Dermatophytic Pseudomycetoma by Microsporum canis in a Persian Cat Associated with Coinfection by Extended-Spectrum β-Lactamase-Producing Escherichia coli

Abstract

Background: Dermatophytic pseudomycetoma is an uncommon deep subcutaneous mycosis that mainly affects Persian cats, in which Microsporum canis invades beyond the epidermis and hair follicles into the dermis and subcutaneous tissue, forming chronic pyogranulomatous nodules mistaken for neoplasia. Cases complicated by secondary or multidrug-resistant bacterial infection are rare and hinder treatment. This report aimed to describe a case of dermatophytic pseudomycetoma caused by M. canis associated with extended-spectrum β-lactamase (ESBL)-producing Escherichia coli in a domestic cat, integrating clinical, diagnostic, microbiological and therapeutic data.

Case: A 5-year-old intact male Persian cat, weighing 4 kg, was presented with chronic ulcerated lesions at the base of the tail, bilateral alopecia and intense pruritus, after previous treatment with cefovecin and antifungal baths without clinical response. Physical examination revealed bilateral popliteal lymphadenomegaly and positive fluorescence under Wood's lamp. Cytology showed pyogranulomatous inflammation with degenerated neutrophils and epithelioid macrophages. Fungal culture isolated M. canis, whereas bacterial culture and biochemical tests identified ESBL-positive E. coli; the phenotypic ESBL profile was confirmed by Kirby-Bauer disk diffusion, and conventional PCR amplified the blaCTX-M gene from a single colony after thermal-lysis DNA extraction. Initial therapy with oral itraconazole (10 mg/kg/day), docosahexaenoic acid and topical ketoconazole plus chlorhexidine produced no significant improvement over 15 days. Owing to progressive tissue necrosis, caudal amputation was performed under balanced general anaesthesia; histopathology revealed diffuse pyogranulomatous dermatitis and panniculitis with PAS-positive hyphae surrounded by eosinophilic material consistent with the Splendore-Hoeppli reaction, confirming dermatophytic pseudomycetoma. The postoperative course was favourable under oral itraconazole and ciprofloxacin, with no recurrence during 2 months of follow-up.

Discussion: This case confirmed that dermatophytic pseudomycetoma is a chronic infection of complex progression, in which bacterial coinfection by multidrug-resistant strains may aggravate the clinical picture and reduce therapeutic efficacy. The repeated administration of cefovecin probably exerted selective pressure favouring the emergence of the ESBL-producing E. coli, while the limited diffusion of itraconazole into the encapsulated lesion likely contributed to the initial therapeutic failure. The pyogranulomatous reaction with PAS-positive hyphae encased in radiate eosinophilic Splendore-Hoeppli material reflected the host attempt to wall off the fungal antigens and was the histopathological hallmark that confirmed the diagnosis. Wide surgical excision complemented by systemic antifungal therapy remained the most effective strategy for definitive control. The detection of ESBL-producing E. coli carrying the blaCTX-M gene in a deep cutaneous infection highlighted a potential zoonotic risk and reinforced the need for microbiological surveillance and antimicrobial stewardship in companion-animal skin infections. To the authors' knowledge, this is the 1^st reported case of dermatophytic pseudomycetoma caused by M. canis associated with ESBL-producing E. coli, expanding current understanding of the interaction between fungal and multidrug-resistant bacterial infections in feline medicine and its impact on public health.

Keywords: blaCTX-M, cat, ESBL, Escherichia coli, Microsporum canis, pseudomycetoma, Splendore-Hoeppli.

Título: Pseudomicetoma dermatofítico por Microsporum canis em um gato Persa associado a coinfecção por Escherichia coli produtora de β-lactamase de espectro estendido

Descritores: blaCTX-M, gato, ESBL, Escherichia coli, Microsporum canis, pseudomicetoma, Splendore-Hoeppli.