Dermatophilosis resources

1. Why should dermatologists be aware of dermatophilosis?

In 2025-2026, two clusters of suspected sexually transmitted dermatophilosis were reported among men who have sex with men (MSM) in Barcelona, Spain, and Lyon/Paris, France, representing the first documented human-to-human transmission of Dermatophilus infection. A total of 18 cases were identified, all occurring in urban settings without animal exposure or tropical travel. Most patients reported recent sauna attendance, suggesting close skin-to-skin contact may facilitate transmission. These reports represent a notable shift in the epidemiology of a pathogen previously considered an exclusively zoonotic infection.(1,2)

2. What is dermatophilosis?

Dermatophilosis is an infection caused by Dermatophilus congolensis, a gram-positive, aerobic actinomycete best known in veterinary medicine as the cause of “rain rot” in horses and “lumpy wool” in sheep. Human infection has historically been rare and almost always associated with occupational or recreational exposure to infected animals. Prior to the 2025-2026 European outbreaks, person-to-person transmission had not been described and was limited to isolated case reports and small case series.(1-5)

3. Why should dermatologists care?

The newly described sexually transmitted form of dermatophilosis can closely mimic more common dermatologic and sexually transmitted conditions. Patients often present initially to dermatologists, sexual health clinics, or primary care physicians with folliculitis-like eruptions involving genital and intertriginous sites. Increased recognition may improve diagnosis and reduce onward transmission, particularly as mupirocin resistance has been reported.(6)

The estimated incubation period appears to be approximately 3-14 days, with a median of 6.7 days.(2) Across both European cohorts, HIV infection, concurrent sexually transmitted infections, and prior STI history were common.(1,2)

4. What are the skin findings?

Clinical presentation varied somewhat between the Spanish and French outbreaks; see Table 1.

5. What else can look like this?

The differential diagnosis includes:

• Bacterial folliculitis

• Secondary syphilis

• Mpox

• Herpes simplex virus infection

• Molluscum contagiosum

• Scabies

• Acneiform eruptions

• Candidiasis involving intertriginous areas

Dermatophilosis should be considered when folliculitis-like lesions occur in genital or intertriginous sites and routine microbiologic testing is unrevealing.

6. How is dermatophilosis diagnosed?

Diagnosis relies on culture and microscopy. On blood agar, Dermatophilus grows as yellow, rough, adherent beta-hemolytic colonies that typically become visible within 24-48 hours.(1,2)

Gram stain demonstrates characteristic:

• Gram-positive coccoid forms

• Branching filaments

• Transverse septa

• “Tram-track” appearance

MALDI-TOF mass spectrometry successfully identified isolates in both outbreaks and may provide rapid confirmation when Dermatophilus is included in the reference database.(4,7,8)

Whole-genome sequencing demonstrated highly related strains within each cluster, supporting direct human-to-human transmission. The Barcelona isolates may represent a previously undescribed Dermatophilus species.(1)

7. How is dermatophilosis treated?(3,5,7,8)

There are currently no formal treatment guidelines for human dermatophilosis. Available isolates demonstrated susceptibility to:

• Beta-lactams

• Tetracyclines

• Macrolides

• Trimethoprim-sulfamethoxazole

Most patients experienced rapid improvement following one week of therapy. Reported successful regimens included:

• Cefadroxil 500 mg twice daily

• Cloxacillin 500 mg every 6 hours

• Doxycycline 100 mg twice daily

• Amoxicillin 1 g three times daily

• Pristinamycin 1 g three times daily

Notably, all Barcelona isolates demonstrated extremely high-level resistance to topical mupirocin, suggesting mupirocin should not be relied upon for treatment. (1) Although spontaneous resolution may occur, antibiotic treatment is advisable to accelerate recovery and may reduce transmission.

8. Take-home points for dermatologists

• The first documented sexually transmitted outbreaks of dermatophilosis were reported in Europe during 2025-2026.

• Patients typically present with folliculitis-like papules, pustules, crusts, or scaly lesions involving genital and intertriginous sites.

• Recent sauna attendance and sexual contact have emerged as potential risk factors.

• Dermatophilosis may mimic common bacterial folliculitis, mpox, syphilis, or herpes infection.

• Diagnosis is confirmed by culture, microscopy, and MALDI-TOF identification.

• Most reported cases respond rapidly to short courses of beta-lactams or doxycycline.

• Awareness is important because this infection is likely underrecognized and may represent an emerging sexually transmissible dermatosis.