Dealing with feline allergic skin disease

There are several problems in the approach to allergic skin diseases
in cats. The first is that it is difficult to make an unequivocal
diagnosis of allergy in this species, largely because allergies have not
yet been well defined in cats. In dogs, atopic dermatitis has been
recognised and well described both clinically and immunologically,
whereas in cats, research is at a very early stage.

Clinical manifestations of allergy in cats are not as site-specific
as in dogs; for example, a cat scratching the neck may have a flea
allergy as well as food allergy, or a cat licking its belly may have a
flea allergy, food allergy or atopic dermatitis. To make things even
more complicated, in cats there are some clinical manifestations of
allergy that may be due to other causes. For example, a “bald belly” may
be due to flea allergy or to psychogenic causes, and a linear granuloma
may be associated with food allergy or may be hereditary or idiopathic.

Clinical appearance of feline skin allergy

The most frequent clinical signs of allergy in cats are facial
pruritus, self-inflicted alopecia, miliary dermatitis, eosinophilic
plaque and eosinophilic granuloma.

The eosinophilic granuloma is a well circumscribed, raised, firm, yellow-pink, linear lesion usually located on the caudal thigh (Figure 1). It is generally asymptomatic but can occasionally ulcerate and show pinpoint white foci of necrosis and become pruritic. It may also be located on the chin (Figure 2), paws or oral cavity (Figure 3). The eosinophilic granuloma has been associated with flea-bite allergy, food hypersensitivity, atopic dermatitis, mosquito bites, insect hypersensitivity, genetic predisposition and bacterial and viral infections (calicivirus).

The eosinophilic plaque is a very pruritic, well circumscribed, round to oval, erythematous, oozing, ulcerated plaque, mostly located on the abdomen and medial thighs (Figure 4). It is found in cats of all ages and breeds, and is often associated with flea allergy, atopic dermatitis and food allergy. Occasionally, it occurs together with feline miliary dermatitis. The eosinophilic plaque probably develops due to chronic trauma caused by the cat’s tongue, when licking pruritic areas. A secondary bacterial infection is frequent.

The lip ulcer (former name: indolent ulcer) is a well circumscribed, necrotic ulcer with raised borders located unilaterally or bilaterally on the upper lip (Figure 5). It is usually not painful and has also been associated with allergies.

Miliary dermatitis is characterised by discrete light brown crusts, diffusely distributed on the trunk (Figure 6).
The animals are often only mildly pruritic. Miliary dermatitis has been
associated with any allergy, but may also, albeit rarely, be associated
with a number of other causes, including bacterial infection,
dermatophytosis, drug reactions, pemphigus foliaceus and ectoparasites.

Self-induced alopecia

Bilateral symmetric alopecia is almost always self-induced due to licking (less frequently plucking) in cats (Figure 7).
The most frequently affected areas are the abdomen, groin and medial
thighs. Less frequently, the alopecia can affect the lateral thighs,
flanks or forearms. While hair plucking is more frequently associated
with psychogenic causes, licking can be induced by pruritus as well as
by pain or stress. It is worth remembering that itch is not only due to
allergy but can be elicited, albeit less frequently, by parasites,
fungi, bacteria and/or yeast, immune-mediated/autoimmune diseases and
tumours.

Head and neck pruritus

Cats can cause severe excoriations in the pre- and/or post-auricular
skin, face, chin and neck due to scratching with their hindpaws (Figures 8 and 9).
Head and neck pruritus is considered a sign of feline allergy –
particularly, but not exclusively, a food allergy. Other less frequent
causes of head and neck pruritus and lesions are ear mites, demodicosis
(particularly Demodex gatoi infestation), pemphigus foliaceus, dermatophytosis and herpes virus infection.

Approach to cats with pruritus and lesions compatible with allergic dermatitis

The approach to cats with signs of allergic disease is essentially based on:

1. Elimination of parasitic or fungal causes of pruritus.
2. Identification and treatment of secondary bacterial and/or yeast infections, if present.
3. Differentiation
   between adverse reaction to food and environmental allergy, if, after
   having completed steps 1 and 2, the cat is still showing excessive
   licking or scratching and/or signs of allergic dermatitis.

