Feline Cutaneous Atopic Syndrome: Diagnostic and Therapeutic Approaches

At the last World Congress of Veterinary Dermatology held in Boston in July 2024, our colleague Domenico SANTORO had the opportunity to give a very good lecture on Feline Atopic Skin Syndrome or FASS. This was an opportunity to review this allergic dermatitis, which is so common in the feline species, yet so poorly understood.

Feline atopic skin syndrome is a complex dermatological disorder, analogous to atopic dermatitis in humans and dogs. Although the study of this pathology remains limited compared to other species, veterinarians observe a growing prevalence in their practices. Unlike dogs, felines often present non-specific clinical manifestations and an absence of pathognomonic signs, making the diagnosis and management of this condition even more challenging. During his lecture, our colleague provided a comprehensive review of FASS, based on current scientific data, to offer clinicians adapted diagnostic tools and therapeutic options.

Evolution of Feline Allergic Disease Nomenclature

In 2021, the International Committee on Allergic Diseases of Animals (ICADA) redefined feline allergies by introducing the concept of Feline Atopic Skin Syndrome (FASS). This designation groups together environmental skin allergic diseases. This syndrome is an integral part of Feline Atopic Syndrome (FAS), which also includes food allergies and asthma. FASS, for its part, excludes allergic reactions to fleas as well as hypersensitivity to mosquito bites. This new arrangement has allowed for a better distinction of allergic conditions and favored a more targeted therapeutic approach.

Epidemiology and Predisposing Factors

Distribution of FASS in the Feline Population

In the case of FASS, unlike dogs, no particular breed has shown a clear predisposition. However, clinical studies report that certain breeds, notably the Abyssinian, Siamese, and Persian, appear to be overrepresented in the studied populations, suggesting a possible genetic susceptibility. Most felines show their first symptoms between the ages of 2 and 3 years, although cases have been documented in kittens as young as 6 months and in adult cats up to 5 years. This age variability often complicates early identification of the disease. The prevalence of FASS between males and females is relatively equal, although some authors report a slight predominance in females (58.4%).

Epidemiological Factors

It is also interesting to note the absence of predominant seasonality in FASS. While allergic dogs frequently show a resurgence of symptoms during certain periods of the year, FASS does not seem to be influenced by seasons, suggesting sensitization to perennial allergens. However, in the rare cases where seasonality has been observed, manifestations were evenly distributed across all four seasons, which suggests a generalized sensitivity to allergens present all year round, such as dust mites and other indoor allergens.

Clinical Manifestations and Diagnosis

Typical Dermatological Symptoms of FASS

The cutaneous manifestations of FASS are varied, making diagnosis based solely on clinical signs difficult. The four most common dermatological patterns are:

• Miliary dermatitis: present in 31% of cases, this condition is characterized by small, crusty papules on the skin, often on the rump and back. This type of dermatitis is often confused with flea infestation, hence the importance of differentiating between the two.
• Self-induced alopecia: observed in 60% of cats, it results from excessive licking, usually on the abdomen, flanks, or limbs. This behavior can often go unnoticed by owners, who consider it normal grooming.
• Head and neck pruritus: affecting 43% of affected felines, intense pruritus can lead to excoriations and scratching lesions around the head and neck, sometimes with localized hair loss.
• Eosinophilic granuloma complex: this condition includes eosinophilic plaques, indolent ulcers, and linear granulomas. It is present in 26% of FASS cases. The eosinophilic complex often manifests as painful and pruritic lesions, which are sometimes accompanied by secondary bleeding due to excoriations.

Very severe abdominal eosinophilic plaques

 

Cutaneous lesions of FASS are distributed mainly on the abdomen, head, limbs, neck, and ears, although other areas of the body may be affected. Severe cases may also include respiratory signs, such as allergic rhinitis and feline asthma, as well as ocular manifestations like allergic conjunctivitis.

Extra-Cutaneous Symptoms and Differential Diagnosis

The diagnosis of FASS relies on careful observation and the exclusion of other possible causes of pruritus, such as parasitic infections (fleas, mites), as well as systemic or neoplastic conditions. Cats with FASS may also present non-cutaneous signs, including respiratory or gastrointestinal disorders. Differential diagnosis notably includes cutaneous lymphoma, which can mimic clinical signs of FASS, and dermatophytosis, especially if it is pruritic.

