It is that time of year again: allergy season. Like humans, our canine and feline patients suffer from the influx of pollen, dust, and other allergens that herald the coming of spring. Pruritic skin disease is one of the most common reasons why pets are brought to the veterinary clinic each year, and allergies are a frequent diagnosis. However, despite its prevalence, atopic dermatitis – also known as an environmental allergy – can be relatively challenging to diagnose.
What is Atopic Dermatitis?
Whether they prefer to call it atopy, atopic dermatitis, or environmental allergy, most general practitioners have seen this condition many times before. Atopy is a catch-all term for an allergic skin disease that is not associated with parasites or adverse food reactions. This condition results from a defect in the epidermal barrier, which is thought to be due to a combination of genetic and environmental factors. In an atopic patient, these epidermal barrier defects allow allergens such as pollen to penetrate the skin surface and bind to immune cells carrying allergen-specific IgE. This triggers an inappropriate immune response resulting in the migration of inflammatory cells and the release of cytokines and chemokines. Once this process has started, it won’t be long before the patient presents to the veterinary clinic red, itchy, and uncomfortable.
The antigens responsible for triggering this inappropriate immune response vary with season and geographic location. However, common culprits include pollens, grasses, dust mites, molds, and dander. In many cases, an individual patient has several different allergy triggers.
Identifying Atopic Dermatitis
Atopy is a diagnosis of exclusion, which can be frustrating for veterinarians and pet owners alike. Because there is no specific test for atopy, it can only be diagnosed once other causes for pruritic skin disease – such as parasitism, food allergy, and infection – have been ruled out. This means the first steps will include year-round parasite prevention medications, an 8-week elimination diet trial, treatment of any secondary dermatitis, and diagnostic testing to rule out endocrinopathies and other underlying medical conditions. Understandably, this detailed workup is often curtailed due to financial concerns or poor owner compliance. In suspected atopy cases, clear communication is necessary to ensure pet owners are aware of what to expect during the diagnostic process.
Many pet owners will ask why a simple allergy test cannot be performed. Often they will be familiar with the concept from human medicine, or they will have seen advertisements from less-than-reputable companies offering mail-order hair and saliva allergy tests for dogs. Unfortunately, even serum and intradermal allergy testing performed by a veterinary dermatologist is not able to accurately diagnose a patient with atopy. Some non-allergic dogs will react positively to these tests, while on the other hand, some atopic dogs will have no reaction at all. However, serum and intradermal allergy tests do still play an important role in managing atopy. Patients that have been diagnosed with atopy via exclusion of other differentials may undergo allergy testing to narrow down which environmental antigens result in a reaction. This allows immunotherapy (hyposensitizing allergy shots) to be tailored to the individual patient, which improves outcomes.
Clinical Presentations of Atopic Dermatitis
Pet owners often think of allergies in terms of the typical “hay fever” presentation as seen in humans: sneezing, runny nose, watery eyes, coughing, and congestion. In our canine and feline patients, this clinical presentation is uncommon. It is far more likely that an atopic patient will present with allergic skin disease. Signs may include:
- Pruritus, which may be seasonal or year-round.
- Erythema, papules, macules, and pustules.
- Self-trauma such as alopecia, excoriations, and saliva staining.
- Recurrent dermatitis and otitis.
- Ocular signs such as conjunctivitis, chemosis, ocular discharge, and periocular alopecia.
As always, cats are unique in their clinical presentations and the disease process remains poorly understood in this species. While some cats may present with the clinical signs outlined above, others may have different clinical presentations such as indolent ulcers, miliary dermatitis, and eosinophilic granuloma complex. As with dogs, the disease does not have a characteristic pattern and therefore other causes of these clinical signs must be ruled out before a diagnosis of atopic dermatitis can be made.
Managing Atopic Dermatitis
Once atopic dermatitis is diagnosed, eliminating pruritus becomes the primary goal of treatment. This may be accomplished with short courses of glucocorticoids or through the use of more targeted therapies such as oclacitinib (Apoquel) or monoclonal antibody therapy (Cytopoint). In many cases, reducing self-trauma secondary to pruritus is critical to control secondary skin infections. Topical therapies, such as medicated shampoos, can help reduce antigen burden by washing away allergens and can also soothe damaged skin. In some cases, dietary therapy with fatty acids can also help strengthen the defective epidermal barrier.
While more and more general practice clinicians are turning to medications such as Apoquel and Cytopoint for long term management of atopic patients, immunotherapy remains a cornerstone of long term treatment. Immunotherapy – commonly referred to as “allergy shots” by pet owners – improves the patient’s tolerance for the offending allergen by repeatedly exposing them to low levels of it. This process can be time-consuming, and often takes six months or more before a response to treatment is seen. Immunotherapy treatments may be formulated based on a patient’s response to serum or intradermal allergy testing or may be formulated based on the Regionally-Specific Immunotherapy (RESPIT) method which utilizes the allergens that are most prevalent in the individual patient’s geographic region. These treatments can be administered via injection or oral medication, and many pet owners can accomplish this at home.
Atopic dermatitis, or environmental allergy, is a common problem in canine and feline patients particularly at this time of year. Diagnosing atopy requires ruling out all other causes of pruritic skin disease, often making it a costly and time-consuming diagnostic process. Atopic veterinary patients will not grow out of their allergies the way some young human patients do, but atopy can be managed. As always, good communication regarding the diagnostic process and the need for lifelong multimodal therapy is essential in order to achieve good owner compliance.
Dr. Elizabeth Racine is a small animal veterinarian with a passion for improving the lives of pets. She has worked in the veterinary field in various roles for more than a decade, with professional interests in behavior, nutrition, and palliative care. As a writer, her work has been featured by several world-renowned pet health and wellness brands. Dr. Racine shares her home with her dog Dasher - a beagle with his own storied career training new veterinary students - and her trouble-making orange cat named Julius. She can be found at TheVeterinaryWriter.com.
This article first published in the Q1 2019 edition of Veterinary Practice News.