In cats that are resistant to all forms of oral and topical treatment, isotretinoin (Accutane®) at 2mg/kg/day can be given
orally. Response should be evident within four weeks and about one-third of cats respond. Long-term treatment is required,
and the dose should be reduced to every two to three days. Periodic chemistry screens and Schirmer tear tests are recommended
on cats on isotretinoin. If excessive chin rubbing due to scent marking in multi-cats households is thought to be related
to outbreaks of feline acne in one or more cats in the household, Feliway®, a pheromone product, can be applied to the environment
to reduce scent marking behavior and may help in some cases.
Photo 2: Atopic cat with extensive truncal alopecia due to over-grooming ("fur mowing").
Feline allergic dermatitis
Evaluating the pruritic/over-grooming cat or the cat with recurrent eosinophilic granuloma complex lesions can be a challenge.
Once parasitic mites (Otodectes, Cheyletiellosis, Notoedres, demodicosis), dermatophytosis, and FeLV/FIV infection have been
ruled out, a search for possible underlying allergies including flea allergy dermatitis, food intolerance/hypersensitivity
and atopy should be undertaken in a logical manner. Cats with allergic dermatitis due to any cause can present in many similar
ways, although some presentations are more commonly seen with one particular type of allergy (see Table 1). Generalized miliary
dermatitis (multiple small crusted papules) and stubble alopecia, especially over the caudal dorsum, tail base, and caudomedial
thighs is most commonly seen with flea allergy dermatitis. Severe head and neck pruritus is most commonly seen in food intolerance/hypersensitivity
(Photo 1). Atopy can prevent as generalized over-grooming with non-inflammatory alopecia ("fur mowing") (Photo 2), especially
of the insides of the forelimbs, groin and medial thighs, as miliary dermatitis, or as head and neck pruritus and dermatitis
due to self-trauma. Blepharitis, chelitis, and ceruminous otitis are most commonly seen with food intolerance/hypersensitivity
and atopy. To make matters more complex, cats may have more than one allergy.
Photo 3: Atopic cat with pruritic eosinophilic plaques in the groin area.
Recurrent eosinophilic granuloma complex lesions (indolent ulcer), eosinophilic plaques (Photo 3) and linear/collagenolytic
granulomas may be seen with flea allergy dermatitis, food intolerance/hypersensitivity and atopy. Most cases of eosinophilic
plaques are associated with underlying allergies, while some cases of indolent ulcers and collagenolytic ulcers, especially
in young cats and kittens, are idiopathic or genetically-based. In many cases, recurrent eosinophilic granuloma complex lesions
are the only underlying sign of allergic dermatitis and a skin biopsy should be performed of the lesion to rule out neoplasia
and fungal infection, and to look for signs of underlying allergy. Skin biopsy may also be helpful early in the work-up of
cases of generalized or localized dermatitis or alopecia, to confirm underlying allergy and to rule out infectious or neoplastic
disease. Histopathology cannot be used to determine the type of allergy, only to confirm that allergic dermatitis is present.
Table 1: Characteristics of Feline Allergic Dermatitis
Flea allergy dermatitis
All cats with non-seasonal or seasonal pruritus or recurrent eosinophilic granuloma complex lesions should first have a trial
of both on-animal and environmental flea treatment to determine if fleabite hypersensitivity (allergy to the saliva of the
flea) is the primary underlying cause. Fleas or flea dirt may or may not be present in flea allergic cats. Due to cats' meticulous
grooming habits and overgrooming due to pruritus, it is not uncommon to fail to find any evidence of fleas in flea allergic
cats. Remember, the more flea allergic the cat, the less likely it is that you will find fleas. If possible, cats that go
outdoors should be kept inside during the duration of the trial. Adulticide treatments with either imidacloprid (Advantage®),
fipronil (Frontline Top Spot®) or selamectin (Revolution®) applied every two weeks for three or four treatments should be
administered to the allergic cat, as well as monthly treatments of all in-contact dogs and cats. The indoor environment should
be treated at least once with a household flea spray that contains an adulticide (preferably permethrin) as well as an insect-growth
regulator. Remove pets and people from treated areas for at least four to six hours until treated surfaces are dry before
re-entering the area. Treated areas should be well-ventilated before pets and people return. If clinical signs in the patient
resolve, then flea allergy was the culprit, and flea prevention with year-round monthly flea treatments on all pets, keeping
the affected cat indoors, and periodic environmental treatment is needed to prevent recurrences of flea allergy dermatitis.
