JPC SYSTEMIC PATHOLOGY
SPECIAL SENSES 
April 2021
S-P02

Signalment (JPC #1754374):  Unknown age and gender New Zealand white rabbit 

HISTORY:  A New Zealand white rabbit had crusting, erythema, and intense pruritus in both ears.

MORPHOLOGIC DESCRIPTION:  Haired skin, pinna:  Diffusely, the hyperplastic epidermis is covered by a crust up to 3 mm thick consisting of loose lamellations of orthokeratotic and parakeratotic hyperkeratosis, entrapped degenerative inflammatory cells (serocellular crust), numerous arthropod mite adults and eggs, and colonies of 1-2 µm diameter basophilic cocci.  Mites are approximately 350 µm wide with a striated, chitinous exoskeleton with few spines, striated musculature, jointed appendages, a haemocoel, and a digestive and reproductive tract.  Within the ovary is a large, developing egg characterized by numerous eosinophilic globules.  Eggs are oval; 60 µm in diameter; have a thin, 3-5 µm, pale eosinophilic shell; and contain numerous 2-3 µm basophilic, round globules and abundant variably-sized eosinophilic globules (yolk material).  Diffusely, the epidermis is moderately hyperplastic with acanthosis, spongiosis, and prominent anastomosing rete ridges.  Multifocally, the underlying superficial dermis is infiltrated by low numbers of heterophils and macrophages.  Diffusely, apocrine glands are ectatic.  

MORPHOLOGIC DIAGNOSIS:  Haired skin, pinna:  Hyperkeratosis, orthokeratotic and parakeratotic, diffuse, severe, with mild heterophilic and histiocytic dermatitis, acanthosis, and numerous superficial intracorneal adult mites and eggs, etiology consistent with Psoroptes cuniculi, New Zealand white rabbit, lagomorph.

ETIOLOGIC DIAGNOSIS: Psoroptic otitis externa; psoroptic otoacariasis

CAUSE:  Psoroptes cuniculi

CONDITION:  Ear canker; psoroptic mange; psoroptic scabies; ear mange

GENERAL:

• P. cuniculi is a non-burrowing, obligate parasite that is the most common and costly ectoparasite disease of rabbits (see also I-P10)
  • Other species: sheep, goats, deer, horses, donkeys, mules, antelope
• Psoroptic mites live on skin covered with hair or wool or in the ears
• In rabbits, the mites are almost exclusively found on the inner epithelial surface of the pinna (concha) of the ear
• Severe cases of ear canker have been reported to be fatal
• There is no zoonotic or vector significance associated with psoroptic mange

PATHOGENESIS:

• All stages of the life cycle are completed on the skin surface in less than three weeks
• Highly contagious and transmitted by direct contact
• Initially, mites feed on lipids from the stratum corneum
• Inflammatory response to saliva or fecal material produces intense pruritus and secondary, self-inflicted trauma from scratching; mites then feed on released serum and hemoglobin released in the epidermal crust
• Scratching spreads mites and predisposes animal to secondary bacterial infections on the face, neck, and legs

TYPICAL CLINICAL FINDINGS:

• Intense pruritic dermatitis; head shaking; ear scratching
• Malodorous discharge
• Ears are painful
• Occasional lesions on the face, head, neck, limbs, abdomen, and back

TYPICAL GROSS FINDINGS:

• Hyperemia and edema of the pinna, may be ulcerated
• Tan to brown crusty exudate along the inside of the pinna often filling the ear canal
• Crusts form on the inner pinna and removal of the debris and exudate reveals moist, erythematous skin

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Marked parakeratotic to orthokeratotic hyperkeratosis with numerous mites and eggs present in a keratin crust
• Superficial dermatitis with eosinophils, mast cells, and lymphocytes
• Mites are large and oval shaped (400 x 700 um) with a chitinous exoskeleton, jointed appendages, striated muscle, simple intestine with cuboidal or columnar epithelium, and females have ova containing brightly eosinophilic globular material (yolk/developing eggs)
• Eggs are oval, 50 X 70 um, with a 5‑8 um anisotropic shell

ADDITIONAL DIAGNOSTIC TESTS:

• Ear swab or skin scraping
• Mite is large enough to be seen with the unaided eye
• Single most important characteristic is long, 3-segmented pedicels with terminal caruncle

DIFFERENTIAL DIAGNOSIS:

• Notoedres cati (I-P08): Smaller size (400 um in length); rear legs that do not extend beyond the margin of the body;  sucker on a short unsegmented stalk;  burrowing mite
• Cheyletiella parasitovorax (fur mite): Smaller than cati (380 um in length); large curved palpal hooks adjacent to piercing mouthparts; can transmit rabbit myxoma virus
• Sarcoptes scabiei (I-P06): Located within the stratum corneum rather than on the surface
• Leporacarus gibbus: High incidence of asymptomatic infections in domestic rabbits
• Trichophyton mentagrophytes: Dermatophyte in rabbits; hyperkeratotic dermatitis

COMPARATIVE PATHOLOGY:

• Horses frequently affected; main distribution includes ears, mane, and tail; less commonly the dorsal trunk
  • Usually asymptomatic
• Notoedres cati (I-P08): Scabies of the cat; affects the head and ears (particularly the margin); causes severe pruritus and is highly contagious
• Other psoroptic mites and their primary hosts include:
  • P. ovis ‑ body mite of sheep (one of the worst parasites of sheep); also affects cattle; reportable disease in sheep and cattle in most states
  • P. equi ‑ body mite of horses (indistinguishable from P. cuniculi)
  • P. natalensis ‑ body mite of cattle (P. ovis also reported), zebu, water buffalo, and horses in South Africa, South America, and Europe
  • P. cervinus ‑ ear canal mite of bighorn sheep and body mite of elk (wapiti) in the western United States
  • P. hippous ‑ ear canal mite of horses
• Knemidocoptes mutans (chickens, I-P12) or pilae (budgerigar) – proliferative and hyperkeratotic dermatitis of the legs and beak/face

References:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: John Wiley & Sons; 2016:303.
2. Bowman DD. In: Bowman DD, ed. Georgis’ Parasitology for Veterinarians. 10th ed. St. Louis, MO: Saunders Elsevier; 2013:392.
3. Gardiner CH, Poynton SL. Morphological characteristics of arthropods in tissue section. In: Gardiner CH, Poynton SL, eds. An Atlas of Metazoan Parasites in Animal Tissues. Washington, DC: Armed Forces Institute of Pathology; 1999:56-58.
4. Hargis AM, Myers S. Integumentary system. In: Zachary JF, Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Mosby Inc; 2016:1247. 
5. Hess L, Tater K. Dermatologic Diseases. In: Quesenberry KE, Carpenter JW, eds. Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. 3^rd St. Louis, MO: Elsevier; 2012:237.
6. Jones MEB, Gasper DJ, Mitchell E. Bovidae, Antilocapridae, Giraffidae, Tragulidae, Hippopotamidae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. St. Louis, MO: Elsevier; 2018:141.
7. Mauldin EA, Peters-Kennedy J. Integumentary Systems. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed. Philadelphia, PA: Elsevier; 2016:675-676.
8. Miller WH, Griffin CE, Campbell KL. Dermatoses of exotic small mammals. In: Miller WH, Griffin CE, Campbell KL, eds. Muller and Kirk's Small Animal Dermatology. 7th ed. St. Louis, MO: Elsevier Mosby; 2013:858.

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