Parasitic skin infections - Pathology

()Map INTEGUMENTARY SYSTEM (Map) SKIN INFECTIOUS

Lice

Pediculosis

Mites

Notoedric mites

• Rare but very contagious
• Caused by Notoedres cati
• Grossly:
  • Erythematous papular rash -> scales, crusts, alopecia -> lichenification when chronic
  • Start on neck and ears -> head, face, paws -> generalised
• Microscopically:
  • Epidermal hyperplasia, spongiosis, crusts
  • Superficial perivascular dermatitis
  • Eosinophils

Otodectic mites

• Caused by Otodectes cynotis
• In external ear canals of carnivores, occasionally elsewhere

Psoroptic mites

• Occurs in cattle, sheep, horses, goats, rabbits and other animals
• Host-specific
• Psoroptes cuniculi in ear canals of rabbits, horses, goats and sheep
• P.equi at base of mane, tail and forelock in horses
• P.ovis in sheep (sheep scab)and cattle
  • Thickened skin and dry scales and crusts
  • Starts at withers and spreads due to self trauma
  • Microscopically:
    • Spongiotic, hyperplastic or exudative superficial perivascular dermatitis
    • Eosinophils

Scabies

• Caused by Sarcoptes scabiei
• Highly contagious
• Mainly in pigs, dogs, also horses, cattle, sheep, goats and cats
• Intense pruritus due hypersensitivity to mites borrowing through stratum corneum
• Usually starts with external pinnae -> head -> neck -> generalised
• Grossly:
  • Erythematous macules, papules, crusts
  • If chronic -> lichenified, hairless
• Microscopically:
  • Hyperplastic, spongiotic, superficial perivascular dermatitis
  • Crusting, eosinophil infiltration
  • Mites are not commonly seen, but eggs and feces may be found in stratumn corneum

Trombiculidiasis

• Harvest mite infestation (chiggers) - Trombicula sp.
• Form tunnels in epidermis by injecting saliva - eat digested tissue fluid
• Intense pruritus
• Grossly:
  • Small, red papules or crusts
  • Orange to red larvae
  • On skin close to plants or the ground
• Microscopically:
  • Hyperplastic, superficial perivascular dermatitis
  • Eosinophils, mast cells
  • Intraepidermal mites

Protozoa

• Cutaneous infection may become a part of systemic infection
• Mostly caused by Leishmania
• Transmitted by sandflies
• Occurs in dogs, cats and rodents (also humans)
• Grossly in dog:
  • Generalised alopecia
  • Silvery white scales
  • Nodules and ulcers if more severe
  • Mainly on muzzle, ears and eyes where flies feed
• Microscopically:
  • Hyperkeratosis, parakeratosis, crusts
  • Granulomatous nodules in dermis
  • Macrophages, some lymphocytes and plasma cells - periadnexal pattern, may affect sebaceous glands
  • Organisms may be found extra or intracellularly

Ticks

• Local reaction depends on host hypersensitivity and tick's secretions
• Grossly:
  • Focal erosions, erythema, crusted ulcers, sometimes alopecia and nodules
• Microscopically:
  • Epidermal and dermal necrosis
  • Perivascular of diffuse inflammation and necrotic margins
  • Eosinophils, macrophages and lymphocytes in exudate
  • May form granulomas containing collagenous fibres and lymphoid follicles within dermis