DIFFERENTIAL DIAGNOSIS AND PATHOGENESIS OF NON INFECTIOUS, NODULAR TO DIFFUSE, GRANULOMATOUS AND PYOGRANULOMATOUS SKIN LESIONS IN DOGS(PATOLOGIE CUTANEE NON INFETTIVE A CARATTERE NODULARE E DIFFUSO DI TIPO GRANULOMATOSO E PIOGRANULOMATOSO NEL CANE: DIAGNOSTICA DIFFERENZIALE E PATOGENESI)

Non infectious, non neoplastic, nodular to diffuse, granulomatous/pyogranulomatous skin lesions are rarely reported in dogs and they often represent a clinical and histological diagnostic challenge. A precise diagnosis is pivotal for their adequate approach and treatment that differs for each entity. Leishmania spp. was reported in canine skin samples diagnosed as sterile in some veterinary studies. Mycobacterium spp. and other infective partially acid fast organisms (e.g. Serratia spp) might represent a possible source of persistent exogenous antigenic stimulation in cases with a previous diagnosis of sterile lesions. For this reason the aims of this project were to select retrospectly/prospectly skin samples with a histopathological diagnosis of cutaneous sterile nodular lesion and: to investigate the presence/absence of microorganisms by haematoxylin and eosin, PAS, Ziehl–Neelsen, Gram and Fite Faraco; to exclude presence of foreign bodies with polarized light examination; to detect the presence/absence of Leishmania spp. antigenic persistence in canine skin samples diagnosed histologically as sterile by immunohistochemistry; to perform identification of Leishmania spp., Mycobacterium spp., Serratia marcescens and Nocardia spp. by means of Real Time-PCR in canine skin samples diagnosed as sterile; to evaluate the effectiveness of RT-PCR technique in detecting Leishmania organisms in comparison with immunohistochemistry (Anti-Leishmania, anti-Bacillus Calmette-Guerin). Forty formalin-fixed, paraffin-embedded skin biopsies were collected from 40 dogs through the Diagnostic Pathology Service of the Veterinary Faculty of Milan, Italy and through the Pathology Service of Ambulatorio Veterinario Associato, Turin, Italy. All samples had lesions corresponding to the sterile granulomatous/pyogranulomatous pattern, in which neither microorganisms nor foreign bodies were detected by H&E and polarized light. PAS, Gram, Ziehl–Neelsen and Fite Faraco were negative for infective agents. All the samples were examined by RT-PCR analysis for Leishmania spp., Mycobacterium spp., Serratia marcescens and Nocardia spp. Furthermore, immunohistochemistry (IHC) was performed for Leishmania spp. Skin lesions were represented by nodules in 25/40, alopecia in 7/40, exfoliation and ulcers 3/40, crusts in 2/40; in 3/40 cases they were not reported. PAS, Ziehl–Neelsen, Gram and Fite-Faraco were negative for infective agents and polarized light failed to reveal any foreign bodies. The histological pattern was nodular to diffuse in 17/40, focally nodular in 12/40 and diffuse in 11/40. The infiltrate was granulomatous in 20/40, pyogranulomatous in 8/40 and granulomatous/pyogranulomatous in 12/40. The histological diagnosis was SPGS in 14/40, granulomatous vs pyogranulomatous dermatitis in 15/40, sterile nodular panniculitis in 4/40, reactive histiocytosis in 5/40 and juvenile cellulitis in 2/40. BCG was negative for infective agents. Immunohistochemistry for Leishmania spp. was positive in 1/40 samples while RT-PCR for Leishmania spp. was positive in 4/40 biopsies collected. RT-PCR for Serratia marcescens was positive in 2/40 samples; RT-PCR for Mycobacterium spp. and Nocardia spp. were negative in all 40 samples. In conclusion, Leishmania spp. and Serratia marcescens were the only infective agent identified in a minority of skin lesions with a diagnosis of cutaneous sterile nodular lesions. Further studies regarding other bacterial or protozoal organisms might lead to different conclusions.