Flea Infestation and Flea Allergy Dermatitis

When associated with mild pruritus and minimal dermatitis, fleas can cause a symmetric alopecia of the dorsum, proximal tail, flanks, perineum, and ventral abdomen. Although a similar presentation can be seen in cats with flea allergy dermatitis, the alopecia and pruritus are much more severe in flea-allergic cats, and they typically have concurrent skin lesions (miliary dermatitis) along the dorsum, tail base, and ventral abdomen (see Chapter 45). Some flea-allergic cats do not have involvement of the typical body locations as mentioned; other body locations maybe involved, and flea allergy dermatitis should remain a differential diagnosis for cats with pruritus and alopecia even if the typical body locations are not involved.

Fleas and/or their feces are usually present on examination. However, in some cases the fleas and feces are removed during the excessive grooming (fleas may be found on fecal flotation in these cases) and the absence of fleas or flea feces should not rule out the possibility of flea allergy dermatitis.

Food Allergy

Food allergy in cats is associated with non-seasonal pruritus and a variety of clinical presentations, including FSA. The skin lesions and severe pruritus may be generalized or confined to the face and ears or trunk. Alopecia, whether diffuse or symmetric, results from excessive pruritus triggered by an allergic reaction to specific food items ingested (see Chapter 47). Stubbled or broken hairs, with or without concurrent focal erythema, papules, and crusting (miliary dermatitis), are usually present. Food allergy should be considered a differential diagnosis in any cat with non-seasonal pruritus and alopecia.

Feline Atopic Dermatitis

Atopic dermatitis in cats may be associated with seasonal or non-seasonal alopecia and pruritus (see Chapter 46). Similar to that seen in feline food allergy, the alopecia is secondary to pruritus and is associated with broken or stubbled hairs and concurrent skin lesions. The alopecia may be generalized or confined to the head and neck or distal extremities. Food allergy and atopic dermatitis are virtually indistinguishable on clinical examination in cats with non-seasonal pruritus.

Dermatophytosis

This is a common cause of localized or generalized alopecia in cats (see Chapter 42). Fungal organisms invade the hair shaft and grow downward but do not penetrate the mitotic region of the hair. Alopecia is not permanent unless the follicle is destroyed by secondary inflammation. A wide spectrum of clinical presentations can be seen with feline dermatophytosis, including the asymptomatic carrier state, particularly in longhaired cats. Classically, the alopecia is focal to diffuse with small numbers of fractured or stubbled hairs and mild epidermal erythema and scaling. The majority of dermatophyte infections in cats are caused by the zoophilic fungus Microsporum canis. Pruritus is variable in dermatophytosis and when present is typically mild and rarely moderate to severe.

Demodicosis

Demodicosis is an uncommon cause of alopecia in cats (see Chapter 43). The pathogenesis of feline demodicosis is reported to be similar to that described in the dog and caused by either Demodex cati (follicular mite) or Demodex gatoi (skin surface mite). D. cati is a normal inhabitant of feline skin, which under favorable conditions proliferates in hair follicles. Lesions consist of focal to diffuse alopecia, erythema, scaling, and crusts that may be localized and self-limiting or generalized. In some cases, these lesions may mimic feline endocrine alopecia. Generalized demodicosis due to D. cati infestation is usually associated with an underlying immunosuppressive disease, such as feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), diabetes mellitus, or neoplasia.

A pruritic, symmetric alopecia of the trunk caused by D. gatoi has been described in cats. Other body locations may occasionally be affected.

Otodectic Mange

Otodectes mites are the most common cause of otitis externa in cats (see Chapter 59). Otodectic mites can live on the skin surface and infrequently cause a symmetric alopecia over the lower back and tail base. Concurrent otitis externa may or may not be present. The degree of pruritus is variable.

Notoedric Mange

Notoedres mites cause a severely inflammatory and pruritic dermatitis involving the head, pinna, and neck in particular. Typically, miliary dermatitis is seen, but FSA may be an uncommon manifestation of infestation with this mite.

Bacterial Infection (Pyoderma)

Bacterial infection (usually staphylococci bacteria) is usually secondary to underlying primary disease, for example, allergic dermatitis, parasitic infestations, or endocrinopathy. Secondary bacterial skin infections are far less common in cats compared with in dogs. However, occasional cats may have a secondary bacterial infection that will mask response to flea control or food trials if not detected and treated to resolution.

Yeast (Malassezia) Dermatitis

Similar to bacterial infections, yeast dermatitis is usually a secondary problem associated with primary skin disease (allergy, parasites, endocrine, etc.) and is also far less common in cats compared with dogs. However, when present, yeast infections may induce moderate to severe pruritus in their own right, even when the underlying primary disease is non-pruritic (e.g., endocrine). Once again, failure to detect and resolve yeast skin infections will make it difficult to appreciate response to flea control, food trials, immunotherapy, etc.

Psychogenic Alopecia

This condition is often an enigma. Alopecia develops because the cat excessively licks, bites, or pulls out hair from those areas normally groomed (perineum, ventral abdomen, flank). In most cases, a stressful event such as moving to a new surrounding, being hospitalized or boarded, loss of a favorite companion, or introduction of a new pet or person (baby) into the environment precipitates the disease. Psychogenic alopecia is most common in nervous or “high-strung” cats, such as Siamese, Abyssinian, or black-colored cats. Typically, the areas affected are regions the cat can easily lick, such as the dorsal lumbosacral region, tail, medial and caudal thighs, ventral abdomen, flanks, and perineum. Partial to complete alopecia and stubbled hairs are observed, and concurrent skin changes (e.g., erythema, erosions, exudation, and crusting) are rare.

Feline Endocrine Alopecia

This is a disease of unknown cause. It is presumed to result from hormonal imbalances or deficiencies based on the positive response observed following treatment with specific hormones. The condition is characterized by non-pruritic, symmetric hair loss on the perineum, ventral abdomen, and caudal or medial thighs. Although the alopecia may spread to the flanks, lateral thorax, and proximal tail, the dorsum is usually spared. A thinning of the hair, rather than complete baldness, with normal, non-inflamed skin is the classic presentation. Feline endocrine alopecia primarily affects neutered females and males. However, this syndrome has been reported in intact cats.

Significant controversy exists regarding the relationship of hypothyroidism to feline endocrine alopecia. Affected cats usually have normal baseline serum thyroxine (T₄) levels; however, serum T₄ levels at 6 hours following stimulation with thyroid-stimulating hormone (TSH) have been low compared with TSH stimulation results in normal cats. These findings suggest that some cats with feline endocrine alopecia may have a low thyroid reserve.

Some cats with hyperthyroidism may develop truncal alopecia, although typically the skin lesions of hyperthyroidism are uncommon and more often associated with a dull, greasy, lusterless haircoat.

Feline hyperadrenocorticism is rare; however, spontaneous and iatrogenic Cushing disease can produce FSA of the trunk or pinnae associated with thin hypotonic skin.