Eruptive inflammatory tinea corporis: a case report highlighting the role of molecular testing on formalin-fixed tissue in confirming the causative species

Introduction

Tinea corporis refers to dermatophyte infections of the stratum corneum involving the trunk and the extremities except flexural surfaces. These highly contagious infections occur worldwide and are acquired by direct skin contact with humans or animals or fomites and secondary spread from other sites of infection. Cutaneous fungal infections often appear as “ringworm” with a well-defined erythematous or hyperpigmented scaling patch which gradually expands with central clearing and an accentuated scaling at the border (1). With time, additional annular lesions may erupt and coalesce to form polycyclic patches. Atypical and widespread presentations of tinea may be seen in immunocompromised patients. The sudden onset of pruritic papulovesicular lesions appeared in our patient. A case of tinea corporis demonstrates the variability of presentations which require the clinician to maintain a high diagnostic index of suspicion, leading to prompt initiation of appropriate testing followed by effective treatment. We present this article in accordance with the CARE reporting checklist (available at https://acr.amegroups.com/article/view/10.21037/acr-2025-184/rc).

Case presentation

A 22-year-old female presented with a slowly expanding, blistering rash on the neck that appeared one week after a gastrointestinal infection, for which she was treated with intravenous fluids. The patient described “popping” the blisters, leaving behind redness and crusting. In the subsequent days, additional papulovesicular patches developed on the abdomen, neck, bilateral arms, and lower extremities (see Figure 1). The patient’s fiancé developed similar lesions a few days after the rash first presented. There was no history of a recent upper respiratory tract infection or any new medications.

Figure 1 Erythematous papulovesicular patches with central clearing appeared on the neck followed within days by 12 other similar lesions on the abdomen, bilateral arms, and lower extremities. A few excoriations were noted, but no honey-colored crusting was present.

A dozen 1–2 cm well-demarcated erythematous pink, crusted, papulovesicular patches with central clearing were scattered on the abdomen, neck, bilateral arms, and lower extremities with sparing of the palms and soles. There was no honey-colored crusting to suggest bullous impetigo. No Herald patch was identified. The differential diagnosis included chickenpox, allergic contact dermatitis, pityriasis rosea (PR), bullous impetigo, and insect bites. Bed bugs were considered in view of the fiancé’s rash. Punch biopsies from the right upper arm, right lower arm, and left lower leg were obtained and demonstrated mild acanthosis, prominent spongiosis, and overlying basket-weave hyperkeratosis with focal parakeratosis and neutrophilic crust. No bacterial colonies were identified (see Figure 2). Hematoxylin & eosin (H&E) sections also showed neutrophils in the horny layer, and an underlying superficial perivascular, periadnexal, and mixed interstitial infiltrate (2). Periodic acid Schiff with diastase (PASD) and Grocott-Gomori methenamine silver (GMS) special stains identified septate hyphae within the stratum corneum and hair follicles (see Figure 3A,3B). Since the patient lived 150 miles from the clinic, returning for a fungal culture or polymerase chain reaction (PCR) of skin scrapings was not possible. Broad range PCR from the formalin-fixed biopsy specimen revealed Microsporum canis.

Figure 2 Histopathologic examination revealed spongiosis with overlying basket-weave hyperkeratosis, parakeratosis, and a neutrophilic crust (H&E, original magnification ×10). H&E, hematoxylin & eosin.

Figure 3 Histopathologic evidence of fungal elements in the tissue. (A) Septate hyphae were present in the stratum corneum (GMS, original magnification ×20). (B) Septate hyphae and spores were similarly present within an inflamed hair follicle (H&E and PASD, original magnification ×40). GMS, Grocott-Gomori methenamine silver; H&E, hematoxylin & eosin; PASD, periodic acid Schiff with diastase.

Treatment was initiated by phone (ketoconazole 2% cream twice daily and terbinafine 250 mg once daily for 1 month). A discussion with the patient about this zoophilic fungus revealed that her dog had acquired a scaling rash with hair loss in the past month. The veterinarian confirmed the presence of tinea and the animal was treated with oral griseofulvin. The patient’s fiancé was also treated with the identical regimen after a teledermatology consultation. Both humans and the dog cleared completely without recurrence over 6 months.

