Rain scald

There are two different manifestations of rain scald: the winter form, which is more severe due to the longer coat of the horse, and the summer form, which is less severe.^[4] Horses are usually affected on the back, head, and neck where insects commonly bite, and the legs, which are commonly infected if the horse is kept in moist footing.^[5] Initially, the horse will display a matted coat and bumps which will progress to crusty scabs and lesions.^[6] The animal may also be pruritic and display signs of discomfort.^{[citation needed]}

In humans, dermatophilosis typically presents as a mild, localised cutaneous disease with symptoms including "papular, pustular or scaly, pruritic skin lesions", or "folliculitis-like eruptions", "mainly in the genital and facial area, without systemic symptoms",^{[3]: 1, 2} with lesion patterns mirroring those observed in sexually transmitted dermatophytosis.^[3]: 4 For cases identified among contact sport practitioners, symptoms were mainly mild occurrences of pustular lesions on the face, arms and torso.^[2]: 11

D. congolensis is a Gram-positive bacterium that is thought to originate from the soil. It commonly causes disease in moist tropical areas, but can also be found in wet northern environments.^[6] Moisture and high temperatures facilitate the dispersal and penetration of zoospores into the skin, contributing to the spread of the disease.^[4][6]

Diagnosis is most commonly done with the identification of bacteria in the lesions by microscopic evaluation.^[7] A positive diagnosis of rain scald can be confirmed if filamentous bacteria are observed, as well as chains of small, spherical bacteria (cocci).^[6] If a diagnosis cannot be confirmed with a microscope, blood agar cultures can be grown to confirm the presence of D. congolensis.^[7] The resulting colonies have filaments and are yellow in colour.^{[citation needed]}

Human diagnostic confirmation likewise relies on lab testing of lesion samples — through microscopy, culture or molecular methods,^[3]: 5 however ECDC reported that "Maldi-TOF may give inconclusive or low-confidence results" and that ribosomal 16S sequencing and whole-genome sequencing may be needed for species confirmation and comparison of isolates".^[2]: 12 In an August 2026 BMJ report, the UK Health Security Agency recommended that "healthcare professionals should be aware of the possibility of Dermatophilosis in GBMSM who present with compatible symptoms",^[8] reiterating advice from ECDC that awareness be raised "among clinicians, sexual health services and laboritories to support timely recognition, testing and reporting of campatible skin infections".^[2]: 13

In addition to wet conditions, exposure to ticks, biting flies, and contact with other infected animals can also cause the spread of rain scald.^[5] Tick and insect control is an effective way to stop the spread of the bacteria from one animal to another.^[7] Separating infected animals will help to isolate bacterial colonies.^[5] Keeping the animal in a dry, well-ventilated area out of the rain and wet conditions will stop the bacteria from growing.^[6]

ECDC reported that there is limited evidence on environmental control of D. congolensis in human settings but recommended the use of "established hygiene practices in shared venues",^[3]: 6 including routine cleaning and disinfection of surfaces "particularly in humid areas" for sex on premises venues, notable in the 2026 European clusters' etiology,^[3]: 2 and that "measures to reduce moisture accumulation ... may also be effective".^[3]: 6 Recommendations did not include strict isolation, but avoiding close contact with pets and livestock is recommended for people with skin lesions; close contacts should self-monitor for skin lesions for 14 days.^[3]: 5 ECDC recommended targetted risk communication and prevention advice for gay, bisexual and other men who have sex with men, and for operators of sex on premises venues.^[2]: 13

Rain scald normally heals on its own; however, the condition can spread, so prompt treatment is recommended. Although some cases can be severe, most rain scald is minor and can be treated at home naturally.^{[citation needed]} Typically the condition is not life-threatening, nor does it impact the welfare of the horse, so treatments are more for the owner's peace of mind and cosmetic appeal of the animal.^[6] Human cases typically see complete recovery following oral or topical antibiotic treatment, and with occasional self-limiting resolution;^[3]: 2 European clusters in 2026 described successful treatment with topical fusidic acid, antiseptic washes, doxycycline or amoxicillin.^[3]: 3

Human infections have only been reported sporadically and have typically been associated with contact with infected animals.^[3]: 2 However, ECDC reported in mid-2026 that "several clusters of genetically and epidemiologically linked cases predominantly affecting gay, bisexual and other men who have sex with men" had been seen in 4 EU countries in the first half of 2026 and that "cases have also been reported in Norway among individuals practicing martial arts, in 2025 and 2026".^{[2]: 1 [3]: 1} ECDC posited that "this indicates a possible shift in transmission dynamics, with human-to-human transmission through close physical contact being the most likely route",^[3]: 1 noting the epidemiological links and genomic relatedness between isolates.^[2]: 12 As of 17 June 2026,^[update] it is thought "the probability of D. congolensis infection among gay, bisexual and other men who have sex with men who have multiple sexual partners and attend sex-in-premises venues is assessed as moderate with a very low impact",^[3]: 5 justifying ECDC's overall risk level of "low" in this community.^[3]: 4