Background

Dermatophilus congolensis infection, colloquially known as "mud fever" or "rain rot," had previously been recorded exclusively in individuals with direct animal contact, including farm workers, veterinarians, and equestrian enthusiasts. Human-to-human transmission had not previously been documented.


In December 2025, Lyon's public hospital dermatology department admitted the first unusual case: a male patient in his fifties presenting with widespread red papules, pustules, yellow crusting, and persistent itching. Initial treatment for folliculitis and scabies proved ineffective. Bacterial culture and genetic sequencing of skin secretions confirmed Dermatophilus congolensis infection.

Epidemiological Findings

Over the following two months, eight additional male patients aged 22 to 63 with highly similar presentations were admitted. Whole-genome sequencing of bacterial samples from all nine core-study patients showed near-identical genetic sequences, confirming a single transmission source and ruling out independent zoonotic infection.


Seven of the nine patients had visited the same Lyon venue in the two weeks before symptom onset. The remaining two had not visited that venue but had engaged in close skin-to-skin contact with multiple partners at other social venues. None of the patients had any history of animal contact.

The research team concluded that Dermatophilus congolensis has achieved human-to-human transmission — the first confirmed instance of a sustained human transmission chain for this zoonotic bacterium.

Geographic Spread

As of mid-June 2026, nearly 30 confirmed cases had been registered across France, concentrated in Lyon and Paris. More than ten similar cases had been identified in Barcelona, Spain, with sporadic cases reported in Germany and Italy. Total linked cases across Europe stood at approximately 40.

Transmission Environment and Public Health Notes

Researchers noted that warm, humid, enclosed environments are conducive to bacterial spore survival and dispersal, with direct skin contact via shared surfaces identified as a likely transmission pathway. Extensive skin-to-skin contact and minor mucosal abrasions were identified as key infection risk factors. Two mechanistic hypotheses have been proposed: an early carrier with animal exposure introduced the bacterium into an enclosed venue, contaminating shared surfaces; or the bacterium underwent adaptive mutation in humid conditions, acquiring the capacity for sustained survival on human skin.


The report advises heightened skin hygiene awareness, reduced unnecessary direct skin contact in public facilities, and prompt medical attention upon the appearance of suspicious skin symptoms.


European Editorial Office: John