How To Tell If The Itch Is Atopic Dermatitis

Managing patients with chronic pruritus requires time, interest and a methodical approach.

Secondary Malassezia and bacterial infection with associated lichenification and hyperpigmentation in a dog with atopic dermatitis.

Courtesy of Rusty Muse, DVM, Dipl. Acv

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Management of the pruritic dog is one of the most common problems general practitioners see. The mistake in general practice that I see time and time again is that most cases of pruritus are handled as if they were the same disease. 

Pruritus is not a disease.
 
Consider three different presentations:

* A 10-year-old dachshund with no previous history of skin disease presents with pruritus and skin lesions on the trunk;
* A 5-year-old standard poodle with pruritus to the head and neck and increased scale and alopecia; and
* A 3-year-old Labrador retriever with seasonal pruritus to the paws and muzzle.

Would you treat them all with antibiotics and steroids? They might temporarily improve with that approach.  But they will all relapse and the conditions will recur until the underlying problem is addressed and managed—or until the clients find their way to the practice down the street and you lose not only the patient but the client as well. 

Managing patients with chronic pruritus requires time, interest and a methodical approach.

A common cause of pruritus in the dog is atopic dermatitis, or AD.

Arguably the most common allergic disease diagnosed in dogs, AD is caused by environmental allergens, such as outdoor pollens, or indoor, allergens such as dust mites or human dander. 

Getting Started
The first step in confirming the diagnosis is taking an accurate and methodical history. Atopic dermatitis is manifested as pruritus to the paws, face, ears, ventrum or axillary areas and may be seasonal or nonseasonal.  Certain breeds are more predisposed to atopic dermatitis than others and clinical signs for the majority of affected dogs will manifest between 1 and 3 years of age. 
 
Determining that these factors are present in the history and signalment is the first step in diagnosing AD.  If your history does not appear compatible with these simple historical factors, atopic dermatitis is much less likely.

Step two is to perform a complete and thorough physical examination of the skin, including the ear canals, lip folds, groin, interdigital webs and ventral aspects of all four paws. Examining the patient in a methodical approach from the planum and oral cavity to the tip of the tail and the perianal area is crucial. 

Know what to look for. Focus on pattern distribution and lesion types. The typical atopic dog will present with erythema of the paws and the outer ear canal and sometimes papules to the trunk. Clinical evidence of pruritus including excoriations or traumatic dermatitis may also be present.
 
Step three involves ruling out other primary causes of pruritus, or the presence of complicating secondary factors that are often associated with atopic dermatitis. This is when the methodical approach is essential.   

Establish the relevance of fleas or other pruritic parasites by flea combing and performing superficial and deep skin scrapings. Many cases of atopic dermatitis have combinations of allergic triggers including flea bite hypersensitivity.  While the clinical presentation of classic FBH is different from atopic dermatitis, the presence of fleas  may complicate the picture and make it more difficult to identify the typical clinical signs of AD. 

Effective flea control products, both topical and oral, have made the management of this complicating factor much easier. Aggressive flea and parasite control is important in any pruritic case. Anything that causes increased itching—even if the pet is not flea allergic—will lower the "itch threshold” and allow for increased pruritus in the AD patient.

Food allergies or adverse food reactions, although much less common than the general public tends to believe, can also cause clinical findings similar to AD. While the exact percentages of adverse food reactions is not known, most dermatologists believe that between 10 and 25 percent of cases of allergy-induced pruritus are at least partially food triggered.  

Testing Involved
No reliable testing exists to distinguish food allergies and simply having clients switch commercial diets is insufficient to confirm food allergies. Many over-the-counter diets share similar ingredients and the potential for contamination with protein sources other than those listed on the label has been documented in several studies. 

Diet changes to either completely novel protein diets (such as kangaroo, soy, rabbit, etc.) or hydrolyzed protein diets in which the proteins are modified to a size less likely to stimulate an immune response can be used to document the presence of adverse food reactions. Home-prepared food trials in which all ingredients the pet has been exposed to previously are removed are also often utilized. 

While grains, corn or wheat can cause allergic responses, other more common triggers are dairy products or meat triggers, which include chicken, beef, egg, fish and lamb.

A common secondary complicating problem of AD is the presence of bacterial or yeast organisms on the surface of the skin as a result of the primary disease. Secondary infections caused by Staphylococcus pseudintermedius or Malassezia pachydermatis are commonly seen with underlying AD because of the microclimate and cutaneous surface alterations present in this condition. 

Abnormal cutaneous barrier function leads to increased colonization of the skin with these organisms, as well as increased adhesion to keratinocytes, allowing for establishment of infection. Increased barrier permeability allows for percutaneous absorption of allergen and transepidermal water loss leads to increased scale and further damage to the skin, leaving it predisposed to chronic recurrent infections.
 
The presence of these complicating conditions will increase the level of pruritus in most patients.
  
A tentative diagnosis can be made once the criteria for atopic dermatitis are met and only when the other primary conditions and complicating secondary conditions have been evaluated for and ruled out or managed. 

Differences
Every patient is different and a diagnostic and treatment plan should be tailored to the individual patient. Many different short-term management strategies can be designed to alleviate the symptoms of allergies, including essential fatty acids (oral supplements, dietary sources and topical applications), antihistamine therapy, topical shampoo therapy, cyclosporine and in some cases oral corticosteroids.
 
For long-term management however, allergy testing and then immunotherapy—either subcutaneous injections or a newer approach of oral drops for immunotherapy—is preferable as it avoids long-term drug use and is a safer way to manage allergies. 

Immunotherapy is not a cookie-cutter process. It takes expertise and practice to achieve the best outcome; referral to a dermatologist can be helpful. Working hand-in-hand with a dermatologist can provide your clients and patients with solutions for chronic and often frustrating allergic skin disease.
 
Educating clients about underlying diseases and diagnostic and therapeutic options for pruritus is the most important part of the process. It is critical for the owner to understand all aspects of allergies, how they manifest and what to expect in treating patients. 

Allergies in animals, as in humans, are not curable. But there are many ways to manage them effectively to make the patient’s and owner’s lives more comfortable and happy. While not every case can be "hyposensitized,” most dogs (and cats) benefit from allergy injections help reduce the clinical signs of these conditions. 

Things can also be done at home to help dogs with AD.

Routine bathing of allergic dogs is very important in removing pollen and reducing allergen exposure.

Bathing with a gentle moisturizing shampoo once to twice weekly will help remove allergens and prevent prolonged exposure to the skin. Bathing with shampoos that contain products to help repair the lipid barrier on the surface of the skin can help to moisturize the skin and provide a strengthened epidermal barrier to prevent moisture loss and allergen absorption. 

In addition, wiping or rinsing off the paws after outside exposure can provide similar benefit.

So let’s revisit our three cases that presented to your clinic.
 
After a thorough history, physical examination and appropriate diagnostics:
* Is the 10-year-old dachshund with no previous history of skin disease that presented with pruritus and skin lesions on the trunk an atopic patient? 
No. That patient has hyperadrenocorticism and a secondary pyoderma. 
* How about the 5-year-old standard poodle that presented with pruritus to the head and neck with increased scale and alopecia—atopic? 
No. That dog has sebaceous adenitis. 
* What about the 3-year-old Lab that presented with seasonal pruritus to the paws and muzzle? 
Yes. That dog is indeed manifesting atopic dermatitis, although it is complicated by secondary Malassezia overgrowth to the paws.

An orderly and methodical approach is always indicated in working through dermatological cases and will almost always lead you to the correct diagnosis. Both your clients and your patients will appreciate your ability to make their lives better.

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