Table 2: Suitable Feline Commercial Food-Elimination Diets
Food hypersensitivity/food intolerance
If there is no response to flea control, then the allergy work-up should continue with a strict food elimination diet. This
is especially important if the cat has severe head and neck pruritus and/or history of gastrointestinal upset (vomiting and
diarrhea which may be associated with food intolerance. The most common food allergens in cats are dairy products, beef and
fish. A ten-week diet either home-cooked or commercially prepared (see Table 2 for acceptable diets) should be initiated.
If a home-cooked diet is used, a nutritionist or nutrition reference should be used to formulate a balanced diet with vitamin
and mineral supplementation, including calcium and taurine. All flavored cat treats, table food and chewable medications should
be discontinued. It is usually easier to feed all cats in the house the same elimination diet if possible. If not possible,
the allergic cat must be fed separately. Access to other pets' food must be prevented. Most food allergic cats will show significant
improvement within four to six weeks on a diet, however, some require the full 10 weeks to respond. If the pruritus and dermatitis
resolve on the diet, then the owner has two choices. The food allergic cat can continue indefinitely on a strict diet, knowing
there is a small chance a new allergy to that food will develop in the future. Alternatively, a dietary rechallenge with the
cat's original diet can be performed to prove food hypersensitivity. Most food allergic cats will show worsening of allergic
signs within hours to several days of rechallenge, with a few taking 10 to 14 days for pruritus to recur. If the food allergic
cat's pruritus worsens with food rechallenge, then the cat is returned to the elimination diet for seven to 14 days until
signs abate once more. Then one home-cooked individual protein or carbohydrate ingredient is added one at a time to the elimination
diet at weekly intervals. If there is no reaction to the added ingredient after one week, this ingredient is recorded as a
"good or safe" ingredient, and the next food is added. If a reaction is seen to that ingredient, it is immediately discontinued,
recorded as an "allergic or bad" ingredient, and the plain elimination diet is fed for three to 10 days until allergic signs
resolve. It is important to let allergic signs resolve before going on to the next ingredient to avoid confusion over which
ingredient is causing the problem. In this way, a list of "good" and "bad" ingredients can be formulated so that a commercially
available balanced cat food can be chosen that is free of the patient's allergic ingredients. Some cats are allergic/intolerant
to only one ingredient, while others are allergic to multiple ingredients, food additives or dyes. Some cats cannot tolerate
any commercial product, and need to stay on a nutritionist-prepared balanced home-cooked diet for life.
Pruritic cats that have had parasitic mites and fungal infections ruled out, and that fail to respond to flea control and
a food elimination diet, most likely are atopic. Feline atopy is caused by a genetic inappropriate immune response involving
allergens-specific IgE, to environmental allergens such as house dust mites, molds and grass, tree or weed pollen. Atopy is
second only to flea allergy dermatitis as a cause of chronic pruritus in cats. Atopy can occur in any age or breed or cat
but is more common in younger cats 6 months to 3 years of age. However, middle aged or older cats can develop the disease.
In most cats, the problem is nonseasonal and involves allergies to house dust mites (Dermatophagoides sp.) Some cats may also
have mold and pollen allergies. The four most common reaction patterns in atopic cats include: self-induced alopecia (fur-mowing),
eosinophilic granuloma complex lesions, miliary dermatitis, and facial otic and neck pruritus and dermatitis. In addition,
some cats will have rhinitis, chronic cough, allergic bronchitis, asthma, and chronic blepharoconjunctivitis alone or in addition
to their dermatologic signs. Lymphadenopathy and eosinophilia are common in chronic cases with severely inflamed and excoriated
skin, miliary dermatitis, or eosinophilic plaques. Cats with atopy may have concurrent food hypersensitivity and fleabite
Photo 4: Cat with indolent ulcer of upper lip.