All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Discussion

The prototypical presentation of dermatophyte skin infections is one or a few annular inflammatory skin lesions with an erythematous and hyperpigmented leading scale. Often, the lesions of tinea corporis expand over the course of weeks as the fungal infection spreads outward, leaving a clear central area. Anthropophilic fungi such as Trichophyton rubrum often demonstrate very little scale while zoophilic fungi (Microsporum canis from dogs, Microsporum nanum from pigs, and others) show marked inflammation and prominent peripheral scale (2). The diagnosis of tinea is more difficult in eruptive, papulovesicular and bullous forms of tinea and the differential diagnosis is broad (3). It includes annular inflammatory lesions such as erythema annulare centrifugum with its erythematous edge and trailing scale. Granuloma annulare is easier to distinguish because it is a dermal inflammatory process with no scaling. PR, especially inflammatory PR, can demonstrate multiple annular lesions usually with an initial Herald patch. The diagnosis in this case was difficult because a dozen papulovesicular lesions erupted together with the primary lesions demonstrating little scale. Bullous tinea is most common on the feet (3). It is not rare, however, for zoophilic fungi to produce atypical patterns of fungal infection including papulovesicles and even bullous tinea (4). Gaining an understanding of the pathophysiology of tinea can help explain these clinical findings.

Geophilic and zoophilic dermatophytes such as Microsporum canis induce increased cytokine production through interleukin (IL)-17 activation leading to more severe inflammation than anthropophilic fungi (4,5). The presence of neutrophils in the horny layer led to PAS and GMS staining that revealed the presence of fungal elements within hair follicles. This would explain the papulovesicles seen in this case. Dermatophytid reaction, or id reaction, is a secondary dermatitis (delayed-type hypersensitivity to fungal infections) that produces a pruritic papulovesicular eruption, but id reactions occur distant from the initial site of infection (6). In this case, special stains identified hyphae in association with the papulovesicles.

In retrospect a KOH preparation and/or fungal culture could have been performed at the time of the initial visit, but tinea was not high on the clinical differential diagnosis because of the rapid onset of multiple, widespread, papulovesicular lesions and absence of typical “ring worm”. Distance precluded the patient from returning for a fungal culture. Molecular testing with broad-range fungal PCR targeting the 28S rDNA and internal transcribed spacer (ITS) rDNA was conducted on the paraffin embedded tissue and identified the fungus as Microsporum canis (7). In the future, molecular testing will be more commonly used because results can be obtained in hours and the tests high sensitivity and specificity (8). Unfortunately, molecular testing is not readily accessible with inevitable delays due to the time required to transport specimens to a reference lab. While molecular testing charges have been coming down, the cost is currently over $500 which is much more expensive than a KOH preparation or fungal culture. In our case, the confirmation of the specific fungus on formal-fixed tissue led to identification of the animal vector in the home. Once the dog’s tinea was confirmed by a veterinarian, there was an excellent response to simultaneous treatment of the patient, fiancé and the pet minimizing the chance for reinfection.

Conclusions

This case highlights an atypical, eruptive, papulovesicular patches of tinea corporis caused by Microsporum canis, emphasizing the importance of maintaining a broad differential diagnosis in patients with unusual cutaneous eruptions. If a discussion of household pets or farm animals identifies a likely source of the fungus, a referral to a veterinarian can confirm the diagnosis and appropriate treatment of patients and animal vector. Molecular diagnostic methods from formalin-fixed tissue confirmed the zoophilic species leading to diagnosis and successful treatment of the patient and her pet minimizing the chance for reinfection.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the CARE reporting checklist. Available at https://acr.amegroups.com/article/view/10.21037/acr-2025-184/rc

Peer Review File: Available at https://acr.amegroups.com/article/view/10.21037/acr-2025-184/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://acr.amegroups.com/article/view/10.21037/acr-2025-184/coif). R.T.B. is an investigator for multi-center clinical trials (Novartis, Sanofi, Lilly and the Corevitas Biologic Registry), receives Royalties for Up-To-Date, received a single consulting fee from Amgen for an Advisory board, and owns stock in Veradermics, Inc. He also provided payment for expert testimony in cases like Rothbloom vs O’Donoghue (Sarasota County, Florida Case No. 2024-CA-002555-NC) and Lok vs The Dermatology Group (Warren, Ohio Court of Common Pleas Case No. 24CV098085), and serves as President-elect of the Noah Worcester Dermatological Society. The other 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. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for the publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

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/.

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