Terminology and classification
ICADA introduced new nomenclature, and a Canadian Veterinary Journal column by Jangi Bajwa summarizes it [3]. Feline atopic syndrome (FAS) is the umbrella term for allergic disease of the skin, gastrointestinal tract, and respiratory tract. FASS is the skin component [1][3].
According to that summary, FASS is "an inflammatory and pruritic skin syndrome of cats manifested by a spectrum of reaction patterns," which may be associated with IgE antibodies to environmental allergens [3]. Food allergy (FA) is an etiological diagnosis covering any clinical signs attributable to immune reactivity to an ingested food. Flea allergy dermatitis and mosquito-bite hypersensitivity are specifically excluded from the atopic designation [3]. Feline asthma is classified within FAS. It is an eosinophilic disease of the bronchioles, and the summary distinguishes it from chronic bronchitis by bronchoalveolar lavage cytology [3].
Older literature uses different terms. A 2017 Veterinary Sciences review by Alison Diesel uses "non-flea, non-food hypersensitivity dermatitis" (NFNFHD). Diesel explains that this term was preferred because IgE involvement had not been conclusively shown [2]. None of the cited sources describe a severity staging system.
Who gets it
Merck reports that FASS usually begins before 5 years of age, although the age of onset varies. Signs may be seasonal or nonseasonal, and purebred cats may be at higher risk than domestic shorthairs [1]. The CVJ summary lists some evidence of a heritable component in the Abyssinian, Somali, Ocicat, Siamese, Persian, Maine coon, Devon rex, and Himalayan breeds. It also notes that, compared with humans and dogs, little is known about heritability [3].
On pathogenesis, Merck describes genetic predisposition, impaired skin barrier function, and immune dysregulation. Merck adds that IL-31, a key itch mediator in dogs, has no established role in cats [1]. Diesel reports that skin barrier function had not been well evaluated in cats at the time of her review [2].
For flea allergy, Merck states that most cases occur in late summer, that it can occur in animals of any age, and that there are no sex or breed predispositions [4].
Clinical presentation
The four reaction patterns described across the sources are miliary dermatitis, self-inflicted alopecia, head and neck pruritus, and eosinophilic granuloma complex [1][2][3]:
- Miliary dermatitis consists of small crusted papules, typically about 1 to 2 mm [3]. Merck notes that in longhaired cats these lesions may be felt more easily than seen [1].
- Self-inflicted alopecia is often bilaterally symmetrical and appears wherever the cat can groom [1].
- Head and neck pruritus can be intense enough to cause excoriation and ulceration, and may require protective collars [3].
- Eosinophilic granuloma complex includes indolent ulcer, eosinophilic granuloma, and eosinophilic plaque [3].
The ICADA review, as summarized in the CVJ, reported these frequencies in 263 cats with FASS:
- miliary dermatitis 31.2%
- self-inflicted alopecia 60.1%
- head and neck pruritus 43.0%
- eosinophilic granuloma complex 25.9%
Of these cats, 37.7% had at least two patterns [3]. Signs outside the skin were less common. Among 230 cats in 7 manuscripts, 8.3% had respiratory disease, 4.8% had conjunctivitis, and 3.9% had gastrointestinal signs [3].
None of the patterns is specific to one cause. Flea allergy and food allergy can produce any of the four [3][4]. Diesel notes that flea allergy most commonly involves dorsal lumbosacral pruritus with miliary dermatitis. Food allergy is often described with severe head, neck, and pinnal pruritus, and one study cited by Diesel found gastrointestinal signs in one-third of cats with cutaneous adverse food reactions [2].
Owners may not see overgrooming. Merck notes that hair trichography can be used to evaluate the tips of hair shafts, which are broken off in cats that overgroom [1]. The CVJ summary describes a study of 21 cats referred for suspected psychogenic alopecia. A primary behavioral cause was found in only 2 of them. Sixteen had pruritic dermatitis alone, and 3 had pruritic disease with a behavioral component [3]. The summary does not discuss limitations of that study.
Diagnosis
Exclusion sequence
Merck lists the first diagnostic steps as dermatological and physical examination plus skin cytology. Cytology may include skin scrapings, direct impressions, and acetate tape impressions [1]. Extremely hypersensitive cats can be virtually free of fleas because of excessive grooming. For this reason, Merck describes using response to ectoparasite control as a diagnostic test [4]. In nonseasonal cases, Merck states that a food elimination trial should be done once flea allergy and infection have been ruled out [1].
Diesel describes a strict elimination diet followed by a provocative challenge with the original food as the only accurate way to diagnose food sensitivity [2]. She cites an evidence-based review that found more than 80% of cats with cutaneous adverse food reactions improved within 6 weeks. Extending the trial to 8 weeks produced complete remission in about 90% of cats and dogs combined [2]. Diesel also advises avoiding hydrolyzed versions of proteins the cat has already eaten. This advice is extrapolated from a canine study, not tested in cats [2].
Merck's list of differential diagnoses for FASS includes:
- other ectoparasites
- mosquito bite hypersensitivity
- autoimmune disease
- infectious diseases such as dermatophytosis
- cutaneous neoplasia
- hyperthyroidism
- feline lower urinary tract disease [1]
Diagnostic criteria
Merck reproduces a 10-item criteria set adapted from Santoro and colleagues (2021). The criteria include:
- at least two reaction patterns
- at least two affected sites
- miliary dermatitis as the dominant pattern
- specific lesion locations, such as symmetrical abdominal alopecia [1]
Merck states that meeting 6 of the 10 criteria gives 90% sensitivity and 83% specificity for non-flea-induced hypersensitivity dermatitis [1]. Merck also states that a diagnosis of FASS is made when the other differential diagnoses have been eliminated and compatible clinical signs are present [1].
