When “Routine Fungal Dermatitis” Isn’t So Routine: A Case Every Veterinarian Will Relate to

A dog with alopecia, erythema, scaling, and pruritus often leads to a working diagnosis of dermatophytosis—and in many cases, that assumption is correct. Standard antifungal therapy typically delivers predictable results. However, an increasing number of cases in clinical practice are not following this expected course. Instead, they relapse, persist, or worsen despite seemingly appropriate treatment. 

Case Presentation: A Familiar Yet Challenging Scenario¹ 

An eight-year-old intact male Spitz (10 kg) presented with an 11-month history of dermatological lesions. Clinical examination revealed multifocal alopecia, erythema, scaling, and pruritus involving the face, trunk, and limbs. 

The dog had previously been diagnosed with fungal dermatitis and treated with oral itraconazole (10 mg/kg pulse therapy for one month), along with topical antifungal therapy using 2% miconazole shampoo and terbinafine dusting powder. Initial improvement was noted; however, relapse occurred within four weeks of discontinuing therapy. 

The treatment protocol was subsequently repeated multiple times by the owner without veterinary consultation, resulting in prolonged antifungal exposure without sustained clinical resolution. 

Progression to Refractory Disease¹ 

With time, the clinical picture became more complex. The dog developed elevated liver enzymes (ALT 275 U/L, ALP 350 U/L), while hematological parameters remained within normal limits. Antifungal therapy was discontinued due to suspected hepatotoxicity. 

At this stage, the case fulfilled the clinical expectation of refractory dermatophytosis, given persistence and recurrence despite repeated antifungal therapy. This highlights a key challenge in practice, distinguishing between routine and refractory disease early enough to alter the diagnostic approach. 

Diagnostic Re-evaluation: Moving Beyond Assumptions 

A structured diagnostic workup was initiated. Deep skin scrapings were negative for ectoparasites, helping exclude parasitic causes. 

The cytological examination revealed multiple fungal spores, indicating active infection. Skin biopsies were performed to rule out underlying immune-mediated conditions. Histopathological examination demonstrated fungal spores within hair follicles, supporting dermatophytosis. Histopathology becomes particularly valuable in recurrent cases, especially when prior antifungal therapy may reduce fungal load and result in false-negative cultures. 

Fungal culture yielded Microsporum canis after 14 days of incubation. Identification of the dermatophyte species is critical, as different organisms may vary in pathogenicity, environmental persistence, zoonotic potential, and antifungal susceptibility¹. 

Antifungal Susceptibility Testing: The Missing Link in Recurrent Cases 

Although antifungal susceptibility testing (AFST) is not routinely indicated in uncomplicated dermatophytosis, it becomes clinically relevant in refractory cases, particularly after repeated antifungal exposure^1,2. 

In this case, AFST revealed resistance to commonly used antifungal agents, including itraconazole, ketoconazole, and fluconazole. In contrast, susceptibility (dose dependent) was observed for posaconazole, voriconazole, and amphotericin B¹. 

These findings provided a clear explanation for repeated treatment failure and reinforced the importance of evidence-based drug selection in non-responsive cases. 

Rational Therapeutic Shift: Why Posaconazole Was Selected 

Following normalization of liver enzyme levels, treatment was initiated with posaconazole at 10 mg/kg once daily for three weeks, followed by 5 mg/kg once daily for an additional three weeks. 

Posaconazole is a second-generation triazole antifungal agent that inhibits fungal cytochrome P450-dependent 14α-demethylase, thereby disrupting ergosterol synthesis and fungal cell membrane integrity³. While its use in veterinary dermatology is limited due to cost and availability, in vitro studies have demonstrated potent activity against dermatophytes, with efficacy comparable to or exceeding that of itraconazole in some cases⁴. 

Although veterinary dermatology-specific data remain limited, its successful use in systemic mycoses and supporting evidence from human medicine in refractory dermatophytosis provide a rationale for its use in selected cases^1,5. 

Monitoring and Multimodal Management 

Given the known risk of hepatotoxicity associated with systemic azoles, baseline and periodic monitoring of serum biochemistry, particularly liver enzymes, was performed throughout therapy¹. A mild elevation in ALT was observed during treatment and managed with hepatoprotective supplementation. 

Adjunctive topical therapy using an essential fatty acid-based shampoo was continued weekly. Topical therapy plays an important role in reducing fungal burden, minimizing environmental contamination, and limiting transmission¹. 

Environmental hygiene and client education were emphasized, given the zoonotic nature of dermatophytosis, particularly for vulnerable populations such as children, elderly individuals, and immunocompromised persons. 

Clinical Outcome 

Progressive improvement was observed within five weeks of initiating posaconazole therapy, characterized by a reduction in erythema and scaling, followed by gradual hair regrowth. 

At the completion of therapy, complete clinical resolution was achieved. Repeat mycological examination was negative, confirming successful eradication of infection. Treatment was continued beyond clinical resolution to minimize the risk of relapse. 

Clinical Relevance for Veterinary Practice 

This case reflects a pattern frequently encountered in practice, where a seemingly straightforward dermatophytosis case evolves into a chronic, non-responsive condition. 

It underscores several important clinical considerations: 

• Recurrent or non-responsive cases should prompt diagnostic reassessment rather than repeated empirical therapy

• Prior antifungal use can influence diagnostic accuracy, necessitating careful sampling and, when required, histopathology

• Antifungal susceptibility testing can be pivotal in guiding therapy in refractory cases

• Multimodal management, including systemic therapy, topical treatment, and environmental control, is essential for successful outcomes

Conclusion: From Empirical Treatment to Diagnostic Precision 

This case highlights the importance of recognizing refractory dermatophytosis and adopting a structured, evidence-based approach to diagnosis and management. Identification of the causative organism, supported by histopathology and antifungal susceptibility testing, enabled targeted therapy and complete clinical resolution. 

In clinical practice, cases that fail to respond as expected should not be viewed merely as treatment failures, but as opportunities to refine diagnosis and improve therapeutic precision. 

References 

1. Tiwari A, Khajuria BK, Plowgian C, Hwang CY. Refractory Dermatophytosis in a Spitz Dog Successfully Managed with Posaconazole in India: A Case Report. https://doi.org/10.20944/preprints202601.2345.v1

2. Martinez-Rossi NM, Bitencourt TA, Peres NT, Lang EA, Gomes EV, Quaresemin NR, Martins MP, Lopes L, Rossi A. Dermatophyte resistance to antifungal drugs: mechanisms and prospectus. Frontiers in microbiology. 2018 May 29;9:1108. https://doi.org/10.3389/fmicb.2018.01108

3. Zhou J, Wei Z, Xu B, Liu M, Xu R, Wu X. Pharmacovigilance of triazole antifungal agents: analysis of the FDA adverse event reporting system (FAERS) database. Frontiers in pharmacology. 2022 Dec 15;13:1039867. https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2022.1039867/pdf 

4. AL-Khikani FH. Itraconazole and posaconazole from antifungal to antiviral drugs. Biomedical and Biotechnology Research Journal (BBRJ). 2022 Apr 1;6(2):164-9. https://journals.lww.com/BBRJ/_layouts/15/oaks.journals/downloadpdf.aspx?an=02058561-202206020-00002

5. Ebert A, Monod M, Salamin K, Burmester A, Uhrlaß S, Wiegand C, Hipler UC, Krüger C, Koch D, Wittig F, Verma SB. Alarming India‐wide phenomenon of antifungal resistance in dermatophytes: a multicentre study. Mycoses. 2020 Jul;63(7):717-28. https://doi.org/10.1111/myc.13091