Eruptive zoophilic dermatophytosis and the expanding role of molecular diagnostics

Introduction

Dermatophytosis, one among the most prevalent infectious conditions encountered across healthcare settings, occupies a paradoxical position in clinical medicine. Dermatophytes are keratin-adapted moulds that utilize human and animal keratin as a nutrient source, enabling infection of skin, hair, and nails. Dermatophytes are classically categorized into anthropophilic, zoophilic, and geophilic species based on their natural reservoirs, with anthropophilic species adapted to humans, zoophilic species primarily harboured by animals, and geophilic species, which are soil-dwelling organisms that only occasionally cause human disease. Dermatophytes, once classified within a limited traditional framework, have undergone substantial taxonomic revision following molecular phylogenetic advances and the adoption of the “one fungus–one name” principle. They are currently recognized under seven genera: Trichophyton, Epidermophyton, Microsporum, Nannizzia, Paraphyton, Lophophyton, and Arthroderma. Despite being among the most common infections encountered in clinical practice, dermatophytoses continue to pose therapeutic challenges, as their epidemiology is dynamic and continually reshaped by demographic, environmental, and behavioural factors rather than geography alone (1-3).

Goode et al. (4) reported an immunocompetent young woman with an acute, widespread papulovesicular eruption involving the trunk and extremities, accompanied by similar lesions in a close contact and concurrent dermatophytosis in a household dog. Histopathology demonstrated fungal elements, and species-level identification of Microsporum canis was achieved using polymerase chain reaction (PCR) on paraffin-embedded tissue, thereby enabling targeted antifungal therapy and coordinated treatment of contacts, resulting in sustained clinical resolution.

Although the organism itself is well recognized, the significance of this report lies in its demonstration of how dermatophyte infections, particularly those caused by zoophilic species, can defy conventional expectations. The case invites reflection on three interrelated themes: the variability of dermatophyte morphology, focusing on zoophilic dermatophytes, the importance of histopathology in atypical eruptions, and the emerging clinical relevance of molecular diagnostics when standard approaches are constrained.

Zoophilic dermatophytosis—etiology and ecological insights into transmission

Zoophilic dermatophytes are primarily adapted to non-human animal hosts, with Microsporum canis, Trichophyton mentagrophytes, and Trichophyton verrucosum accounting for most human infections. In animals, dermatophytosis “ringworm” commonly presents as circular alopecic lesions with minimal pruritus and asymptomatic carriage (3,5). Microsporum canis is a zoophilic dermatophyte with cats (most common) and dogs as its principal reservoirs. In cats, particularly young or immunocompromised animals, Microsporum canis infection may manifest as patchy alopecia, scaling, crusting, or erosions, especially with the animals housed in shelters, poor hygiene and overcrowding, acting as important risk factors. Infectious arthrospores shed from feline fur can persist in the environment for up to a year, enabling human infection through direct contact with affected or asymptomatic carrier animals, as well as via contaminated fomites, dust, or clothing. Transmission from cats to humans has been reported in up to 30% of cases, whereas secondary human-to-human spread, although documented, remains distinctly uncommon (6,7). Trichophyton erinacei is a zoonotic dermatophyte acquired from hedgehogs, including pet African pygmy hedgehogs (7). Livestock-associated species such as Trichophyton verrucosum in ruminants, Nannizzia nana in pigs, and Trichophyton equinum in horses pose occupational risks to farmers and handlers, while rodents serve as important reservoirs for Trichophyton mentagrophytes. Evolutionarily, zoophilic species retain greater potential for sexual reproduction than anthropophiles, although increasing host adaptation favours asexual propagation, further blurring the ecological boundaries between zoophilic and geophilic dermatophytes (3,5). In contrast to classical zoonotic transmission, Trichophyton mentagrophytes infections appear to occur almost exclusively via human-to-human spread, with little evidence to support ongoing animal-to-human transmission. This epidemiologic shift may reflect a process of “anthropization”, whereby dermatophytes previously adapted to animal or environmental reservoirs evolve to colonize human skin, as has been described for Trichophyton rubrum (8). It is also important to consider the ecological context of infection. Dermatophytosis is often conceptualized as an isolated cutaneous disease, yet it frequently exists within a broader network of transmission involving household contacts, pets, and shared environments. In the report by Goode et al. (4), the identification of concurrent disease in the patient’s fiancé and dog highlights the importance of comprehensive history-taking that extends beyond the individual patient. The coordinated treatment of all affected individuals and the animal reservoir was central to the reported successful outcome. Six months of recurrence-free follow-up suggests that addressing the full transmission ecosystem, rather than treating the index patient alone, can significantly reduce the risk of reinfection. This holistic approach is particularly relevant for zoophilic dermatophyte infections, where untreated animal reservoirs may perpetuate disease despite appropriate treatment of the index case.