Allergy testing pitfalls
Merck describes intradermal and serological testing as diagnostic aids that measure IgE. Their main purpose is to select allergens for immunotherapy, and results matter "only if the offending allergens identified are compatible with the history or seasonality of pruritus" [1]. Intradermal reactions in cats are less dramatic and fade faster than in dogs, which makes them harder to read [1]. The CVJ summary notes that serology is more convenient but has variable reliability [3].
For flea allergy, Merck states that intradermal and serological testing for flea saliva antigens is no longer recommended [4]. Diesel cites one study in which about 36% of cats without skin abnormalities tested positive to flea extract [2]. Diesel also reports that serum and intradermal food allergen testing has not been evaluated in cats. She does not recommend it, based on canine data [2].
Secondary infections
Diesel reports that bacterial pyoderma has historically been considered uncommon in allergic cats but may be more prevalent than previously reported [2]. She also cites one study in which Malassezia was isolated much more often from cats with NFNFHD than from cats without allergic disease. In that study, pruritus improved with itraconazole [2]. Merck states that treating secondary bacterial infections when cytology confirms infection can help decrease lesion severity, and that topical treatment is preferred over systemic treatment whenever feasible [1].
Treatment of FASS
Merck describes a multimodal approach that combines control of flare factors, antipruritic drugs, and allergen-specific immunotherapy (ASIT). Merck also notes that avoiding allergens is difficult or impossible in practice [1]. The ICADA evidence grading, as summarized in the CVJ, rated the evidence as good for systemic glucocorticoids and cyclosporin and limited for topical glucocorticoids, oclacitinib, and ASIT. It pointed to low-to-moderate efficacy for antihistamines, fatty acids, and palmitoylethanolamide [3].
Merck lists the following agents and doses. All doses below are as given by Merck [1].
| Agent | Dose per Merck [1] | Points noted by Merck [1] |
|---|
| Methylprednisolone | 1.4-1.5 mg/kg, PO, once q 24 h until remission, then tapered | Remission within 14 days; long-term use requires monitoring |
| Hydrocortisone aceponate 0.0584% spray | 2 sprays, once q 24 h for 28 d, then q 48 h as tolerated | Effective in 7 of 10 cats in a pilot study |
| Modified cyclosporine | 7 mg/kg, PO, once q 24 h, then tapered to q 48 h or twice weekly | Mostly GI adverse effects; fatal toxoplasmosis reported in naive cats |
| Oclacitinib | 1 mg/kg, PO, q 12-24 h | Extralabel; no long-term safety data |
| Chlorpheniramine | 2 mg/cat, PO, q 12 h | Small, limited benefit; sedation |
| Ultramicronized palmitoylethanolamide | 15 mg/kg, q 24 h | Moderate efficacy, moderate evidence |
| Maropitant citrate | 2 mg/kg, PO, q 24 h for 4 wk | Extralabel; single study; hypersalivation |
Several of these entries rest on small studies. The hydrocortisone aceponate data come from an open-label pilot study of 10 cats. Merck notes that topical glucocorticoids have been studied less in cats than in dogs and that absorption can still cause adverse effects [1]. The maropitant data come from one study, which Merck's reference list identifies as an open-label, uncontrolled pilot study [1]. Merck reports that cats responding to oclacitinib usually do so within the first month [1].
Diesel cites a prednisolone starting range of 1-2 mg/kg every 24 hours, then tapered. She notes that cats tend to be less sensitive to steroids than dogs and often require a higher starting dose [2]. She also states that lokivetmab, the caninized anti-IL-31 antibody, is not recommended for cats because of the likelihood of severe reactions to the foreign protein [2].
For ASIT, Merck lists subcutaneous and sublingual routes. For subcutaneous ASIT, onset can take up to 12 months, and the most common adverse reaction is worsening of pruritus. For sublingual ASIT, the most common adverse reactions are face rubbing, transient worsening of pruritus, and GI signs [1]. Diesel reports that both conventional and rush induction protocols have been successful in cats [2].
Flea allergy and insect bite hypersensitivity
Merck's management of flea allergy dermatitis rests on three steps: eliminating fleas on the pet, eliminating the flea biomass in the home, and preventing reinfestation in every animal in the household [4].
For supportive care in cats, Merck states that prednisolone can be given at 0.5-1 mg/kg, PO, every 24 hours for 7-14 days, then tapered on alternate days, and that the glucocorticoid can be discontinued as soon as flea control is accomplished. Merck states that anti-inflammatories "should never be used as a substitute for flea control" [4]. Diesel notes that many veterinary dermatologists recommend year-round flea prevention to avoid confounding diagnoses [2].
For mosquito bite hypersensitivity, Diesel describes avoidance as the mainstay of therapy, which includes keeping affected cats indoors. She warns that DEET-containing human repellents and permethrin products are unsafe for cats [2].
Monitoring, prognosis and client communication
Merck advises tapering long-term antipruritic therapy to the lowest effective dose and frequency. For long-term glucocorticoid use, Merck recommends regular hematology, serum biochemistry, and urinalysis. Merck also advises close monitoring of cats on oclacitinib [1].
On prognosis, Diesel writes that "there is no cure for NFNFHD in cats," and she frames the goal as reducing the severity and frequency of flares [2]. The CVJ summary notes that asthma may be underrecognized in cats with FAS [3].
The sources identify several points relevant to client conversations:
- Merck states that owners should commit to 1 year of ASIT before judging whether it works [1].
- Clearing an infestation from the home often takes up to 3 months, and flea control must continue for life [4].
- Owners also may not notice overgrooming at home [1].