A typical diagnostic protocol begins with parasite exclusion through appropriate tests, such as skin scrapings, tape strips, or trichograms. If in doubt, a skin biopsy may be performed to rule out neoplastic disease. The food challenge test is often necessary to rule out food allergy, although this procedure is sometimes complex to perform in cats.

Therapeutic Options for FASS

Systemic Drug Therapies

Systemic glucocorticoids are the treatment of choice for rapid control of pruritus and inflammation. Among the most commonly used are prednisolone, methylprednisolone, triamcinolone, and dexamethasone. These medications generally provide a rapid response, with notable improvements within 7 to 14 days. However, side effects, such as increased liver enzymes and the risk of hyperglycemia, limit their long-term use.

Cyclosporine has become an increasingly popular alternative for FASS cases refractory to glucocorticoids or when the latter are contraindicated. Approximately 500 cats have been successfully treated in various clinical trials, with response rates ranging from 40% to 100%. The most common side effects include gastrointestinal upset and decreased appetite.

Antihistamines, although widely used, have shown mixed results. Chlorpheniramine is the most effective drug in this group, but overall, antihistamines only moderately relieve symptoms in approximately 36% of treated felines.

Topical Therapies and their Impact

Topical glucocorticoids, such as hydrocortisone aceponate, have shown notable efficacy without the side effects associated with systemic treatments. Approximately 50% of treated cats showed significant improvement in pruritus and skin lesions after 56 days of regular application. This type of treatment is particularly useful for localized lesions and can be used as a complement to systemic therapies.

Allergen-Specific Immunotherapy (ASIT)

ASIT, although more common in dogs, is increasingly being explored in felines. It can be administered subcutaneously (SCIT), sublingually (SLIT), or orally (OIT). Clinical results show success rates varying between 45% and 75%, depending on the administration method and the individual cat’s response. SLIT, for example, has been tested in a trial involving dust mite-allergic cats, with encouraging results, showing a significant reduction in clinical signs without notable side effects. SCIT remains the most studied approach, although additional studies are needed to evaluate its long-term efficacy in felines【5†source】.

Alternative Therapies and New Horizons

Use of Essential Fatty Acids and PEA

Essential fatty acids, particularly omega-3 and omega-6, are often recommended as complementary therapy for their role in reducing skin inflammation. Palmitoylethanolamide (PEA), an endocannabinoid, is also being studied for its anti-inflammatory effects and its ability to prolong remission periods. In one clinical study, PEA demonstrated superior efficacy to placebo, with doubled remission time and improved owner satisfaction regarding pruritus symptom management.

Complementary Treatments and the Role of CO2 Laser

For severe eosinophilic granulomas, particularly when they obstruct the oral cavity, the CO2 laser can offer immediate relief. This method helps reduce discomfort and prevent serious complications, such as difficulty eating. Although the laser is primarily palliative, it offers a valuable solution for situations where pharmacological treatments are insufficient【5†source】.

Perspective and Future Research

Research Gaps and Clinical Challenges

The main difficulty in managing FASS is the lack of specific clinical criteria for diagnosis, which leads to great variability in treatments. The need for further research is particularly acute regarding understanding the pathogenesis and variability of clinical manifestations of this pathology.

New Medications and Potential Therapies

Research is looking towards new molecules, such as cytokine receptor antagonists, which could offer more targeted and less invasive alternatives to current therapies. Studies on interleukin receptors are underway, and clinical trials on Janus kinase (JAK) inhibitors are planned to explore new therapeutic avenues in FASS.

Conclusion

Feline atopic skin syndrome is a complex and still poorly understood condition in feline veterinary medicine. A rigorous and personalized approach is essential for accurate diagnosis and effective therapeutic management. Veterinarians must stay informed of the latest advances to best adapt their practices and offer a better quality of life to affected cats.

D Santoro. Feline atopic skin syndrome – diagnosis & treatment. WCVD10