonseasonal pruritus. In approximately 10% of the dogs, there will be concurrent gastrointestinal symptoms. The average age of onset is similar to atopy (1-3 years). However, ARF may be observed in a dog less than 6 months of age or in the older dog with no prior history of pruritus. The majority of food allergic dogs will respond to antiinflammatory doses of glucocorticoids. The distribution of lesions for ARF and atopy is similar. In most surveys 10% to 15% of the allergic dogs have a food allergy. Up to 50 % of the dogs with food allergies also have concurrent atopy. A hypoallergenic diet trial is the only reliable diagnostic test to confirm a food allergy.

Flea infestation or flea bite allergies must be considered for any pruritic dog of any age that has lesions on the dorsal lumbar area and is not maintained on adequate preventative flea control. There may be a more generalized distribution of lesions as the result of a flea infestation or caused by other conditions. The finding of fleas or flea excrement with visual observation or flea comb is diagnostic. Maintenance of adequate preventative flea control is indicated whether or not fleas have been identified as a cause of the pruritus,

The last common differential for a primary complaint of pruritus is Sarcoptic mange. A dog with Sarcoptic mange has a very predictable profile including the sudden onset of intense pruritus at any age, poor response to anti-inflammatory doses of glucocorticoids and often multiple pruritic dogs with similar lesions. The initial lesions usually involve crusts on the pinnal margins, lateral elbows and hocks. This distribution is in contrast to atopy or food allergic dogs that have lesions involving the medial pinnae, axillae and forearm. The lesions may become generalized. In approximately 30% of the Sarcoptic mange cases, one or more family members will have pruritic lesions starting about the same time as the pruritic symptoms of the pet. Scrapings yield a positive diagnosis in approximately 50% of the dogs. A therapeutic trial with a scabicide is indicated if Sarcoptic mange is on the initial diagnostic list and scrapes are negative.

Pruritus with exfoliative dermatitis:

Exfoliative lesions may be either scales or crusts. In many cases both lesions are observed. The initial list of differential diagnoses for dogs in which excessive scaling accompanies pruritus includes Cheyletiellosis, Demodicosis, sebaceous adenitis, zinc responsive dermatosis, dermatophytosis and epitheliotropic cutaneous lymphosarcoma. Although the potential list long, there are a limited number of differentiating factors that help narrow the differential diagnoses list.

Cheyletiella infestation (walking dandruff mite) is associated with surface irritation and scaling on the dorsal trunk. There may be mild inflammation and variable alopecia and pruritus. Cheyletiella must be considered if there is a history of no preventative flea control, It is most common in dogs that have been in a shelter or a shelter animal has been introduced into the home environment. The administration of glucocorticoids may suppress the mild pruritus associated with Cheyletiellosis. A diagnosis can usually be made with a tape preparation or surface scrapes. A therapeutic trial with flea products may be indicated. The incidence in dogs in a home environment is low.

Sebaceous adenitis is characterized by a tightly adhered scale that can be multifocal, regional or generalized. There is variable alopecia from minimal to marked, variable pruritus and erythema. Typical lesions in the predisposed breeds (Standard poodle, Vizla, Akita, Samoyed) of 1 to 5 years of age warrant high consideration. Sebaceous adenitis may be seen in other breeds. There is generally a poor response to corticosteroids. Biopsies are the definitive diagnostic test. The incidence is relatively low.

Dermatophytosis is more prevalent in the immature dog (<1 yr old). If there is scaling associated with alopecia or short broken hairs in an older dog that has been on immunosuppressive drugs (chemotherapy, glucocorticoids), a fungal infection should be considered. The lesions may be localized or generalized. There will be variable erythema and pruritus. If there is a Trichophyton infection, the inflammation and pruritus is much more severe. A Wood's Lamp examination may be positive in up to 50% of the M. canis infections. A fungal culture is the most definitive diagnostic test.

Multifocal heavy accumulation of tightly adhered scales with alopecia on the face, pinnae and pressure points of a 1 to 3 year old Siberian husky or Malamute would be consistent with a zinc-responsive dermatosis. Erythema and pruritus are variable. If secondarily infected, it may be difficult to differentiate between scale and crust as the predominant lesion.

Epitheliotropic cutaneous lymphosarcoma (Mycosis fungoides) is an uncommon disease that occurs in the older dog. Slowly progressive patchy or generalized scaling is one of several presentations. Plaques and dermal nodules may also be observed. The degree of alopecia, erythema and pruritus is variable. A consistent finding is poor response to glucocorticoids and antibiotics. A biopsy is the definitive diagnostic test.

Superficial pyoderma is one of the most common causes of focal or multifocal scales and crusts. Since it is almost always a secondary problem, the associated history and clinical signs will be variable. However, it is generally easily recognized by the sequence of lesions starting with papules and progressing to pustules, crusts, epidermal collarettes and hyperpigmented macule. In haired areas of the body, there will be a follicular or moth-eaten pattern of raised tufted papules or alopecia. In more advanced cases, the lesions coalesce into larger lesions. The history usually includes a resolution of lesions with appropriate antibiotic therapy. Superficial pyodermas are frequently mildly to moderately pruritic. Antibiotic therapy often results in a mild to marked decrease in the pruritus.

