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Skin: contact dermatitis

ISSN 2398-2977

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Synonym(s): Contact allergy

Introduction

  • Although only few publications exist on contact dermatitis in horses, this disease is recognized by clinicians in the field.
  • Contact dermatitis may be irritant or allergic in nature.
  • Irritant reactions are noted in all exposed animals and are detectable the first time of exposure, while allergic reactions develop in a small number of animals and are not noticeable when animal is first exposed as hypersensitivity requires time to develop.
  • Cause: direct contact between the horses skin and an irritant substance, eg shampoo, fly spray, etc.
  • Signs: pruritus, scaling, alopecia.
  • Diagnosis: confinement, patch testing, biopsy.
  • Treatment: avoidance, anti-inflammatory drugs.
  • Prognosis: good.

Presenting signs

Contact allergy or hypersensitivity

  • Pruritus.
  • Erythema.
  • Crusting and scaling.
  • Papules.
  • Alopecia   Hair: alopecia areata  .
  • Excoriations (secondary to self trauma).
  • Oozing.
  • Pustules.

Acute presentation

  • Depending on the level of sensitization of the animal signs may develop from a few hours after exposure to several days after contact with the offending substance.

Geographic incidence

  • Dermatitis secondary to exposure to sprays, shampoos can occur worldwide.
  • Dermatitis secondary to exposure to plants appear to be more common in warm, tropical climates.

Age predisposition

  • It can occur at any age.

Breed/Species predisposition

  • None reported.

Public health considerations

  • None since it is not a contagious disease.

Cost considerations

  • Affected horses will need treatment of secondary infections and anti-inflammatory therapy   Therapeutics: skin  .

Pathogenesis

Etiology

  • Small proteins (called haptens).
  • Substances reported to cause contact allergy include plants, bedding, shampoos, blankets, fly sprays   Skin: contact dermatitis 01    Skin: contact dermatitis 02  , topical antibiotics such as neomycin   Neomycin  .

Predisposing factors

General
  • Previous trauma to the skin, whether related to other pruritic conditions or the development of secondary infections, may predispose to increase allergen penetration thus the development of a hypersensitivity.

Pathophysiology

  • Contact hypersensitivity is a type IV hypersensitivity in which small allergens (haptens) are thought to bind to epidermal proteins in order to become a complete antigen. 
  • During the sensitization phase the allergen is processed by antigen presenting cells (Langerhan's cells) and presented to lymphocytes in regional lymph nodes. This phase can last from weeks to months and during this time no clinical signs are noted upon exposure.
  • Once sensitization has occurred any subsequent exposure to the allergen triggers release of pro-inflammatory cytokines, eg TNF-a, and activated memory T-cells are recruited at the site of the challenge.

Timecourse

  • Incubation (also called sensitization phase) varies from a few weeks to several months.
  • In some cases animals might have been exposed to allergen for up to years before developing any problem.

Diagnosis

Presenting problems

  • Pruritus is a consistent sign.
  • In early cases papular eruption is present.
  • More chronic cases present with secondary excoriations, lichenification,  self-inflicted alopecia.

Client history

  • Development of skin reactions after known exposure to offending substance.
  • Lag phase depends on the severity of the sensitization.

Clinical signs

  • Pruritus.
  • Erythema.
  • Crusting and scaling.
  • Papules.
  • Alopecia.
  • Excoriations (secondary to self-trauma).
  • Oozing.
  • Vesicles.
  • Pustules.

Diagnostic investigation

Confinement

  • For example, avoidance of suspected allergen   →   resolution of clinical signs within 7-10 days if no secondary infections are present.
  • Rechallenge to confirm diagnosis is important.
  • Worsening of signs should occur within 48 h upon re-exposure to the offending allergen.

Patch testing

  • Can be done on the side of the neck.
  • The suspected plants are minced and applied under a gauze, directly to a shaved area on the lateral neck.
  • Patch test is kept in place by bandaging.
  • It should be evaluated for the presence of papules and erythema after 48 h.

Biopsy 

  • This should reveal a lymphocytic inflammatory response.

Confirmation of diagnosis

Discriminatory diagnostic features

  • Confinement and resolution of clinical signs.
  • Rechallenge and worsening of clinical signs.
  • In order to identify offending allergen, patch test can be performed. For this purpose the suspected substance is applied on the side of the neck in an area previously clipped.

Definitive diagnostic features

  • There is no pathognomic feature.

Differential diagnosis

Treatment

Initial symptomatic treatment

  • Avoidance of the offending substance, and therapy with anti-inflammatory doses of glucocorticoids.
  • Combination with pentoxifylline to decrease severity of signs upon rechallenge. Pentoxifylline has been shown to effectively prevent clinical signs of contact allergy in both dogs and humans. It has been empirically tried in a few equine cases with good success at 10 mg/kg BID.
  • It is important to treat secondary infections that developed as a consequence of the inflammation and self-trauma.

Prevention

Control

  • Avoidance of offending allergen.

Outcomes

Prognosis

  • Good.

Expected response to treatment

  • Good.

Reasons for treatment failure

  • Continued exposure to allergen.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Fadok V A (1995) Overview of equine pruritusVet Clin North Am Equine Pract 11 (1), 1-10 PubMed.