Feline Atopic-like Skin Syndrome: Understanding Cause and Therapy

Introduction

Feline atopic-like skin syndrome (FASS) is a relatively common presentation in the veterinary clinic. The feline patient often presents as a very challenging and frustrating case to the veterinary clinician. The clinical signs of the disease are often confusing and are not pathognomonic for any single underlying etiology. It is for these reasons that an accurate, timely diagnosis and a successful, comprehensive treatment plan are essential, but can be difficult to manifest. Many pet owners seek out immediate relief, but fail to understand that this condition is a life-long struggle for their pet. It should be paramount that the veterinary clinician be able to identify clinical signs consistent with FASS and then determine which treatment plan is best for the individual case.

Cutaneous reaction patterns in felids

The veterinary clinician should be able to distinguish between the more common reaction patterns seen in cats with FASS. While none of these patterns are indicative of the exact underlying etiology, they should suggest that the individual patient has a hypersensitivity disorder and that further investigation into this is warranted. What all four common reactions have in common is that the patient is experiencing some degree of inflammation and pruritus. The four cutaneous reaction patterns in cats are: i) selfinduced alopecia, ii) head/neck/pinnal pruritus, iii) eosinophilic granuloma complex (indolent ulcers, eosinophilic plaques), and iv) miliary dermatitis.

Underlying etiologies in FASS

There are three main allergic dermatoses found in the feline species: flea bite hypersensitivity, cutaneous adverse food reactions, and environmental allergies. Clinically, these three conditions can present identically. It is for this reason that a good history and clinical examination are imperative. The data collected with these two activities will help to lay the foundation of the treatment plan for the patient. Feline patients differ from their canine counterparts in that there is substantially more study information documented in dogs. The following information is broken down into the individual etiologies:

I. Flea bite hypersensitivity

Flea bite hypersensitivity (FBH) has been implicated as the trigger in the vast majority of cases of FASS. The incidence of FBH in patients exhibiting at least one of the cutaneous reaction patterns described above has been reported to be as high as 70% in one study. This is often dependent on the area of the country where the patient resides, as FBH is more common in flea endemic regions. It is often difficult to find evidence of fleas on the feline patient as they are meticulous groomers. The production of pro-inflammatory cytokines seen with FBH in cats is due to a salivary protein termed flea salivary antigen 1. The introduction of this protein into a sensitized individual will lead to a type I (immediate-type) and/or type IV (delayed-type) hypersensitivity reaction. Treatment should be focused around preventing exposure to the antigen and reduction of inflammation. The isoxazoline class of drugs works by antagonizing ligand-gated chloride channels and is the author’s prevention of choice. These products are often applied topically and on a monthly basis to provide sustained protection against the reintroduction of antigens. It is important to treat all animals in contact with the patient as they could be asymptomatic carriers of fleas. Environmental decontamination could also be considered. Glucocorticoids, such as prednisolone administered at 1 mg/kg once daily or a methylprednisolone injection administered as 20-40 mg/cat, or oclacitinib 0.7-1.2 mg/kg every 12 hours (extra-label use) are effective at reducing pruritus and providing comfort in these patients and should be used short-term.

II. Cutaneous adverse food reactions

The incidence of cutaneous adverse food reactions (AFR) in cats varies greatly in the reported literature. In general, the reported incidence is between 1-6% of feline patients. It is the author’s opinion that this condition is likely under-reported and/or under diagnosed. As with canine patients, the pathogenesis of AFR in cats in not completely understood. There are varying reports of allergen-specific IgE involvement, as well as the exact type of hypersensitivity reaction identified. The patient will present with a nonseasonal pruritus in the case of strict AFR. Again, there may be the presence of one or more of the previously noted cutaneous reaction patterns. Clinical signs may include gastrointestinal disturbances in up to 33% of feline patients presenting for AFR. Unusual clinical signs have been noted in these patients, including plasma cell pododermatitis, conjunctivitis, and respiratory abnormalities. The diagnostic of choice for this condition is a strict elimination diet trial. The most commonly reported food allergens in cats with AFR are beef, dairy products, chicken, and fish. Together, these groups account for roughly 90% of documented cases of AFR in cats from various reports. Once the patient has been fed the strict elimination diet trial and clinical resolution of lesions has been obtained, the patient is then fed a provocation diet to confirm suspicion of the underlying cause. Serum allergy testing, intradermal allergy testing, and gastroscopic testing have all been shown to be inaccurate in the cat. The elimination diet trial should be kept strict throughout the duration. It is cited in the literature that more than 80% of cats with known AFR showed resolution within 6 weeks of beginning a strict elimination diet. That percentage increased to almost 90% by the 8 week time point. It is generally recommended that an elimination diet last between 8-12 weeks. There are many commercially available diets containing limited ingredients and novel proteins that can be utilized. A home-cooked diet may also be used provided that the owner works with a veterinary nutrition expert to ensure that the diet is balanced. As the diet trial may last several weeks, control of pruritus is paramount and consideration should be given to treatment options that are safe for that time period. Glucocorticoids and/or modified cyclosporine at 7 mg/kg once daily are effective at controlling the pruritus for the initial phases of the diet trial. Oclacitinib may also be attempted, but is not very effective for this condition in the author’s experience.