Clinical presentation—a diagnostic dilemma

Dermatophytes implicated in bullous presentations include Microsporum canis, Trichophyton rubrum, and Trichophyton schoenleinii. When transmitted to humans, these species typically induce a more acute and inflammatory disease course than anthropophilic dermatophytes, likely reflecting limited host-pathogen adaptation. Bullous tinea has also been reported with Trichophyton tonsurans, an anthropophilic species typically associated with scalp infections, without any history of animal exposure (9). Blistering can occur in tinea pedis, which is classically attributed to Trichophyton rubrum and Trichophyton mentagrophytes. However, vesiculobullous lesions have been repeatedly linked to zoophilic dermatophytes, with reports describing targetoid papulovesicular lesions due to Microsporum canis following cat exposure (10), extensive bullous tinea corporis caused by the cattle-associated species Trichophyton verrucosum in immunocompetent individuals following close animal contact (11), bullous tinea caused by Microsporum canis mimicking linear immunoglobulin A (IgA) bullous dermatosis (12) and bullous lesions on the hand following guinea pig contact, caused by Trichophyton mentagrophytes (13).

In a nutshell, zoophilic dermatophytosis is characteristically more inflammatory, frequently exhibiting vesiculation or pustulation, erythema, induration, and the distinctive “red-rubber ring” morphology, a finding consistently reported in literature. The vesicular lesions in zoophilic infections often mimic eczema, viral exanthems, allergic or irritant contact dermatitis, pityriasis rosea, bullous impetigo, or arthropod reactions, predisposing to inappropriate use of topical corticosteroids and further contributing to diagnostic delay and atypical clinical appearance (14).

Role of histopathology

Histopathology plays a pivotal role in the diagnosis of zoophilic dermatophytosis, particularly in bullous presentations that frequently mimic eczematous or autoimmune blistering disorders. Bullae in dermatophytosis result from intense host inflammatory and hypersensitivity responses, rather than deep fungal invasion. In a large series of biopsy-proven bullous tinea, dermatophytosis was rarely suspected clinically, underscoring the diagnostic value of skin biopsy. Routine hematoxylin-eosin sections often fail to reveal fungal elements, necessitating the use of periodic acid-Schiff staining in a substantial proportion of cases. Fungal hyphae were most consistently identified in the stratum corneum adjacent to the blister rather than within the blister cavity itself, representing an important diagnostic clue. The predominant histologic pattern comprised marked spongiosis with intraepidermal blister formation and prominent neutrophilic infiltration, reflecting an exaggerated host inflammatory response typical of zoophilic infection. Dermal neutrophils, even when not the dominant inflammatory cell population, served as an additional histopathologic indicator of dermatophytosis. Importantly, the presence of deep dermal inflammation did not exclude a superficial fungal etiology and should not divert consideration toward immune-mediated blistering diseases. The classically described “sandwich sign” was inconsistently observed, limiting its diagnostic reliability in bullous or intensely inflammatory cases. Apparent acantholysis was occasionally encountered and represents a potential pitfall that may prompt misclassification as an autoimmune blistering disorder. Collectively, these findings highlight the necessity of maintaining a high index of suspicion and routinely employing fungal special stains when evaluating vesiculobullous eruptions in which zoonotic dermatophytosis is a diagnostic consideration (15).

Molecular and rapid diagnostic techniques in zoophilic dermatophytosis (16)

Traditional diagnostic approaches for dermatophytosis, such as potassium hydroxide preparation and fungal culture, are effective and widely used, but they depend on fresh material and patient availability. In the report by Goode et al., logistical constraints prevented repeat visits for skin scraping and culture, a scenario that reflects real-world practice, particularly in geographically dispersed or resource-limited settings. By targeting conserved ribosomal DNA regions, fungal PCR enabled species-level identification from tissue already obtained for diagnostic biopsy. The identification of Microsporum canis was not merely confirmatory; it prompted recognition of a zoonotic source, guided veterinary evaluation, and facilitated coordinated treatment of all affected hosts. While molecular diagnostics remain more costly and less accessible than conventional methods, their clinical utility in select scenarios is increasingly evident. These molecular and rapid diagnostic techniques complement conventional microscopy and culture by shortening diagnostic delays and improving species-level identification, a critical consideration in zoophilic dermatophytosis, where atypical, inflammatory presentations often obscure the underlying fungal etiology.

