Urticaria pigmentosa is a generalized cutaneous form of mast cell hyperplasia (1). Mast cell tumors are the most frequently occurring mast cell disease in cats. By contrast, urticarial pigmentosa is uncommon (15). Urticaria pigmentosa describes maculopapular cutaneous mastocytosis (MPCM), urticaria pigmentosa-like dermatitis, or papular eosinophilic/mastocytic dermatitis (11,12). Hairless or hypotrichosis cats like the Sphynx breeds and Devon Rex are typical, but cases occurring in Himalayan and domestic shorthair cats have also been described (8,12,13). Pathogenesis is unknown (8). In humans, urticaria pigmentosa is associated with cutaneous mast cell tumors and could be due to a metabolic disorder of mast cell differentiation and a mutation of c-kit, a membrane receptor for the stem cell factor that causes mast cell proliferation expressed in the surface membrane of mast cells (4,8).

Urticaria pigmentosa is commonly observed in young animals and causes erythematous macules, papules, nodules, or inconsistent hyperpigmentation with severe pruritus on the head, limbs, neck, and trunk (11). The lesions in affected cats can spread over the body (2). Lymphadenopathy can be present (7). Skin biopsies of lesions are characterized by moderate to severe perivascular dermatitis with numerous mast cells and few eosinophils (2,7). Clinical management with corticosteroids can be effective with or without antihistamines (10). This case report describes the successful treatment of feline urticaria pigmentosa with antihistamines only.

A six-month-old, intact female, 1.18 kg mixed-breed cat, was presented with a three-week history of significant pruritus and erythematous lesions on the ear margin. The cat received Fluralaner (Bravecto, MSD, USA) to exclude an ectoparasites infestation. The cat showed no response to oral antimicrobial treatments (Cephalexin in a dosage of 20 mg/kg twice daily for five days, Doxycycline in a dosage of 5 mg/kg twice daily for two weeks, and Metronidazole in dosage of 10 mg/kg twice daily for two weeks) and oral prednisolone at 0.5 mg/kg twice daily for five days prescribed by the referring veterinarian. The owner reported that the lesions showed gradually progressed exudation with crust formation on both ear pinna. The bilateral ear margin was removed by surgical excision. On the other hand, recurrence occurred approximately ten days later, and the left hind limb developed. Therefore, the cat was referred to the Chungnam National University Veterinary Teaching Hospital.

The physical examination revealed macular lesions and bilaterally crusted eruption on the ear margin and left medial hind limb (Fig. 1). Based on the clinical findings of crusted ear margins, differential diagnoses include vasculitis, mosquito bite hypersensitivity, feline scabies, and dermatophytosis. An enlargement of the left popliteal lymph node was detected. Cytological samples obtained from the lesions on the ear margin and the left medial hind limb, using direct impression smears and acetate tape strips, revealed the presence of degenerated neutrophils and cocci. Wood’s lamp and skin scrapping were negative. A microscopic examination of plucked hairs taken from the margin of the crusted areas was unremarkable for the presence of fungal elements and Demodex mites. Blood analysis showed no abnormalities. A mixed lymphoid population with nondegenerate neutrophils was observed from the cytology of the left popliteal lymph node. Two skin biopsies for a histopathological examination were collected from the margin of the right ear and medial hind limb using a 3 mm biopsy punch.

Figure 1. Six-month-old, 1.18 kg, intact female mixed-breed cat presented with (A) crusted eruption on the ear margin and (B) erythematous lesions with crust on the left medial hind limb.

The histology findings revealed epidermal hyperplasia with diffuse and mild hyperkeratosis (Fig. 2A). The lesions comprised a moderate perivascular diffuse infiltrate of well-differentiated, nonmalignant mast cells with numerous neutrophils and eosinophils in the dermis. The mast cells had a central nucleus, cytoplasmic granules, and no mitotic figures (Fig. 2B). A toluidine blue stain identified the presence of metachromatic purple granules (Fig. 3). No multinucleated giant cells were observed. A diagnosis of urticaria pigmentosa was made, based on these results.

Figure 2. Histopathologic examination of the right ear margin. (A) Epidermal hyperplasia and mild hyperkeratosis. (B) Diffuse mast cells and eosinophils infiltration of the dermis. H&E, ×100 (A) and ×400 (B).

Figure 3. Well-differentiated mast cells with metachromatic granules infiltration of the dermis. Toluidine blue stain, ×400.

