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Dermatophilosis
Synonym(s): Rain scald, Mud fever, Greasy heel, Scratches, Dew poisoning
Introduction
- Superficial bacterial dermatitis.
- Cause: Dermatophilus congolensis Dermatophilus spp.
- Signs: alopecia and crusting
. - Diagnosis: microscopic identification ofDermatophilus congolensis
. - Treatment: keep dry +/- topical antibacterials +/- systemic antibiotics.
- Prognosis: good.
Presenting signs
- Localized or generalized alopecia and crusting.
- No or mild pruritus.
- Lesions are frequently mildly painful.
- Distribution of lesions in areas subject to wetting by rain or sweat (particularly head, neck, dorsum, sides of abdomen, thorax and lower legs).
- Following wet weather, or associated with standing water or wet, low-lying pastures.
- Rain scald: dorsal trunk.
- Mud fever
, mud rash, greasy heel: distal limbs, particularly pasterns, worst in white skin areas. - Dew poisoning: infections on white-skinned and haired areas, especially the muzzle and legs (often with severe erythema).
- Generalized alopecia and crusting, papular eruptions.
- Lesions may coalesce to cover large areas
. - Depression, poor appetite, weight loss, enlargement of the lymph nodes, fever in severe cases.
Geographic incidence
- Temperature, wet areas - may occur during winter or summer.
Breed/Species predisposition
- No breed predisposition reported in horses, but some individuals may be more susceptible than others.
Public health considerations
- Dermatophilosis has been reported in humans, but the self-limiting nature of the lesions and the paucity of reports suggest a very low risk. Hand-hygiene and the use of coveralls and disposable gloves should be adequate.
- Dermatophilosis is uncommonly reported in humans, most commonly in tropical regions.
- Possible clinical signs in humans: pustules, erythema, scaling exudative lesions, folliculitis, pitted keratolysis, recalcitrant verrucae and intertriginous lesions with maceration and fissuring of the skin. In immunocompromised patients, also nodules and 'hairy' leukoplakia are reported.
- Skin lesions in humans are usually self-limiting without treatment.
- Direct transmission from horses to humans is documented only in a single case report.
Pathogenesis
Etiology
- Dermatophilus congolensis infection Dermatophilus spp.
Predisposing factors
General
- Exposure to persistently wet weather/wet ground.
- Trauma, secondary to allergies or insect exposure.
- Intercurrent disease.
- Malnutrition.
- Stress.
Specific
- 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.
Pathophysiology
- 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
. - 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.
Timecourse
- Depends on time taken between exposure to organism and exposure to wet environment, and on the health and immune status of the host.
Diagnosis
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
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.