Immune-mediated dermatoses may be associated with both scales and crusts. Pruritus is most often mild to moderate. The distribution of lesions in immune-mediated dermatoses is a predictable finding. Pemphigus foliaceus and pemphigus erythematosus usually involve the dorsal muzzle, periorbital regions and pinnae. Heavy crusts are typical. However, in mild cases, scaling may be seen. Pruritus and inflammation are variable. Most of PF and PE cases are in middle to older aged dogs. Discoid lupus erythematosus (DLE) has similar distribution with lesions being more variable. Pruritus is most often mild. The lesions may be primarily scales. At times superficial erosions with crusts predominate. If there is hypopigmentation of the anterior nares and nasal planum, DLE is more likely than PF. The drug history of antibiotics usually reveals a poor to partial response if secondarily infected. Antiinflammatory doses of glucocorticoids are usually ineffective in PF and have variable response in DLE. Biopsies are required to make a definitive diagnosis.

Deep pyoderma is often associated with crusts. The pruritus may be mild to intense in the local areas of infection. Pododermatitis with interdigital pyoderma is the most common presentation of a deep pyoderma. A deep pyoderma may be associated with Demodicosis. There may be a history of a penetrating wound or a severe acute moist dermatitis. Deep pyoderma is characterized by a marked deep dermal inflammation, exudation and often draining tracts in a focal or regional area. In many cases, there has been a poor response to antibiotic therapy. Cytology will reveal severe inflammation and variable bacteria. A biopsy will reveal a severe pyogranulomatous lesion. Bacterial cultures may be obtained from the mid dermal level of the biopsy tissue.

Adult onset demodicosis is often associated with some scaling along with the patchy or more generalized alopecia. Erythema may be minimal to marked (red mange). Pruritus may be observed when superficial bacterial folliculitis or other concurrent problems are present. In cases of deep pyoderma, the predominant lesions are exudation and crusting with alopecia. There may be a drug history of poor response or worsening of lesions with glucocorticoid administration. The onset of demodicosis and associated scaly lesions may occur after treatment with immunosuppressive drugs (chemotherapy, glucocorticoids) for other problems. Localized demodicosis is seldom pruritic. Generalized demodicosis in a young dog may have mild pruritus due to secondary bacterial folliculitis. Scrapings are always indicated in any scaly condition to rule out demodicosis.

Pruritus with Malassezia infection:

Malassezia infections are a very common secondary complication of pruritus. They are responsible for an exacerbation of pruritic symptoms in many dogs. A favorable environment Is created with excessive licking or biting or in the presence of lesions in occluded areas such as feet, skin folds, axillae and groin. If there is an excess of sebum production resulting in seborrhea oleosa, there may be an overgrowth of yeast. Malassezia should be considered anytime there is excessive licking or biting of the feet and if the skin or skin folds are greasy and inflammed. In interdigital and skin fold areas, moderate to marked erythema with or without excessive keratosebaceous debris or greasiness is observed. In a limited number of Malassezia infections, there is minimal inflammation. In many dogs, a Malassezia infection reduces the effectiveness of glucocorticoids and antihistamines in controlling pruritic symptoms.

A tape preparation or surface scrape is indicated if there is increased erythema or pruritus of the feet, axillae, groin or skin folds. A small number of yeast per 1000X is significant if it is associated with pruritus or inflammation.

Pruritus with alopecia:

Endocrine dermatoses are usually considered to be nonpruritic. However, they may predispose to secondary problems that cause some pruritic symptoms. Hyperadrenocorticism, either iatrogenic or naturally occurring, may predispose to relapsing or chronic superficial pyoderma, dermatophytosis, demodicosis and dry skin. The alopecic lesions usually precede the onset of secondary pruritic problems. Hypothyroidism may predispose to a superficial bacterial infection. A history of skin and hair coat changes including alopecia followed by variable scaly or crusty lesions and mild pruritus would support adding endocrine disease to the initial differential diagnoses list. Age of onset, distribution of lesions, and history of systemic signs will help differentiate the specific endocrine etiologies.

A limited number of congenital or acquired dermatoses are pruritic. Epidermal dysplasia in West Highland White Terriers may be intensely pruritic. It usually occurs in the young dog. Marked alopecia, scaling or crusting is usually observed. There is often a poor response to glucocorticoids. Biopsies are needed to confirm the diagnosis.

Follicular dysplasia is usually associated with an initial onset of alopecia that may have a regional distribution. Scaling associated with dry skin or bacterial folliculitis and mild to moderate pruritus may follow. Color mutant alopecia is an example. A definitive diagnosis is based on biopsies.

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