PCR

PCR-based approaches, such as restriction fragment length polymorphism and hybridization assays, are described as molecular tools for dermatophyte species identification, primarily used in specialized or reference laboratory settings rather than routine clinical practice. They allow rapid and sensitive detection of fungal DNA directly from clinical samples. Targeting conserved regions such as the internal transcribed spacer (ITS) enables accurate species-level identification, which is widely regarded as the reference molecular method for dermatophyte identification and is particularly useful for accurate species-level delineation, including among zoophilic dermatophytes.

Real-time/quantitative PCR (qPCR)

Real-time PCR is emphasized for its enhanced sensitivity, specificity, and rapid turnaround time compared with conventional PCR. The technique allows simultaneous detection and quantification of dermatophyte DNA, offering potential utility in monitoring treatment response and distinguishing active infection from colonization, although higher costs and limited availability remain constraints. Lin et al. (17) demonstrated that multiplex real-time PCR significantly outperforms conventional microscopy and culture in the diagnosis of skin and nail dermatophytosis, with nearly two-fold higher sensitivity and excellent specificity. Importantly, this technique reduced diagnostic turnaround time from several weeks to a few hours and enabled reliable species-level detection, including zoophilic dermatophytes such as Microsporum canis, supporting its utility in inflammatory or atypical presentations where rapid etiologic confirmation is critical.

Loop-mediated isothermal amplification (LAMP)

LAMP is presented as a promising rapid molecular technique that amplifies dermatophyte DNA under isothermal conditions without the need for sophisticated equipment. Its speed, simplicity, and visual readout make it particularly attractive for point-of-care testing and resource-limited settings, including situations where zoonotic dermatophytosis is suspected. Müs Tak et al. (18) showed that LAMP and multiplex real-time qPCR targeting the CHS-1 gene enable rapid and highly sensitive detection of Microsporum canis and Trichophyton mentagrophytes, with LAMP providing results within 30 minutes and multiplex qPCR within 45 minutes. Both methods demonstrated 100% specificity, high sensitivity (>93%), and low limits of detection, offering practical alternatives to conventional culture for diagnosing zoophilic dermatophytosis. Jiang et al. (19) developed a LAMP-based microfluidic assay capable of rapidly detecting six clinically important Trichophyton species, including the zoophilic pathogen Trichophyton verrucosum, with 100% specificity and high analytical sensitivity. The integrated platform completed amplification and detection within approximately 60 minutes, markedly reducing turnaround time compared with conventional culture. Such rapid molecular diagnosis has important implications for early, targeted therapy in dermatophytosis, particularly in atypical or zoonotic infections.

Recombinase polymerase amplification (RPA)

RPA is another isothermal amplification method capable of detecting dermatophytes within a short time frame. Its low-temperature requirement and rapid amplification kinetics position it as a potential bedside or field diagnostic tool, though clinical validation remains limited.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS)

MALDI-TOF MS enables rapid species-level identification based on protein spectral profiling from fungal cultures, with accuracy contingent on curated reference databases. It is useful in settings where conventional morphology is inconclusive and molecular sequencing is not readily available. In the study by Baumbach et al., among zoophilic species, Microsporum canis was consistently and reliably identified using standard commercial databases, likely reflecting the availability of multiple high-quality reference spectra. In contrast, closely related Trichophyton species (T. erinacei, T. verrucosum, T. benhamiae) frequently yielded borderline or incorrect identifications due to spectral similarity and underrepresentation in reference libraries. The authors highlight that database completeness is the principal determinant of diagnostic accuracy rather than inherent limitations of the technology itself. Extension of in-house spectral libraries using well-characterized isolates markedly improved species-level identification, including uncommon zoonotic pathogens such as Trichophyton erinacei. The study further shows that fresh cultures (≤5 days) and optimized extraction-based sample preparation significantly enhance identification scores. Importantly, the technique enables early identification before characteristic fungal morphology develops, shortening diagnostic turnaround time. However, the authors caution that MALDI-TOF MS should be interpreted in conjunction with sequencing for rare species or unexpected zoonotic infections (20).

Conclusions

Clinicians should always maintain a high index of suspicion for dermatophytosis, even when morphology deviates from classical descriptions. Histopathology remains indispensable in the evaluation of atypical cutaneous eruptions and should be considered when diagnostic uncertainty persists. Finally, molecular diagnostic techniques represent a valuable adjunct in select circumstances, particularly when conventional testing is impractical. Effective management of dermatophyte infections also requires ecological awareness, encompassing household contacts, animal reservoirs, and environmental factors. Recognizing atypical presentations of dermatophytosis and incorporating emerging diagnostic modalities when appropriate can improve diagnostic accuracy, guide effective treatment, and ultimately enhance patient outcomes.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, AME Case Reports. The article did not undergo external peer review.

