Crusting dermatoses are common in cats. Crust formation occurs when blood, an exudate and/or pus dry on the skin surface and become adherent to it, eventually including hair and scales.

Crusts seen in feline dermatoses

Three types of crusts are found in the cat:

1.  Spontaneously occurring large crusts (SOLC). These are generally coalescent, thick and occur frequently on the face. Good examples are notoedric mange and pemphigus foliaceus.

2.  Crusts secondary to excoriations (Exc). These have variable size and location. They can be seen in all pruritic dermatoses and particularly in allergic dermatitis.

3.  Small circumscribed (punctiform) crusts of feline military dermatitis (FMD). Papulocrusts seen in the typical feline dermatological complex may be follicular or extrafollicular. In dermatophytosis or bacterial pyoderma (rare in cats), there is a true intrafollicular pustule which can lead to furunculosis and granulomatous reaction. Serum exudation and pus formation lead to the formation of a small crust above the papular lesion. Most often, however, papulocrusts of miliary dermatitis are not follicular. This is the case in allergic dermatitis. First, there is dermal inflammation with infiltration of eosinophils and mast cells and epidermal spongiosis leading sometimes to vesicle formation, then polynuclear exocytosis occurs. A spongiotic pustule appears, dries quickly and leads to a small crust above the papular area.

Crusts are yellowish in superficial exudative processes and become darker when the lesions are deeper, progressing up to large adherent black crusts in cases of deep dermal lesions with haemorrhage.

Aetiology, clinical aspects, diagnosis and therapy of feline crusting dermatoses (see table below)

1. Allergic dermatitis

The most common allergic dermatitis in cats is flea allergy dermatitis, which is often expressed clinically as a miliary dermatitis. More than 50% of cases of miliary dermatitis are due to flea allergy dermatitis. There are also crusts secondary to excoriations in the dorso-lumbar area and less frequently on the neck and face. There may be also lesions of the eosinophilic granuloma complex (EGC). Skin-testing with flea extracts is not as reliable as in the dog and the therapeutic response to a thorough flea-control of the animals and the environment is of primary importance.

Food allergy/intolerance is often characterized by a severe pruritus of the face and neck with erosions and crusts in these areas as well as the ear pinnae and eventually the extremities. Miliary dermatitis and lesions of the EGC can also occur. Diagnosis is based on the result of an elimination diets and provocations.

Feline atopic dermatitis ("catopy") due to hypersensitivity to aeroallergens has a variable clinical presentation: pruritus of the face and extremities, otitis externa, lesions of the EGC, miliary dermatitis and extensive alopecia. Skin testing is not as reliable as in the dog and the value of serological testing is controversial. Glucocorticoids at moderate doses, antihistamines, essential fatty acids, cyclosporine and hyposensitization can be used, eventually as combination therapy.

Contact dermatitis is rare in the cat. Crusting dermatosis of the neck due to flea collars containing dichlorvos has been described. The author has seen a similar case due to a leather collar.

2. Dermatophytosis

Clinical aspects of dermatophytosis due to Microsporum canis are variable. Crusts can occur, particularly miliary papulocrusts, which are follicular. Diagnosis is based on Wood's lamp examination, direct examination of hairs, fungal culture and eventually cutaneous histopathology. Treatment is systemic (griseofulvin, itraconazole) and topical (enilconazole, miconazole and chlorhexidine) until cultures remain negative. All the animals must be treated and the environment must be disinfected (enilconazole, undiluted bleach).

3. Ectoparasitoses

Cheyletiellosis due to Cheyletiela blakeiis a contagious and pruritic disease which can cause miliary dermatitis. Brushing, tape-strip tests and scrapings can be used to find the parasite. Topical (fipronil) or systemic (ivermectin, selamectin) acaricides are effective.

Trombiculosis is a seasonal acariasis due to the infestation by larvae of Trombicula autumnalis. It can cause miliary lesions and crusting. Larvae, often visible at the naked eye, are very sensitive to acaricides.

Otodectosis due to Otodectes cynotis is a contagious disease mainly characterized by otitis externa but miliary dermatitis can occur because mites can also infest the whole body surface. Cerumen examination and scrapings will identify the mites, which are very sensitive to aural/topical or systemic acaricides. Topical treatment of the whole body surface should be combined with auricular treatment.

Pediculosis or phthiriasis is due to the infestation with Felicola subrostratus and can be the cause of excoriations with crusting and miliary dermatitis. Parasites are easily seen with the naked eye or with a magnifying lens. Topical (fipronil, imidacloprid) or systemic (selamectin) insecticides are effective.

Notoedric mange is a contagious and very pruritic acariasis which has become rare in Europe where only a few endemic areas persist (e.g., Lombardia, Latium, Slovenia, Andalucia). It is enzootic in some regions of Americas and Australia. Thick crusts occur, particularly on the head ("notoedric helmet"). Parasites are easily found in skin scrapings. Topical or above all systemic acaricidal treatment is effective.

