Researchers are raising concerns that a rare bacterial skin infection, primarily affecting animals, may have evolved to spread between humans. Two distinct clusters of dermatophilosis have been identified in Spain and France, affecting nearly two dozen individuals.
Dermatophilosis, also known as rain rot or rain scald, is caused by a bacterium that primarily infects livestock such as cattle and sheep, as well as wildlife. Human cases are uncommon and usually linked to direct exposure to infected animals or contaminated environments. Human-to-human transmission had not been previously documented.
Spanish Cluster
In Spain, researchers identified a cluster of dermatophilosis cases among men who visited primary care physicians or a clinic for sexually transmitted infections between December 2025 and March 2026. All patients were men who reported having sex with men. Crucially, none of these individuals reported contact with livestock or wildlife, nor did they travel to tropical regions where the infection is more common.
Four patients did report travel to other European cities for sexual activity. All patients reported visiting venues for sexual encounters in the week preceding symptom onset, and eight had visited a sauna. Two patients were regular sexual partners, and two others reported partners with similar symptoms. The men experienced itchy, red rashes with scabs, inflamed skin, hair follicles, and red papules, primarily on their genitals, thighs, groins, and bearded areas. Following treatment with antibiotics, all patients recovered fully. Lab testing confirmed the bacterium causing dermatophilosis. The researchers concluded that attendance at sexual venues may have contributed to transmission and that direct skin-to-skin contact during sexual activity likely served as the primary transmission route. Genomic findings supported recent transmission among humans, suggesting dermatophilosis might be emerging as a sexually transmitted infection, although environmental transmission could not be ruled out.
French Cluster
A second cluster was detailed in France, where nine men sought treatment for infections later identified as dermatophilosis between December 2025 and February 2026 at clinics in Lyon. These patients were also men who had sex with men, and none reported animal contact or travel to tropical regions.
Their symptoms included papules on their genitals, abdomen, legs, and the area around their mouths. Seven of the nine patients reported recent sexual encounters at a gay sauna in Lyon before their symptoms appeared. Another patient reported visiting multiple saunas in Paris, one of which had also been visited by another patient in the study.
Similar to the Spanish cases, all men in the French cluster received antibiotic treatment and experienced full recoveries. The researchers noted that the clinical presentation in this group differed from classical symptoms, suggesting a possible distinct clinical phenotype. The close genomic relationship between the sequenced bacterial isolates and the shared sexual exposures led the French researchers to conclude that interhuman transmission within sexual networks is likely.
Dermatophilosis is transmitted through direct contact or via intermediaries like ticks or biting flies. Symptoms typically include scaly crusts, inflamed skin, hair follicle inflammation, and red papules, often appearing on the face, genitals, legs, and abdomen. While the condition usually resolves without treatment, oral antibiotics are prescribed for severe or persistent cases.
