The itchy cat—fleas aside—is not a common sight. But when cutaneous manifestations of allergy do occur in cats, they can be quite complicated and require multimodal therapy, according to Amelia White, DVM, MS, DACVD, assistant clinical professor of dermatology at Auburn University College of Veterinary Medicine. Dr. White offered attendees her insight about managing several types of allergies in cats this week.
Feline allergies: causes and imposters
The excessive groomer looks allergic, and indeed may be. “But a cat that grooms excessively might be doing so for reasons other than being pruritic,” Dr. White explained. Cats can respond to uncomfortable conditions like urinary tract infections and anal sacculitis by pulling out their hair.
Overgrooming can also be psychogenic; in many cases, it kicks off because of a true dermatologic condition (e.g. parasite infestation) but morphs into habit even after the underlying problem has resolved. “Allergic can become psychogenic very quickly,” Dr. White said.
Cats and their eosinophils
Most true allergies in cats fall under eosinophilic granuloma complex (EGC), which can manifest as different lesion types:
- Miliary dermatitis: small, raised, crusty papules
- Eosinophilic plaque: elevated, flat thickening with a red-orange center and crusting
- Rodent/indolent ulcer: eosinophilic infiltrate that occurs as bilateral, raised lesion(s) at the upper lip margins
- Eosinophilic granuloma: linear plaques, often on one or both hind legs, and dome-like masses that may occur in the interdigital tissues.
EGC is treated with multimodal therapy that sometimes includes antibiotics. “Steroids have a very strong inhibitory effect on eosinophils,” Dr. White said.
Feline pruritus can be rooted in parasites. The most common pruritic skin disease in cats is flea allergy dermatitis (FAD), which may or may not be seasonal depending on geographic region. While flea bites themselves are itchy, most cats develop type I and type IV hypersensitivities to flea salivary antigens. The head, neck and caudal half of the body typically manifest bilateral, symmetric alopecia as well as miliary dermatitis.
While there are no age, gender or breed predilections for FAD, atopic cats—as well as cats with intermittent flea exposure—are most severely affected; cats that are exposed to fleas continuously tend to build up tolerance.
Diagnosis of FAD is generally made by the distribution of lesions and itchiness on the cat, as well as the presence of fleas and/or flea dirt. Trichograms may show broken-ended hairs. Serologic and intradermal skin testing may be diagnostic, although false-positives can occur.
The treatment mainstays for FAD are corticosteroids, but immunosuppressants such as cyclosporine (Atopica—Elanco) and oclacitinib (Apoquel—Zoetis) are effective as well. Atopica, which inhibits the inflammatory activator calcineurin, requires 30 to 90 days for onset of action. Twice-weekly dosing has been shown to be 63% effective in cats with EGC. Apoquel can be used off-label in cats at a dose of 1 to 2 mg/kg twice daily, but Dr. White added that it should not be the first choice for treating allergies in cats.
The affected cat, the home environment and all other pets in the household should be treated with flea control products. Prevention can be accomplished through the use of parasiticides effective against fleas as well as through flea exposure avoidance.
Like flea bites, mosquito bites can also produce hypersensitivity (type I) in cats, with no predilections. Bloody, oozing, pruritic lesions typically develop on the rostrum, ears, face and paw pads. Corticosteroids and insect repellents are effective.
When food’s the culprit
Food allergy is an uncommon cause of nonseasonal pruritus in cats. Raised, crusty, erythemic plaques typically appear on the head, neck, pinnae and abdomen. Signs are typically first seen when a cat is quite young or geriatric, but a food allergy could manifest at any age, Dr. White said.
Once parasites and primary skin infection are ruled out, diagnosis can be made by food elimination trial, conducted over 8 to 12 weeks. The smoking gun is generally 10- to 70-kD proteins found in beef, fish or dairy, although dietary carbohydrates, preservatives and artificial flavors are lesser-known triggers. Food allergy is managed with steroids, antibiotics and dietary change to novel/hydrolyzed protein diets.
Atopy: allergies “on steroids”
Once fleas, food allergy and infectious dermatitis have been ruled out as the cause for nonseasonal pruritus, consider atopy, Dr. White said.
Atopy is cutaneous inflammation often associated with production of IgE against environmental allergens. Atopic cats may have a genetically predisposed abnormal cutaneous barrier to begin with; antigens easily penetrate the skin and are escorted to the lymph nodes by sentinel cells called Langerhans cells. Here, the lymphocytes produce antibodies that initiate the inflammatory cascade.
Atopy is the underlying state that “greases” the immune system: It makes every allergic condition worse, be it flea allergy or food hypersensitivity. It also intensifies allergic responses with subsequent exposures, and seasonal flares can turn into everyday issues. In fact, said Dr. White, 75% of atopic cats have year-around pruritus. “Even in cats that started out seasonal, over time they eventually become nonseasonal,” she said.
Atopic cats have widely distributed lesions, and diagnosis can be made by history, clincal signs and the process of diagnostic exclusion.
Allergen-specific immunotherapy is the gold standard of treatment for feline atopy, Dr. White said. Immunotherapy is developed from results obtained on an intradermal allergy test or serology (which generally shows high levels of IgE in circulation). Vials typically are limited to 10 to 12 antigens apiece, possibly because there is cross-reactivity between different antigens and to avoid diluting therapy to the point of making it ineffective.
Treatment is most effective with a multimodal protocol that includes allergen-specific immunotherapy, anti-inflammatories (corticosteroids, Atopica, Apoquel, antihistamines and essential fatty acids) and allergen avoidance. Because stress can exacerbate pruritic behaviors, Dr. White also recommended the use of behavior modulators like amitriptyline, fluoxetine and clomipramine in cats with intense pruritic behaviors.
Location, location, location
When it comes to cutaneous manifestation of allergies, Dr. White advised, lesion location is the biggest clue to allergy type:
- Head and caudal part of body: Think fleas.
- Head, neck and belly: Think food-induced.
- Diffuse pattern: Think atopy.
Immune hypersensitivities can also encompass locations below the skin surface. Associated signs include upper respiratory abnormalities, nasal/ocular discharge, asthma and inflammatory bowel disease.