Feline demodicosis is caused by the multiplication of the intrafollicular Demodex cati or the surface-dwelling Demodex gatoi. The disease is rare and may be associated with an underlying disease (e.g., diabetes mellitus, FeLV and/or FIV infection). Clinical signs are variable and pruritus exists particularly with Demodex gatoi. Therapy may be tried with amitraz or with systemic macrocyclic lactones (e.g., doramectin).

4. Drug eruptions

Erythema multiforme can be triggered by various drugs in cats: antibiotics (penicillin, cephalexin), griseofulvin, sulfasoxazole and aurothioglucose. The dermatosis can be severe with involvement of mucocutaneous junctions. Toxic epidermal necrolysis has been described in the cat, with ampicillin, hetacillin, penicillin and aurothioglucose as causes. Diagnosis is based on history, clinical signs and cutaneous histopathology. Prognosis is poor with a mortality around 50%. Therapy is based on drug eviction, cutaneous cleaning and applications of antiseptics, as well as general reanimation. Pemphigus foliaceus can be due to a drug reaction (see below).

5. Autoimmune dermatoses

Systemic lupus erythematosus exists in cats. Autoantibodies are directed against a variety of tissues and organs, with the skin affected in 20% of the cases (erosions, ulcers and crusts). Discoid (cutaneous) lupus erythematosus has also been described in the cat, as a seborrhoeic and crusting skin disease, triggered or aggravated by sunlight. Diagnosis is based on the finding of serum antinuclear antibodies in the systemic form, with cutaneous histopathology and direct immunofluorescence in both diseases.

Pemphigus foliaceus is the most common autoimmune skin diseases in the cat. A few cases have been attributed to a drug reaction (e.g., cimetidine and ampicillin). Primary lesions are pustules that evolve rapidly toward a crusting dermatosis. Facial and pedal lesions are rather suggestive. Diagnosis is based on history, clinical signs, cytology, cutaneous histopathology and direct immunofluorescence which can be positive in 50% of the cases. Cytological examination of the pus shows typical acantholytic keratinocytes surrounded by neutrophils, sometimes arranged in rosettes. Looking for such cells with this cytological examination of pustules is called a Tzanck' test. Pemphigus erythematosus is a rare variant with lesions mainly on the face and a histopathological examination showing a combination of pemphigus and lupus. Pemphigus vulgaris is extremely rare with variable erosions and crusts affecting the skin and mucocutaneous junctions. Histopathology and direct immunofluorescence can be diagnostic.

The treatment of autoimmune dermatoses is difficult. Immunosuppressive doses of short-acting glucocorticoids are required (e.g., prednisolone, 1 to 3 mg/kg BID). In case of poor response other glucocorticoids can be used (e.g., dexamethasone, 0.1 to 0.3 mg/kg BID). Doses are decreased progressively down to the minimal necessary level. Azathioprine is very toxic in cats and should not be used. Chlorambucil (0.1 to 0.2 mg/kg/day) may be helpful. Aurothioglucose is not available any more, unfortunately. The prognosis of autoimmune diseases should always be guarded even if cats can tolerate high doses of glucocorticoids better than dogs or humans.

6. Pyoderma

True bacterial infections of the skin are rare in cats, except cellulitis (abscesses) and acne. Cases of true folliculitis and furunculosis have been reported with follicular miliary dermatitis and crusted excoriations as the clinical presentations. Pathogenic Staphylococci sp are isolated. Serology testing for FeLV and FIV infections should be done. Diagnosis is based on history, physical examination, cytology which shows phagocytosis of cocci by neutrophils, bacteriology, eventually cutaneous histopathology and complete response to antibiotic therapy with cephalexin, Clavamox, trimethoprim-sulfa or fluoroquinolones.

7. Rare causes of crust formation in cats

A rare hypereosinophilic syndrome has been described in cats. Blood eosinophilia is extremely high (>20000 E/mm³) and skin lesions may occur (miliary dermatitis and facial excoriations). Glucorticoids are not very effective. Megestrol acetate may be helpful but causes severe side effects.

Viral infections can cause crusting lesions. In poxvirus infection erythematous areas and papules are readily covered with crusts, usually at a bite site. The lesions extend progressively but spontaneous healing occurs frequently. In herpes virus infection persistent ulcerative to necrotizing lesions develop on the dorsum of the muzzle. Diagnosis is based on cutaneous histopathology, electron microscopy, serology and viral isolation for poxvirus and PCR for herpes virus. Glucocorticoids are contra-indicated. Conservative treatment (antibiotics, topical antiseptics) and alpha interferon and lysine for herpes virus infection may be helpful.