Ectoparasites are currently best eliminated with broad spectrum parasiticides, able to kill both fleas and mites (including Notoedres cati, Otodectes cynotis and Cheyletiella
spp.), such as selamectin, imidacloprid/moxidectin or fluralaner spot
ons. At the same time, in case the lesions are compatible with a
dermatophytosis, a fungal culture should be initiated with hairs
collected from both the centre and periphery of the lesions.

Bacterial skin infection is not considered to be as frequent in cats
as in dogs. Recently, a report of 52 cases of feline superficial
bacterial skin infections was published (Yu and Vogelnest, 2012).
Interestingly, skin lesions affected the face and the neck in 62 and 37
percent of the cases, respectively, with 92 percent of the animals being
pruritic. The authors concluded that in most cases, pyoderma was
associated with an underlying allergic disease. This observation
underlines the importance of looking for and treating a complicating
pyoderma in cases of excoriations due to allergy, as well as looking for
and controlling an underlying allergy in cases of feline pyoderma.

Every exudative lesion, particularly if subject to licking by the
cat, should be sampled cytologically for the presence of bacteria
(and/or Malassezia spp. yeasts).

Neutrophils containing bacteria indicate a real bacterial infection (Figure 10), while absence of neutrophils with large amounts of microorganisms is defined as a “bacterial overgrowth” (Figure 11).
Antibiotics can be indicated in cases of eosinophilic plaque, severe
excoriations and ulcers (neck lesion), only if cytological examination
reveals the presence of intracellular bacteria.

In cases of first occurrence superficial pyoderma caused by cocci (Figure 10),
an empiric treatment with amoxicillin/clavulanate 12.5-25 mg/kg q12h
or cephalexin 15-30 mg/kg q12h or clindamicin 5-10 mg/kg q12h can be
given for two to four weeks, or until one week after complete healing of
the lesions. In cases of the presence of rods in the cytological
preparation (Figure 12), a bacterial culture and
susceptibility test should be performed for the choice of a suitable
antibiotic. In cases of bacterial overgrowth or presence of Malassezia spp. yeasts (Figure 13),
a topical disinfecting treatment (eg chlorhexidine foam, gel or spray)
is usually sufficient. In cases of severe generalised yeast infection,
systemic itraconazole (5 mg/kg PO q24h for two weeks) can also be
administered. Antimicrobial therapy can be administered concurrently
with the previously mentioned antiparasitic therapy.

If thanks to antiparasitic (with or without antimicrobial) therapy, pruritus has disappeared, the owner should be advised to continue rigorous flea control permanently. Should pruritus persist after the elimination of skin parasites and pathologic microorganisms, the cat probably suffers from a non-flea-induced hypersensitivity, either due to food or environmental allergens. The recently published criteria for the diagnosis of feline non-flea-induced hypersensitivity dermatitis may aid in the diagnosis of this condition (Table 1) (Favrot et al., 2012).

To differentiate adverse reaction to food from environmental allergy
(equivalent to canine atopic dermatitis), an eight-week long dietary
elimination trial should be started. This should be rigidly enforced by
means of a home cooked or a commercial limited antigen diet containing
protein and carbohydrate sources unknown to the pet, or with a
hydrolysed food. Care should be taken that the cat does not scavenge
scraps from other animals. It may be necessary to stop the cat leaving
the house. If the cat is not cured but has improved after eight weeks,
continue the food trial for a further two to six weeks to see if
resolution occurs. If the food trial is negative or if the cat or owner
refuse to participate, the clinician must proceed with the hypothesis
that the cat may be suffering from atopic dermatitis.

Intradermal skin testing or serum IgE testing may indicate the
allergens responsible for the reaction, but should not be used to
diagnose atopic dermatitis, as many healthy cats show positive results
(Diesel and DeBoer, 2011). If the test results are positive,
allergen-specific immunotherapy (ASIT) can be undertaken with the
appropriate allergens, with reported efficacy similar to that of atopic
dogs.

In dermatology, as in other disciplines, cats cannot be considered
“small dogs”. Cats have specific clinical manifestations, such as the
eosinophilic granuloma complex, differential diagnoses and diagnostic
and treatment modalities. The latter will be described in a future
article dedicated to therapy of feline allergic dermatitis.