During December 2025–February 2026, a total of 9 men sought care at the sexually transmitted infections (STI) clinics of the University Hospital in Lyon, France, for skin infections that were determined to be caused by D. congolensis. All patients were MSM living in urban areas; they were 22–63 years of age (median 50, interquartile range [IQR] 34–59.5 years). None reported occupational exposure to livestock or direct contact with farm animals or horses, although some did report regular or occasional contact with domestic pets (cats or dogs) were reported. None reported recent travel to tropical regions.

Figure 1

Figure 1. Dermatophilosis lesions in patients in cluster of suspected sexual transmission of dermatophilosis among men who have sex with men, Lyon and Paris, France, 2025–2026. A) Papular lesions of the neck...

All patients exhibited nonspecific erythematous papules, occasionally pustular or squamous, mainly located in the genital region (penis, scrotum, pubic area; n = 8), trunk (n = 5), perioral region (beard area; n = 4), lower limbs (n = 4), and, less frequently, anal margin (n = 1) (Table; Figure 1). Lesions predominantly involved areas exposed during sexual contact, without mucosal involvement. Pruritus was variably present. No systemic symptoms were reported except for patient 7, who experienced fever and vomiting 2 days before medical consultation. Given the clinical manifestations and epidemiologic context, differential diagnoses included staphylococcal folliculitis, sexually transmitted dermatophytosis (Trichophyton mentagrophytes genotype VII), Klebsiella aerogenes folliculitis, secondary syphilis, mpox, and molluscum contagiosum.

Figure 2

Figure 2. Results of sample testing in cluster of suspected sexual transmission of dermatophilosis among men who have sex with men, Lyon and Paris, France, 2025–2026. A, B) Dermatophilus congolensiscolonies...

For all patients, we cultured lesion swab samples from involved areas on nonselective media: blood agar plates (aerobic atmosphere) and chocolate agar plates (with 5% CO₂). After 40 hours, cultures yielded β-hemolytic, rhizoid, adherent, rough, shiny, and yellowish colonies on both culture media (Figure 2, panels A, B), and occasionally along with colonies from the skin microbiota. Gram staining revealed identical filamentous, branching gram-positive bacilli with transverse and longitudinal septations producing a characteristic tram-track appearance (Figure 2, panel C). Bacterial identification by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identified D. congolensis with a high confidence score in all cases. In 4 patients, bacterial cultures also yielded pyogenic pathogens: Staphylococcus aureus (patients 3, 7 and 9) or S. lugdunensis (patient 8). Whole-genome sequencing (NextSeq550; Illumina, https://www.illumina.com) and pairwise alignment of D. congolensis isolates from patients 1–8 revealed 1–5 single-nucleotide polymorphisms, covering >94% of the genome, supporting close relatedness and recent direct or indirect transmission from a common source. Reads are available for consultation on the European Nucleotide Archive database (Table). Concomitant STIs were diagnosed in 2 patients (pubic pthiriasis in patient 1 and syphilitic reinfection in patient 7).

Epidemiologic interviews revealed that 7 of the 9 patients reported recent sexual encounters at a gay sauna in Lyon within days or weeks before symptom onset (Table). Patient 5 reported multiple sexual partners in various saunas in Paris during the same period, including 1 that patient 3 visited 2 days earlier. Patient 9 did not report any sauna attendance. On the basis of reported exposures, incubation period ranged from 3 to 14 days; survival modeling with interval censoring estimated a median of 6.7 (95% CI 4.3–10.1) days. However, multiple potential contacts limited precision (Table).

Amoxicillin MIC determined for 1 isolate using Etest (bioMérieux, https://www.biomerieux.com) was 0.064 mg/L, supporting high susceptibility to β-lactams. All patients received oral amoxicillin (1 g 3×/d) or pristinamycin (1 g 3×/d) for 7 days, sometimes combined with topical antiseptic care, with rapid improvement. No patient relapsed; median follow-up was 10 (IQR 5–52) days after treatment. However, on follow-up, patient 1 showed with a new D. congolensis infection on the buttocks, occurring 8 weeks after the first occurrence (Figure 1, panel D). His ongoing visits to the same sauna after recovery and the different infection site suggest reinfection rather than relapse.

Compared with previous reports of dermatophilosis, the predominantly papular and noncrusted manifestation observed here might differ from the classical description, raising the possibility of a distinct clinical phenotype. Moreover, lesions mainly affected areas exposed during sexual intercourse (face and genitals), mirroring patterns seen in sexually transmitted dermatophytosis or K. aerogenes folliculitis (12–14). Whereas no mucosal lesions were observed in this cluster, rare literature reports indicate that D. congolensis can infect human mucosa, justifying detailed clinical examination (6,11). In some occurrences, S. aureus and S. lugdunensis were also isolated from lesion sampling and might have modified clinical manifestations. Their involvement complicates the interpretation of D. congolensis pathogenicity in the given occurrences. Secondary infection of Dermatophilus-induced lesions by such pathogens is the most probable explanation, although co-transmission remains possible.

Although no direct sexual contact between patients could be formally established, the temporal clustering, overlapping sexual exposures, shared multiple STI history, lesion distribution, and close genomic relatedness of isolates strongly support transmission occurring within shared exposure networks, likely involving close physical or sexual contact. The presence of viable bacteria within lesions is consistent with the hypothesis of contact-driven transmission, although the exact transmission route remains uncertain. Environmental or indirect transmission within shared venues remains possible; no environmental sampling or carriage screening was performed at the time of the study to clarify transmission pathways.

We describe a large occurrence of human dermatophilosis cases in an urban population of sexually active MSM without reported livestock exposure. The combination of close genomic relatedness between the 8 sequenced isolates and shared sexual exposures suggest interhuman transmission within sexual networks. Systematic microbiologic evaluation of atypical cutaneous lesions is essential for identifying similar cases and clarifying transmission of emerging cutaneous STIs among MSM. Evolving sexual practices in the HIV preexposure prophylaxis era (15) could lead to emergence of new transmissible dermatoses (12–14). Microbiologists should ensure that D. congolensis is included in reference spectral libraries, be able to recognize its colonies in the context of skin infection, and report its identification to clinicians.