Intermediate and systemic mycotic infections (e.g., cryptococcosis, sporotrichosis) can cause cutaneous lesions (nodules, ulcers) covered with crusts. Cutaneous histopathology and fungal culture are helpful to confirm the diagnosis.

Erythema, alopecia and crusts appear on the ear pinnae in case of solar dermatitis. Cutaneous histopathology will differentiate these lesions from squamous cell carcinoma towards which they can evolve. Sun avoidance and topical glucocorticoid therapy may be useful.

Idiopathic ulcerative dermatosis is a rare and poorly understood condition characterized by a well-demarcated ulceration in the interscapular area, eventually caused by injection of a substance. Diagnosis is based on history, physical examination and cutaneous histopathology.

8. Neoplasms

Many neoplasms can develop from the skin in cats. Crusts appear when ulceration occurs e.g., in squamous cell carcinoma and fibrosarcoma. More rarely this can happen also in basal cell carcinoma, mast cell tumor, sebaceous epithelioma and mycosis fungoides. Diagnosis and prognosis are based on histopathology. Treatments include surgery, chemotherapy (e.g., glucocorticoids for mast cell tumor, glucocorticoids and retinoids for mycosis fungoides) and radiotherapy (squamous cell carcinoma, mast cell tumor).

Practical diagnostic approach of crusting dermatoses in cats

As for all dermatoses, crusting dermatoses in cats require a thorough approach: history, physical examination, and appropriate complementary exams. Specific therapy will then be possible.

1. History is often very helpful.

 Age: kittens and young adults should be suspected of suffering from ectoparasitic diseases, dermatophytosis or allergic dermatoses whereas aged animals will be more prone to develop autoimmune or neoplastic diseases.

 Breed: Persian cats are predisposed to dermatophytosis.

 Contagion to other animals (e.g., in groups) or man is suggestive of a parasitic disease or dermatophytosis.

 Pruritus before the onset of the lesions is suggestive of allergic skin disease or a parasitic disease.

 A previous medical treatment is compatible with a drug reaction.

 A good response to a previous treatment may be an important diagnostic criteria, e.g., response to glucocorticoids or anti-parasitic agents.

2. A thorough physical examination, identifying the rare primary lesions (pustules, papules, nodules) and the nature and colour of the crusts, is particularly important, as well as the localization of lesions e.g.:

 Face, neck will be affected in food allergy, autoimmune diseases, demodicosis or otodectosis.

 Ear pinnae can be affected exclusively in otodectosis, solar dermatitis or dermatophytosis.

 Simultaneous involvement of mucocutaneous junctions and mucous membranes occurs in autoimmune diseases, drug reactions or viral diseases.

 Systemic signs can be helpful, e.g., respiratory signs (herpes virus infection), digestive signs (food allergy) etc.

3. Complementary examinations should be carefully selected and performed.

 Skin scrapings and fungal culture should always be performed.

 Cytological examination of pustules can suggest pyoderma or superficial pemphigus.

 Skin testing is not as easy and rewarding as in the dog but should be attempted when indicated.

 Elimination diet should last at least 2 months in case of suspicion of food allergy.

 Cutaneous histopathology can orientate the clinician in excluding some hypotheses or in establishing the diagnosis, e.g., in case of autoimmune skin disease or neoplasia in the aged cat.

Symptomatic treatment of crusting dermatoses in cats

The treatment of crusting dermatoses must be specific. However symptomatic therapy can be useful. Essential fatty acids are supposed to be helpful in miliary dermatitis, perhaps in contributing to the control of an allergic process. The value of topicals is probably under-estimated in feline dermatology. Topical therapy can help in removing crusts and improve the aspect of the skin and hair coat. Water is beneficial by itself in softening crusts and cleansing agents contribute to their removal. Shampoos are useful if the cat tolerates them well. Medicated shampoos are then beneficial.

Conclusion

Crusts can have many causes in cats. They are not specific. A classification in 3 types can help in establishing the diagnosis. Therapy of crusting dermatoses is specific but also symptomatic.

Further reading (detailed references are available on request)

References

1.  Bensignor E (2000) Diagnostic approach to crusting dermatoses. In Guaguère E, Prélaud P, Eds A Practical Guide to Feline Dermatology, Merial, Lyon, p20.1-20.4.

2.  Carlotti DN, Bensignor E (1995) Les dermatoses crouteuses du chat. Prat Méd Chir Anim Comp 30: 249-261.

3.  Scott DW, Miller WH, Griffin CE (2001) Muller and Kirk's Small Animal Dermatology, 6th edition, WB Saunders Company, Philadelphia.

4.  Sousa CA (1995) Exudative, crusting and scaling dermatoses. Vet Clin North Am Small Anim Pract 25: 813-832.