III. Environmental allergies

Environmental allergies in the feline patient can present with either a seasonal or nonseasonal course. The terms “feline atopy” and “feline atopic dermatitis” are now defunct and have been replaced with the term non-flea, non-food hypersensitivity dermatitis. This is due to the fact that there is a lack of conclusive allergen-specific IgE involvement, which is the hallmark of true atopic dermatitis as seen in both the dog and man. While it has been noted that there is an increase in CD4+ T cells in the skin of allergic cats, the cytokine profile that accompanies this condition is still under investigation. The vast majority of these patients present with varying degrees of pruritus surrounding the times of allergen exposure. As previously discussed, the feline patient may present with at least one of the cutaneous reaction patterns. Environmental allergies are typically a diagnosis of exclusion, so it is often arrived at once flea bite hypersensitivity and adverse food reactions are ruled out. Along with the cutaneous manifestations of the disease, the patient may also present with signs of feline small airway disease, such as allergic rhinitis or sinusitis. Sneezing may be a prominent finding in cases of environmental allergies in cats with some reports approaching nearly 50% of cats with the condition. Unlike the canine patient, bacterial and/or fungal infections of the ears and skin are uncommon to rare in the feline patient with environmental allergies. This may be related to the reduction in adherence of the microbes to the feline corneocytes.

The goal with therapy for feline patients with environmental allergies is to decrease the frequency and severity of the allergic flares as safely as possible. A cure should not be expected and this should be relayed to the owner early in the treatment plan. Treatment should be evaluated on a case-by-case basis to determine which modality is best for that individual. Conservative treatment plans should revolve around antihistamines, although success rates remain relatively low with this modality. Medications, such as cetirizine 5 mg per cat given once every 24 hours, should be tried for a minimum of two weeks to determine efficacy. More aggressive systemic therapy may consist of glucocorticoids (prednisolone 1-2 mg/kg every 24 hours and then tapered) and/or modified cyclosporine (Atopica 7 mg/kg every 24 hours). These therapies should be used at as low of a dose and for as briefly as possible to provide comfort. Allergen-specific immunotherapy (ASIT) remains the treatment of choice for feline patients with environmental allergies. Allergens are identified using intradermal allergy testing and/or serum allergy testing and then formulated into either a subcutaneous injection or a sublingual drop. Response rates vary, but most of the literature places them between 70-80%. ASIT should be given for at least 12 months to determine if it will be effective and administration may be tapered at that point.

Conclusion

There is still much that is unknown when evaluating the pathogenesis of FASS. There also appears to be considerable overlap of clinical signs amongst the known etiologies, making the determination of the underlying cause difficult and frustrating for the veterinary clinician. The cause of the cutaneous manifestations of the condition should be ruled out in a consistent, methodical manner until the correct diagnosis is discovered. Treatment should focus on the underlying issue(s) while providing relief through systemic medications. It is important that the treatment not be made worse than the disease itself. The goal of the comprehensive treatment plan is to improve the quality of life for the feline patient and the pet owner.

References

• Miller W, Griffin C, Campbell K. Muller & Kirk’s Small Animal Dermatology VII. St. Louis, MO: Elsevier; 2013.
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• Reinero C. Advances in the understanding of pathogenesis, and diagnostics and therapeutics for feline allergic asthma. Vet 2011; 190: 28-33.
• Hobi S, Linek M, Marignac G, et al. Clinical characteristics and causes of pruritus in cat: a multicenter study on feline hypersensitivity-associated dermatoses. Vet Dermatol 2011; 22: 406-413.
• Porcellato I, Giontella A, Mechelli L, et al. Feline eosinophilic dermatoses: a retrospective immunohistochemical and ultrastructural study of extracellular matrix remodeling. Vet Dermatol 2014; 25: 86-e26.