The allergic cat is a cat that scratches, licks, bites or pulls out its hair. It manifests its allergies by dermatological problems and sometimes by asthma. There are several possible causes of cat allergies.

Cat allergies and veterinary dermatology

Flea bite allergy (DAPP) is the most common. It mainly affects adult cats. Fleas are not always seen because cats are very good at maintaining their coat. They remove many fleas by efficient grooming. However, 25% of the fleas have already bitten the cat within 5 minutes and 97.2% are eaten after 1 hour on the cat's body. It is therefore very difficult to kill fleas before their first meal.
Food allergy also causes pruritus. The foods (protein sources) most often responsible for allergies are beef, dairy products, chicken or fish. These allergies occur non-seasonally. Although the face and neck are regularly affected, the clinical appearance does not differentiate this type of allergy from others.
Feline "Atopy" is poorly characterised. Feline atopy may be defined as an allergic pruritic cat for which a food allergy or an allergy to flea bites could not be identified. Seasonal involvement may be seen when pollens are responsible for the sensitivity. If dust mites are the cause, the symptoms can be observed all year round.
Contact allergy is much rarer. It is frequently induced by the application of sensitising products to the cat's skin (badly chosen shampoos, ointments to treat an ear infection, etc.).
Allergy to the stings of other insects is rare. However, some cats may develop lesions on the auricles and muzzle from mosquito bites or certain flies.

Clinical signs of allergy in cats. 51% of allergic cats show clinical signs before the age of 3 years. The head, neck and abdomen are affected in over 50% of cases. Abyssinian cats are predisposed to allergies.

Bilateral symmetrical alopecia or extensive feline alopecia: Affected cats appear partially alopecic (naked) but in reality the hair has been broken by licking or sometimes plucked by the cat's mouth. Affected areas are the abdomen, thighs, perianal area, tail and sometimes the forearms. Allergies, but also localised pain or behavioural problems can be the cause of this clinical picture. The hormonal problems mentioned in the past are not responsible for the lesions observed and treatments using sex hormones are contraindicated.
Pruritus of the neck causes ulcerative lesions which are often difficult to manage. Allergies but also local nerve stimulation or unidentified causes should be investigated.
Miliary dermatitis is characterised by the formation of small papules (pimples) with crusts on their surface. Miliary dermatitis is mainly seen on the back and is caused by an allergy (fleas, etc.), a parasitic disease (cheyletiellosis, etc.) or a mycotic infection (ringworm).
The feline eosinophilic complex includes several clinical entities such as indolent ulcer (labial cancroid), eosinophilic plaque and eosinophilic granuloma. These lesions contain large numbers of eosinophils (a type of white blood cell) which is the origin of the name "feline eosinophilic complex". Allergies are in the majority of cases responsible for these clinical entities. The indolent ulcer is characterised by erosive to ulcerative lesions of the upper lip. These lesions appear to be minimally pruritic but the cat's tongue regularly irritates the affected area. The eosinophilic plaque is an erosive, oval to circular, well-defined, oozing, slightly raised lesion (plaque). It is often seen on the abdomen, inner thighs but also on the face. The eosinophilic granuloma can be seen on the back of the thighs in a linear pattern, on the chin (localised pink-yellow swelling), on the feet or in the mouth. This type of lesion is allergic in origin, although a hereditary component or unknown origin is possible in linear granuloma affecting the posterior thighs.

Differential diagnosis

Extensive feline alopecia: identify the allergy responsible (fleas, food, "atopy") or look for a behavioural disorder or localised pain (cystitis, back pain...)
Miliary dermatitis: look for an allergy but also rule out the possibility of a bacterial or mycotic infection, a parasitic infection...
Indolent ulcer or labial cancroid: in addition to an allergy, a deep bacterial infection or a tumour may present in a very similar way
Eosinophilic plaque: identify an allergic cause with or without superinfection and, in the case of facial involvement, rule out feline herpes virus infection or hypersensitivity to insect bites

Complementary examinations and diagnosis

Microscopic examination of the removed hairs can prove that they are broken. This technique allows the detection of dermatophytes or skin parasites
Skin scrapings and microscopic examination of adhesive cellophane applied to the skin help to search for certain parasites (demodex, cheleytiella, notoedres, otodects...)
Wood's light examination identifies dermatophytosis (mycosis) due to Microsporum canis
Fungal cultures allow the identification of mycoses
Cytology allows the visualisation of eosinophils but also other cells, bacteria or yeasts
Skin biopsy confirms the diagnosis of eosinophilic complex. It can be useful when a tumour or a herpes infection is suspected...
Allergy tests (intradermal test or IgE serology) have a very low reliability in cats. They are not a good way to establish the diagnosis
The only reliable method of establishing the diagnosis of food allergy/intolerance is to use an avoidance diet using either a household food (proteins that have never been administered to the sensitive cat), or a commercial veterinary food using either a special protein or a hydrolysed protein. Blood or skin tests are not reliable in this case.

Treatments should be tailored to the cause of the allergy

Management of flea infestation by treating all contact animals and the environment
Diet avoiding the sensitising protein
Antihistamines (often not very effective)
Essential fatty acids
Desensitisation when the causative allergen can be identified
Antibiotics (sometimes useful to control indolent ulcers)
Locally used cortisone (with low skin resorption: hydrocortisone aceponate...)
Ciclosporin or oral glucocorticoids (cortisone)

Allergy in cats : Miliary dermatitis, feline extensive alopecia, eosinophilic complex