Equis ISSN 2398-2977


Synonym(s): Rain scald, Mud fever, Greasy heel, Scratches, Dew poisoning

Contributor(s): Dawn Logas, Rosanna Marsella, Sue Paterson, Simon Peek, David Senter, Beatrice Funiciello


  • Superficial bacterial dermatitis.
  • CauseDermatophilus congolensis Dermatophilus spp.
  • Signs: alopecia and crusting Dermatophilosis 02: mud rashDermatophilosis 01: chronic and severe.
  • Diagnosis: microscopic identification ofDermatophilus congolensis Dermatophilosis 04: cytology.
  • Treatment: keep dry +/- topical antibacterials +/- systemic antibiotics.
  • Prognosis: good.
Print-off the Owner factsheets on Mud fever and Rain scald to give to your clients.



Predisposing factors


  • Exposure to persistently wet weather/wet ground.
  • Trauma, secondary to allergies or insect exposure.
  • Intercurrent disease.
  • Malnutrition.
  • Stress.


  • Prolonged heavy rain.
  • Muddy paddocks.
  • Sweating.
  • Non-pigmented areas.
  • Trauma (grooming, rubbing tack, gravel thrown up by hooves, sharp vegetation).
  • Use of contaminated equipment allowing spread from infected horses.
  • Forceful spraying with water.
  • Ectoparasites, particularly biting flies Biting and nuisance flies.
  • Primary or secondary photodermatitis Dermatitis: solar.


  • Source: chronically infected carrier animals, or environment (saprophytic soil organism).
  • Transmission: direct contact, fomites or biting and non-biting flies and ticks.
  • Dermatophilus can be isolated in the soil where affected animals are kept.
  • Organic matter was found to have a protective effect on the micro-organism. Dermatophilus can also survive as a commensal and opportunistic pathogen in the skin of animals that are clinically normal, therefore potentially acting as a source of infection once favorable conditions are present.
  • Entry: requires abrasion in skin.
  • Infection: requires moist environment.
  • Dermatophilus congolensis from chronically infected carrier animals or saprophytic infection from soil → transmission by direct contact, fomites (persists for up to 42 months in crusts in the environment) or biting and non-biting flies and ticks → entry into epidermis through abrasion in skin → germination of motile coccoid zoospores in a moist defect of stratum corneum → proliferation mycelium within living layers of epidermis → primarily neutrophilic inflammatory response → barrier that Dermatophilus congolensis organisms are unable to penetrate → elimination of organisms as epidermis re-epithelializes Dermatophilosis 03: healed.
  • Non-pigmented areas with dermatophilosis infection may be associated with, and exacerbated by, photodermatitis.
  • Occasional severe cases of deeper, generalized exudative, neutrophilic dermatitis may be seen in immunocompromised horses.
  • Lymph nodes and/or systemic involvement may also occur with possible fever and weight loss.


  • Depends on time taken between exposure to organism and exposure to wet environment, and on the health and immune status of the host.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Frank L A, Kania S A & Weyant E (2016) RT-qPCR for the diagnosis of dermatophilosis in horses. Vet Derm 27 (5), 431-e112 PubMed.
  • Weese J S & Yu A A (2013) Infectious folliculitis and dermatophytosis. Vet Clin North Am Equine Pract 29 (3), 559-575 PubMed.
  • Colles C M, Colles K M & Galpin J R (2010) Equine pastern dermatitis. Equine Vet Educ 22 (11), 566-570 VetMedResource.
  • Pilsworth R C & Knottenbelt D C (2007) Dermatophilosis (rain scald). Equine Vet Educ 19 (4), 212-214 VetMedResource.
  • Pilsworth R C & Knottenbelt D C (2006) Pastern and heel dermatitis. Equine Vet Educ 18 (2), 93-95 VetMedResource.
  • Risberg A I, Webb C B, Cooley A J, Peek S F & Darien B J (2005) Leucocytoclastic vasculitis associated with Staphylococcus intermedius in the pastern of a horse. Vet Rec 156 (23), 740-743 PubMed.
  • White S D (2005) Equine bacterial and fungal diseases: a diagnostic and therapeutic update. Clin Tech Equine Pract 4 (4), 302-310 VetMedResource.
  • Towersey L, de Castro Soares Martins E et al (1993) Dermatophilus congolensis human infection. J Am Acad Derm 29 (2 Pt 2), 351-354 PubMed.
  • Zaria L T (1993) Dermatophilus congolensis infection (dermatophilosis) in animals and man! An update. Comp Immun Microbiol Infect Dis 16 (3), 179-222 PubMed.
  • Evans A G (1992) Dermatophilosis - diagnostic approach to non-pruritic, crusting dermatitis in horses. Comp Cont Educ Pract Vet 14, 1618-1623 AGRIS.
  • Evans A G & Stannard A A (1986) Diagnostic approach to equine skin disease. Comp Cont Educ Pract Vet 8, 652-661 VetMedResource.
  • McGavin M D & Fadok V A (1984) Factors limiting the usefulness of histopathologic examination of skin biopsies in the diagnosis of large animal dermatoses. Vet Clin North Am Large Anim Pract 6 (1), 203-213 PubMed.
  • Pascoe R R (1984) Infectious skin diseases of horses. Vet Clin North Am Large An Pract 6 (1), 27-46 PubMed.

Other sources of information

  • Scott D W & Miller W H (2011) Bacterial Skin Diseases. In: Equine Dermatology II. Saunders, USA. pp 130-170.
  • Knottenbelt, D C (2009) Bacterial Diseases. In: Pascoe’s Principles and Practice of Equine Dermatology. 2nd edn. Ed: Knottenbelt D C. Saunders, USA. pp 141-166.
  • Scott D W (1988) Large Animal Dermatology. W B Saunders, USA.