Funding: None.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://acr.amegroups.com/article/view/10.21037/acr-2026-0065/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Segal E, Elad D. Human and Zoonotic Dermatophytoses: Epidemiological Aspects. Front Microbiol 2021;12:713532. [Crossref] [PubMed]
2. de Hoog GS, Dukik K, Monod M, et al. Toward a Novel Multilocus Phylogenetic Taxonomy for the Dermatophytes. Mycopathologia 2017;182:5-31. [Crossref] [PubMed]
3. Fratti M, Bontems O, Salamin K, et al. Survey on Dermatophytes Isolated from Animals in Switzerland in the Context of the Prevention of Zoonotic Dermatophytosis. J Fungi (Basel) 2023;9:253. [Crossref] [PubMed]
4. Goode SS, Quiring RA, Awais M, et al. Eruptive inflammatory tinea corporis: a case report highlighting the role of molecular testing on formalin-fixed tissue in confirming the causative species. AME Case Rep 2026;10:44. [Crossref] [PubMed]
5. Moskaluk AE, VandeWoude S. Current Topics in Dermatophyte Classification and Clinical Diagnosis. Pathogens 2022;11:957. [Crossref] [PubMed]
6. Capoor MR, Sharma S, Goenka S, et al. Tinea capitis caused by Microsporum canis: A case study of three family members in India, a non-endemic region. Indian J Med Microbiol 2024;50:100621. [Crossref] [PubMed]
7. Nenoff P, Krüger C, Ginter-Hanselmayer G, et al. Mycology - an update. Part 1: Dermatomycoses: causative agents, epidemiology and pathogenesis. J Dtsch Dermatol Ges 2014;12:188-212.
8. Verma SB, Panda S, Nenoff P, et al. The unprecedented epidemic-like scenario of dermatophytosis in India: II. Diagnostic methods and taxonomical aspects. Indian J Dermatol Venereol Leprol 2021;87:326-32.
9. Peris-Espino V, Prat-Colilles E, Marsol IB. Bullous Tinea Manuum. JAMA Dermatol 2025;161:1273-4. [Crossref] [PubMed]
10. Lim SYD, Lee JSS, Chong WS. Targetoid bullous tinea corporis: Unusual presentation of a dermatophyte infection. Indian J Dermatol Venereol Leprol 2021;87:101-3. [Crossref] [PubMed]
11. Sahu P, Dayal S, Mawlong PG, et al. Tinea Corporis Bullosa Secondary to Trichophyton Verrucosum: A Newer Etiological Agent with Literature Review. Indian J Dermatol 2020;65:76-8. [Crossref] [PubMed]
12. Aalfs AS, Jonkman MF. Tinea corporis bullosa due to Microsporum canis mimicking linear IgA bullous dermatosis. Eur J Dermatol 2012;22:805-6. [Crossref] [PubMed]
13. Albanell-Fernández C, Muñoz-Castro D, Revert Fernández Á. Bullous Tinea. Actas Dermosifiliogr 2025;116:181. [Crossref] [PubMed]
14. Bunyaratavej S, Kiratiwongwan R, Komoltri C, et al. Predictive equation to identify infection due to anthropophilic or zoophilic dermatophytes based on clinical features and risk factors: A ten-year retrospective study. Indian J Dermatol Venereol Leprol 2022;88:416-9. [Crossref] [PubMed]
15. Drolshagen H, Zoumberos N, Shalin S. Bullous Tinea: Single-Center Retrospective Histopathologic Review of 25 Skin Biopsies. Arch Pathol Lab Med 2023;147:1327-32. [Crossref] [PubMed]
16. Aboul-Ella H, Hamed R, Abo-Elyazeed H. Recent trends in rapid diagnostic techniques for dermatophytosis. Int J Vet Sci Med 2020;8:115-23. [Crossref] [PubMed]
17. Lin BB, Pattle N, Kelley P, et al. Multiplex RT-PCR provides improved diagnosis of skin and nail dermatophyte infections compared to microscopy and culture: a laboratory study and review of the literature. Diagn Microbiol Infect Dis 2021;101:115413. [Crossref] [PubMed]
18. Müs Tak HK, Ünal G, Müs Tak INBA. Detection of Microsporum canis and Trichophyton mentagrophytes by loop-mediated isothermal amplification (LAMP) and real-time quantitative PCR (qPCR) methods. Vet Dermatol 2022;33:516-22. [Crossref] [PubMed]
19. Jiang W, Hu D, Xu Y, et al. Loop-mediated isothermal amplification-microfluidic chip for the detection of Trichophyton infection. Front Microbiol 2022;13:1031388. [Crossref] [PubMed]
20. Baumbach CM, Müller S, Reuschel M, et al. Identification of Zoophilic Dermatophytes Using MALDI-TOF Mass Spectrometry. Front Cell Infect Microbiol 2021;11:631681. [Crossref] [PubMed]