Parasitic/Immune

• Parasitic skin diseases

  • Cause disease directly by inflammation, blood sucking, or toxin injection
  • Cause disease indirectly as important vectors or by predisposing to other infections
• Parasitic skin diseases

  • Result in annoyance, reduced production, or unthrifty/blemished hides
• Diagnosis of parasitic skin diseases

  1. History and clinical signs (especially pruritus)

  2. Parasite identification

  3. Skin biopsy
• Demodectic mange

  • Mainly affects dogs
  • Genetic predisposition and immunodeficiency
  • Localized form is usually self-limiting in juvenile dogs
  • Generalized form occurs in juveniles following localized form or in older dogs with internal disease/immunosuppression
  • Causes folliculitis, furunculosis, and secondary pyoderma
• Demodectic mange

  • Localized demodecosis
  • Generalized demodecosis
• Sarcoptic mange (scabies)

  • Highly contagious, with host-specific varieties
  • Zoonotic, usually limited
  • Lesions due to mechanical damage from burrowing, irritation from mite saliva/excreta, and hypersensitivity to mite products
• Sarcoptic mange

  • Sarcoptic scabiei variety suis
• Other parasitic skin diseases

  • Notoedric mange
  • Otodectic mange
  • Psoroptic mange
  • Chorioptic mange
  • Cheyletiellosis
  • Psorergatic mange
  • Trombiculidiasis
• Hard (Ixodid) ticks


  • Can cause paralysis in humans
• Soft (Argasid) ticks

  • Can cause paralysis in humans
• Fleas

  • Most important cause of skin disease in small animals
  • Ctenocephalides felis, also C. canis
  • Can cause asymptomatic carriage, flea-bite dermatitis, or flea allergy dermatitis
• Lice (pediculosis)

  • Sucking lice feed on blood and tissue fluids
  • Biting lice feed on exfoliated epithelium and debris
• Fly-related skin conditions

  • Fly bite dermatitis
  • Vectors of infectious disease
  • Myiasis
  • Warbles (Hypoderma)
  • Cuterebriasis
  • Screwworm myiasis
• Cutaneous larval migration

  Adults live in non-cutaneous sites while larval stages migrate through skin
• Cutaneous larval migration

  • Cutaneous habronemiasis in horses
• Filarial dermatitis

  • Onchocerciasis
  • Stephanofilariasis
  • Elaeophoriasis
  • Dirofilarial (heartworm) dermatitis
• Protozoal skin diseases

  • Sarcocystosis
  • Leishmaniasis
  • Besnoitiosis
• Hypersensitivity reactions

  • Reactions to normally harmless foreign compounds
  • Most cutaneous hypersensitivities are mediated by type I or IV reactions
  • Pruritus is a common feature
• Diagnosis of immune-mediated skin diseases

  1. History and clinical signs (especially pruritus)

  2. Skin biopsy

  3. Intradermal skin testing, elimination of antigens, response to therapy
• Atopic dermatitis


  • Common/familial in dogs, less so in cats and horses
  • Complex type I (± IV) hypersensitivity to predominantly percutaneously absorbed allergens
  • Possible T cell dysfunction with overproduction of IgE
  • Mast cell degranulation leads to pruritus and self-trauma
• Atopic dermatitis

  • Pruritus, erythema, excoriation, alopecia, hyperpigmentation, lichenification
  • Early perivascular/interstitial dermatitis, later hyperplastic perivascular dermatitis, secondary pyoderma/Malasseziasis
• Flea allergy dermatitis

  • Most common hypersensitivity in cats and dogs
  • Combination of type I and IV hypersensitivity to flea saliva antigens
  • Intense pruritus leads to self-trauma and secondary infections
• Flea allergy dermatitis

  • Erythematous papules/wheals, alopecia, crusts, hyperpigmentation, lichenification
  • Early perivascular dermatitis with eosinophils and mast cells, later mononuclear cells, spongiosis, eosinophilic microabscesses
• Other hypersensitivity reactions

  • Urticaria/angioedema
  • Allergic contact dermatitis
  • Food hypersensitivity
  • Bacterial (staphylococcal) hypersensitivity
  • Hormonal hypersensitivity
  • Dirofilarial (heartworm) hypersensitivity
  • Equine insect hypersensitivity (Culicoides)
• Autoimmune reactions

  • Autoantibodies or T cells react against self-antigens
  • Rare in domestic animals, more common in dogs than other species
• Pemphigus

  Autoantibodies bind to keratinocyte surface antigens (desmogleins), leading to inflammatory mediators, acantholysis, and intraepidermal pustules
• Pemphigus

  • Pemphigus foliaceous
  • Bullous pemphigoid
• Discoid (cutaneous) lupus erythematosus

  UV light alters keratinocyte antigens, leading to autoimmune reaction and interface dermatitis
• Other immune-mediated disorders

  • Immune-mediated vasculitis
  • Erythema multiforme
  • Toxic epidermal necrolysis
  • Vogt-Koyanagi-Harada-like syndrome
  • Plasma cell pododermatitis
  • Cutaneous amyloidosis