Feline atopic syndrome: What it is and how to diagnostically approach it

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When it comes to feline patients, diagnosing them can be tricky due to their nature to hide that they are hurt or sick, and this also rings true for diagnosing skin diseases in a pruritic cat. The signs of skin disease can sometimes be unintentionally overlooked because they may not scratch but overgroom instead. Besides overgrooming, these patients may also have hair loss due to self-induced biting or licking, but clients may bring them in thinking the hair loss is spontaneous.

During his lecture “Unique Considerations in Feline Dermatologic Disease” at the 2024 New York Vet Show in New York City, New York, Andrew Rosenberg, DVM, DACVD, director of medical operations for the Animal Dermatology Group, explained to attendees multiple skin reaction patterns as well as what veterinary professionals can recommend for diagnostic options.

Reaction patterns

Feline atopic syndrome has 4 reaction patterns, miliarydermatitis, facial neck and pruritus, Eosinophilic Dermatoses, and Self-Induced Alopecia, but they are not a diagnosis. Each of the patterns is a recognized manifestation of skin diseases that feline patients will exhibit and can be caused by a plethora of differentials, but the most common are hypersensitivity reactions.

Miliary Dermatitis

Papulocrustous dermatitis, also known as miliary dermatitis, is often pruritic, widespread, small, erythematous, crusted papules that tend to be nonfollicular. The disease usually presents itself on the dorsum but can be located anywhere on the feline patient. This pattern is often seen with allergies, however underlying causes can produce this same type of reaction pattern as well.

“What's interesting about these is they aren't associated with follicles, so these are nonfollicular, crusted papules [and] very often they're infected. So if you see these, do a cytology, look for bacteria,” explained Rosenberg.

“This is very commonly seen on the dorsum of a cat, especially the caudal dorsum. Besides allergies, ectoparasites can do this, especially flea allergy can do this. Dermatocytosis can do this. Different types of cancer can look like this as well. And then certainly autoimmune diseases, pemphigus foliaceus is the most common, widespread crusting on a cat, and you can get these little papulocrustous lesions with pemphigus very frequently. I f that's the case, on the if it's pemphigus, most commonly seen on the face and sometimes seen on the ventral abdomen,” he continued.

Eosinophilic dermatoses

Eosinophilic dermatoses is the new proposed term that covers the group of skin diseases that were formerly known as eosinophilic Granuloma Complex (EGC) and includes eosinophilic plaques, granulomas, and indolent ulcers. Rosenberg explained they present as pink to yellow pruritic lesion that are raised, erythematous, and exudative. The lesions usually develop in the inguinal region and ventral abdomen of the feline patient but can sometimes be found on the neck or face.

They could present as linear or as nodular firm lesions that occur within the chin or oral cavities. In severe cases, indolent ulcers can affect the upper lip of cats and could affect their nasal planum. When it comes to eosinophilic dermatoses, it is almost always are aligned with hypersensitivity reactions.

The diagnostic strategy

According to Rosenberg, none of the reaction patterns he listed during his lecture are pathognomonic for hypersensitivity reactions or feline atopic syndrome specifically. Because of this, veterinary professionals need to rule out other diseases that are differentials for reaction patterns. Diagnosing feline atopic syndrome is based on compatible clinical signs once other different diagnosis is ruled out such as ectoparasites, such as fleas and lice, Otodectes, Notoedres, and Demodex spp, as well as dermatophytic infections. Rosenberg also explained to attendees that in past years, psychogenic alopecia/pruritus was assumed to be common, but now it is exceedingly rare, meaning it should only be considered once teams have ruled out all other possible causes of the patient pruritus, including feline atopic syndrome.

To help veterinary professionals with diagnosing feline atopic syndrome, criteria was created to help diagnosing patients. The criteria is:²

• Presence of at least 2 affected body sites
• Presences of at least 2 of the 4 reaction patterns
• Presence of lesions on lips
• Presence of symmetrical alopecia
• Presence of erosions or ulcerations on the neck or chin
• Absence of lesions on rump
• Absence of nonsymmetrical alopecia on rump and tail
• Absence of nodules or tumors

With the criteria, if 5 out of the 8 are present, there is a sensitivity of 75% and specificity 76% for diagnosing nonflea hypersensitivity dermatitis (feline atopic syndrome).2 Once veterinary professionals can use the following additional criteria when a flea hypersensitivity reaction is ruled out:

• Pruritus is present at onset of disease
• At least 2 of the 4 reaction patterns are present
• At least 2 body sites are affected
• There is a presence of miliary dermatitis as a dominant pattern
• Eosinophilic dermatitis or symmetrical alopecia or erosions/ulcerations are present on the ears, face, head, lips, or neck
• Nonsymmetrical alopecia is present on the hindlimbs, tail, or rump
• Symmetrical alopecia is present on the abdomen
• There is an absence of erosions/ulcerations on the forelimbs
• Absence of lesions on the sternum or axilla
• Absence of nodules or tumors

When 6 of the 10 criteria above are met, there is a sensitivity of 90% and specificity 83% for the diagnosis of nonflea hypersensitivity dermatitis (feline atopic syndrome).²

Ruling out other illnesses

Flea allergy dermatitis (FAD) is the most common type of allergic dermatitis in feline patients, so it is a mandatory first step to be ruled out according to Rosenberg. Often clients will not see fleas and make the assumption their pet is not suffering from a flea allergy.Affected cats can be a tricky to diagnose for even clinicians because they are fastidious groomers, so they typically groom the fleas off during a mild infestation, so no fleas or flea feces is observed during the exam.

[Cats] are also not like dogs in the sense that they're not just effective in the caudal rump if they have a flea allergy. Verry often it's miliary dermatitis and self-induced alopecia as a reaction pattern, and very often it is the lumbosacral malaria and the caudal medial thighs. But it doesn't have to be. And I actually think when I do see flea allergic cats, they have more of those indolent ulcers and my guess is that that's burning those fleas off, the reaction in their mouth from self-burning themselves,” Rosenberg continued.

Along with ruling our FAD, it is important to rule out dermatophytes, even though they are not technically on the diagnostic criteria list, and food allergies. If all other common causes of pruritus are ruled out, veterinary teams can conduct a skin biopsy in order to diagnose other potential conditions like feline herpes ulcerative dermatitis, or pemphigus.

References

1. Rosenberg A . Unique Considerations in Feline Dermatologic Disease. Presented at: New York Vet Show; November 7-8, 2024; New York, NY.
2. Favrot C, Steffan J, Seewald W, et al. Establishment of diagnostic criteria for feline nonflea-induced hypersensitivity dermatitis. Vet Dermatol. 2012;23(1):45-e11. doi:10.1111/j.1365-3164.2011.01006.x