Dermatophilosis

Description

A group of diseases affecting the epidermis caused by dermatophilus congolensis . Causes a range of conditions in large animals including rain scald in horses and strawberry foot rot in sheep. Lesions typically involve exudative dermatitis with scab formation. A zoonosis affecting humans in close contact with infected animals.

Signalment

Can be seen in animals of all ages but most commonly occurs in young animals who are chronically exposed to moisture. Affects horses, sheep, cattle, goats, pigs and rarely dogs and cats.

Diagnosis

Can often make a diagnosis on history and physical exam. Impression smears can also be useful when stained with either gram stain or Giemsa and examined microscopically. Additionally it is possible to culture material from the crusts however this can be difficult due to the slow- growing nature of the pathogen.

History and Clinical signs

Lesions commonly occur followng heavy rainfall and commonly affects the dorsum of animals. Any previous trauma or damage to the skin can predispose to infection. Blood-sucking insects are also thought to be involved in transmission.

Equine dermatophilosis

Three clinical syndromes have been documented:

• rain scald
• Mud fever
• Grease heel.

Typically hair becomes matted and focal lesions develop into crusts and thick scabs covered by layers of yellow-green pus. When scabs are removed the hair is taken with them and alopecia results. Rain scald commonly affects the dorsum, shoulders and neck. It can however extend to lower portions of the body and abdomen. It typically begins as small rough raised bumps and can go un-noticed in0 long haired regions. Mud fever looks similar to rain scald but affects the pasterns and distal limbs. White legs and white- skinned areas are most commonly affected. Grease heel is a disease complex which presens in the same way as rain scald and mud fever over the heels. It produces a grey exudate which feels greasy to touch. The disease complex can involve dematophilosis as well as a number of other conditions including dermatophytosis, pastern folliculitis, pyoderma, photosensitisation, contact dermatitis and chorioptic mange.

Prolonged wetting of inected areas can lead to secondary bacterial infection which can result in limb oedema and cellulitis.

Ovine dermatphilosis

• • Papules, serous, exudative matting of hair, raised crusty scabs
  • Scab formation more prominent in sheep and cattle than in horses
  • Lesions may resolve within weeks if dry weather, or may progress
• Treatment:
  • Parenteral antibiotics e.g oxytetracycline, pr penicillin-streptomycin combinations

Dermatophilosis in a cow (Courtesy of Bristol BioMed Image Archive)

• Caused by Dermatophilus congolensis
• Affects cattle, horses, sheep mainly
• More common in wet and warm weather
• Transmotted from animal to animal
• Lesions tend to form on dorsal back and extremities
• Associated with skin trauma, prolonged wetting or parasites
  • -> penetration of zoospores
• Bacteria proliferate in outer sheath of hair follicles and superficial epidermis
• Gram-positive, filamentous branching organisms, subdivided longitudinally and transversly
• Causing:
  • Acute inflammatory response -> neutrophil migration through dermis and epidermis -> formation of microabscesses
  • Further penetration of bacteria is thus prevented
  • Regenerated epidermis is invaded again by remaining organisms
  • Repeated reinfection -> multilaminated pustular crusts
• Grossly:
  • Papules, pustules, crusts may coalesce and mat the coat
• Microscopically:
  • Hyperplastic superficial perivascular dermatitis
  • Multilaminated crusts, alternating keratin and inflammatory cell layers