JPC SYSTEMIC PATHOLOGY
INTEGUMENTARY SYSTEM
September 2022
I-M12
Signalment (JPC #1945775): 6-year-old female Yorkshire terrier
HISTORY: This dog developed multiple skin nodules over an eight-month period.
HISTOPATHOLOGIC DESCRIPTION: Haired skin: Expanding the deep dermis and subcutis, separating deep (anagen) hair follicles and subcuticular adipocytes, and infiltrating the panniculus carnosus are multiple coalescing nodules composed of numerous epithelioid macrophages, fewer neutrophils, lymphocytes, and reactive fibroblasts with scant fibrous connective tissue. This inflammatory infiltrate extends through the panniculus carnosus and often separates and surrounds skeletal muscle fibers which are often shrunken with angular margins (atrophy) or occasionally swollen and pale with vacuolated sarcoplasm and loss of cross striations (degeneration). In the less affected superficial dermis there are multifocal periadnexal and perivascular infiltrates of low numbers of lymphocytes and plasma cells with rare eosinophils, hemorrhage, fibrin and edema, as well as dilated lymphatics and increased clear space that separate collagen bundles (edema). Few hair follicles in the superficial dermis are ectatic and filled with keratin. Apocrine glands are mildly ectatic.
MORPHOLOGIC DIAGNOSIS: Haired skin and subcutis: Panniculitis, nodular, granulomatous, multifocal to coalescing, marked, Yorkshire terrier, canine.
CONDITION: Idiopathic sterile nodular panniculitis
GENERAL DISCUSSION:
- Idiopathic sterile nodular panniculitis is a diagnosis of exclusion seen rarely in young dogs (less than one year of age), cats, horses, and cattle
- Breed predilection:
- Some older literature reports that dachshunds and poodles are over-represented; more recent publications favor Australian shepherds, Brittany spaniels, Dalmatians, Pomeranians, and Chihuahuas
- There is some disagreement in the literature regarding the occurrence of concurrent systemic disease: older studies report several potential concurrent systemic diseases (e.g. pancreatic disease, liver disease, protein losing enteropathy, arthropathy, renal disease, systemic lupus erythematosus, inflammatory bowel disease, keratoconjuncitivis sicca), while according to a more recent study, 82% of affected animals had no concurrent systemic disease identified (the remaining 18% had concurrent arthritis)
PATHOGENENSIS:
- Idiopathic
TYPICAL GROSS FINDINGS:
- Nodules may be firm and well circumscribed or soft and ill-defined; typically on the trunk, but may occur anywhere
- Lesions may become cystic, ulcerate, and develop draining tracts that discharge an oily, yellowish‑brown to bloody substance
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Nodular inflammation in the panniculus and deep dermis composed mainly of macrophages and neutrophils; progresses to diffuse involvement and inflammation (i.e., the addition of lymphocytes and plasma cells); may be necrotizing
- Discrete granulomas and pyogranulomas may be present
- Chronic lesions have variable fibrosis that may extend to the overlying dermis
- Overlying epidermis may be acanthotic or ulcerated
- Impossible to distinguish histologically from panniculitis caused by infectious agents
ADDITIONAL DIAGNOSTIC TESTS:
- Special stains for acid-fast bacteria
- Histochemical stains and/or molecular diagnostics to rule out an infectious cause including fungal infection
DIFFERENTIAL DIAGNOSIS (nodular skin lesions):
- Infectious panniculitis:
- Mycobacterium spp. and mycotic infections are the most likely infectious differential diagnosis
- Juvenile sterile granulomatous dermatitis and lymphadenitis: occurs exclusively in puppies (3-weeks to 4-months old); periadnexal pyogranulomatous inflammation helps to ddx from nodular panniculitis.
- Cutaneous neoplasms
- Deep bacterial folliculitis and furunculosis
- Foreign body reactions (likely to be more focal than multifocal)
- Reactive histiocytosis (I-M10): histiocytic infiltrates often centered around vessels
- Sterile pyogranuloma syndrome
- Vaccine-associated granuloma (I-M35)
COMPARATIVE PATHOLOGY:
- Cats: Lesions are solitary and occur most commonly over the ventral abdomen and ventral thorax
- Horses: Lesions most often found on the neck, thorax, abdomen, and proximal limbs. Shetland ponies may be predisposed
- Cattle: Lesions have been described over the neck, trunk, and proximal limbs
REFERENCES:
- Gross TL, Ihrke PJ, Walder EJ, et. al. Diseases of the panniculus. In: Gross TL, Ihrke PJ, Walder EJ, eds. Skin Diseases of the Dog and Cat. 2nd ed. Ames, IA: Blackwell Science Ltd; 2005:548-551.
- Mauldin, ED, Peters-Kennedy, J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:701-702.
- Welle, MM, Linder, K. The integument. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier Mosby; 2022: 1189-1190.