The cat was treated with cetirizine (Zyrtec, UCB, Korea) at a dosage of 1.7 mg/kg once daily. A topical 0.05% chlorhexidine solution was applied twice daily to treat a bacterial infection. A marked improvement was noticed after two weeks of treatment. The same treatment was continued for seven days, which resulted in the complete resolution of all lesions (Fig. 4). No relapse occurred after the withdrawal of medication for two months.

Figure 4. After treatment, complete resolution of lesions on (A) the ear margin and (B) left medial hind limb.

In human medicine, mastocytosis indicates a pathologic increase in mast cells in cutaneous tissue and virtually all other organs, such as the bone marrow, gastrointestinal tract, liver, spleen, and lymph node (1,4). Mastocytosis is classified in pediatric and adult forms (4). Most pediatric cases of cutaneous mastocytosis do not include systemic findings and usually consist of skin lesions on the face, neck, and scalp, with urticaria pigmentosa being the most common (1,4).

Cutaneous mast cells in cats are uncommon in the epidermis, but they can increase in the epidermis during inflammatory diseases (9). They are abundant in the perivascular dermis and adnexa, with 4 to 20 cells per high-power field (×400) (9). The number of mast cells is higher in the cat skin of the ear pinna than in other body areas (6). Mast cells originate in the bone marrow and move to the peripheral tissues through the vascular system (7). They produce and release a variety of mediators, including histamines, cytokines, coagulation factors, and prostaglandins (7,14). Their cytokines may lead to peripheral eosinophilia (15). After mast cell stimulation, their cytoplasmic granules degranulate (6). This process plays a vital role in immunoglobulin E (Ig E)-mediated allergic disease (6). This is considered a sign of underlying allergic skin disease because of the large number of mast cells and eosinophils (12). Underlying allergies can overlap with the histological appearance of urticaria pigmentosa (13). Hypersensitivity reactions by dermatophyte allergens are in the progression of urticaria pigmentosa are also possible (5). Cases of urticaria pigmentosa were reported in some cats affected by dermatophytosis (5). In this case, the fungal culture was negative, and the histopathological examination detected no fungal organism. Nevertheless, strict food trials were not performed, so food allergies could not be ruled out.

A diagnosis of cutaneous mastocytosis is based on Darier’s signs, histological skin lesions, and an absence of systemic involvement (14). Darier’s sign is the development of flare and urtication after physical trauma to the affected skin, and is observed in children, but infrequent in cats (11,15). The clinical signs categorize it as urticaria pigmentosa, diffuse cutaneous mastocytosis, and mastocytoma of the skin (14). Urticaria pigmentosa is the most common cutaneous mastocytosis, while diffuse cutaneous mastocytosis (DCM) is less frequent (14). DCM consists of more generalized erythema and blisters (11). Some patients may have diffusely lichenification skin or nodular lesions (1). In cats, only one DCM case has been reported (3,11). Cutaneous mastocytoma manifests as a nodular lesion that is not 1 cm in diameter (14). They are all common at young ages and have a favorable prognosis (1,14).

A recent retrospective study of 13 cats diagnosed with MPCM proposed a new classification based on the clinical signs, prognosis, and current human consensus (11). The proposed classifications are subdivided into monomorphic, polymorphic, and pigmented variants (11). In polymorphic form, the lesions manifested as large wheals and papules localized to the head, neck, shoulders, and axilla with moderate pruritus (11). Erythematous dermatitis with a small macule but no hyperpigmentation was characterized as a monomorphic form (11). The pigmented form was chronic dermatitis associated with generalized lesions of hyperpigmentation and high pruritus (11). Different treatments, such as systemic glucocorticoids, antihistamines, or cyclosporin, have been recommended (11). Doxycycline can produce regression because of its inhibitory effect on mast cell activation (11). In one case, doxycycline was reported to be effective when used in conjunction with cyclosporine (11). It is not known why there was a lack of response to oral prednisolone in this case; however, the authors suspect this may be due to the low dose and short duration of prednisolone use. The previous case reported that the lesions were successfully controlled with prednisolone at anti-inflammatory doses (2 mg/kg orally once daily, followed by 1 mg/kg every other day for 1 month) in five Devon Rex cats (12). Antihistamines produce good results but only in cases of the polymorphic form cases (11). Based on the analysis, the cat reported in this paper was diagnosed with cutaneous mastocytosis and subclassified as the polymorphic form, according to the classification provided above. In this case, it was confirmed that the prognosis was good with the use of antihistamines alone for a short period.

The cat presented in this case report was treated the successfully for urticaria pigmentosa with only oral antihistamine for three weeks. No further clinical signs were encountered during the following months.

This work was supported by research fund of Chungnam National University.

The authors have no conflicting interests.