Cutaneous bacterial pseudomycetoma is a chronic pyogranulomatous infection caused by nonbranching, nonfilamentous bacteria. It appears to be very rare in dogs, with only 3 reported cases being well-documented. Skin lesions are characterized by deep-seated nodules and plaques with draining tracts. Tissue grains may not be grossly visible. Diagnosis is confirmed by skin biopsies and cultures. Prognosis appears guarded, with surgical excision and prolonged antibiotic therapy not always being curative.

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Melanocytes transfer the melanosomes to the neighboring keratinocytes via dendritic projections, however, mechanism of melanin transfer in canine hair follicles have not been fully elucidated. In this study, we investigated the ultrastructure of hair follicles, focusing on melanin transfer in a clinically healthy dog and a dog with color dilution alopecia (CDA), in which melanosome transporting dysfunction has been proposed. In the normal dog, melanocytes in the hair matrix extended dendrites to the intercellular space and inserted into keratinocytes, but fusion of plasma membranes between melanocytes and keratinocytes were not observed. In the dog with CDA, oval shaped melanocytes without dendritic processes were filled with mature melanosomes. In addition, keratinocytes in the hair matrix did not contain melanosomes. In CDA dog epidermis, a few melanosomes were observed in the extraspace with the decreased number of melanosomes in keratinocytes. These findings suggested that the cytophagocytosis pathway seems to be the mainstream of melanosomes transfer in canine hair follicles and exocytosis pathway may work, in part, only in the epidermis.

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A three-year and ten-month-old, castrated male Miniature Dachshund presented with generalized pruritus and crusts, pustules and papules on dorsal, precordia, abdomen and inguinal area. The lesions except for pruritus did not respond to predonisolone. Bacterial cultures from intact pustules were negative. Histopathological examination revealed the formation of neutrophilic subcorneal pustule without acantholytic cells, infiltration of neutrophils in epidermis and superficial dermis. In addition, the results of direct and indirect immunofluorescence for IgG, IgA, IgM and C3 were negative. Based on these findings, the patient was diagnosed as a subcorneal pustular dermatosis. Dapson was administered at an initial dosage of 1 mg/kg every 8 hours. The response to the treatment was slow, however, after four months, pustules and crusted lesions diminished. The condition of skin lesion is well controlled.

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