JPC SYSTEMIC PATHOLOGY
Signalment (JPC #1380452): Adult male Siamese cat
HISTORY: This cat presented with a small circular raised nodule on its head. Similar, occasionally ulcerated nodules later developed on the head, ears, nose and forelimbs. The animal was euthanized and this section taken from the ear at the time of necropsy.
HISTOPATHOLOGIC DESCRIPTION: Haired skin, pinna: Infiltrating, expanding, and effacing the dermis, separating fragmented mature collagen fibers and remaining adnexa, and elevating the multifocally hyperplastic and focally ulcerated epidermis are multiple coalescing nodules composed of numerous macrophages and neutrophils with fewer perivascular and periadnexal lymphocytes and plasma cells, and rare melanomacrophages (pigmentary incontinence). There are numerous intrahistiocytic and extracellular 4‑10 µm diameter, round to oval, lightly eosinophilic yeast that are occasionally surrounded by a clear halo. Epidermal hyperplasia is mild, with rete ridges, acanthosis, prominent intracellular bridges and intercellular edema (spongiosis), and rare neutrophilic exocytosis.
PAS: There are numerous intrahistiocytic and extracellular pleomorphic (round, oval and occasional cigar-shaped) PAS-positive yeast.
MORPHOLOGIC DIAGNOSIS: Haired skin, pinna: Dermatitis, pyogranulomatous, nodular, multifocal, moderate, with numerous intrahistiocytic and extracellular PAS-positive yeast, etiology consistent with Sporothrix schenckii, Siamese, feline.
ETIOLOGIC DIAGNOSIS: Aural cutaneous sporotrichosis
CAUSE: Sporothrix schenckii
· An opportunistic, dimorphic, saprophytic fungus found in moist organic debris, sphagnum moss, and hay
· Grows as a mycelium (hyphae) at environmental temperature and as yeast in tissue
· Disease is uncommon; seen most frequently in cats, horses, dogs and humans, with increased incidence in roaming intact male cats and hunting dogs
· Disease occurs more frequently in temperate and tropical zones
· Zoonotic; transmission reported from cats to humans, thus infected cats present a significant public health danger
· Three clinical forms (see gross findings):
· Infection acquired by wound contamination or inoculation by puncture wounds caused by thorns, wood splinters, contaminated claws, cat bite, etc.
· S. schenckii organisms commonly cultured from the blood in cats with both widespread and localized cutaneous lesions
· Pulmonary infections are rare; result from inhalation of the organism
TYPICAL CLINICAL FINDINGS:
· Typically limited to the skin/subcutaneous fat, but systemic dissemination occurs more often in cats than dogs
· Cats with the disseminated form are febrile, depressed and/or anorexic
· Respiratory signs are the most frequent extracutaneous clinical finding
TYPICAL GROSS FINDINGS:
· Primary cutaneous form
o Frequent form in dogs and cats; multi-focal raised, hairless, crusted and/or ulcerated nodules or plaques
o Lesions confined to point of entry of organism
o May result from a high degree of host immunity which prevents spread
· Cutaneous‑lymphatic form
o Most common in horses and humans but has been reported in the dog; lesions occur on head, pinnae, and trunk, and are confined to skin and subcutis
o Lesions follow the lymphatics proximally from the point of entry; lymphatics become thick and corded
· Extracutaneous/Disseminated form
o Develops as a sequela to the cutaneous-lymphatic form, or inhalation; rare in dogs; the disseminated form frequently occurs in cats; occasionally seen with corticosteroid use in dogs
TYPICAL LIGHT MICROSCOPIC FINDINGS:
· Diffuse and/or nodular pyogranulomatous dermatitis and panniculitis; epidermis is acanthotic or ulcerated, with variable fibrosis and necrosis
· Yeast are pleomorphic, round, oval or cigar-shaped; single or budding cells; 2-6 µm in diameter and 2-10 µm in length; refractile cell wall shrinkage artifact may cause yeast to appear encapsulated
· Organisms are numerous in lesions in cats and rare in dogs and horses
· Found extracellularly with regions of necrosis or within the cytoplasm of
macrophages, multinucleate giant cells and neutrophils
· Yeast may be surrounded by a brightly eosinophilic, stellate radial corona known as an asteroid body or Splendore-Hoeppli reaction
ADDITIONAL DIAGNOSTIC TESTS:
· Yeast are gram-positive and PAS, GMS positive
· Grocott silver stain more sensitive than cytologic examination or IHCs (Silva JN, et al., J Comp Path 2018)
· Cytology of exudate with PAS or GMS stain
· Cultures well, even when few organisms present
· Indirect immunoperoxidase or direct fluorescent antibody staining with S. schenckii antiserum
· Polyclonal anti-Mycobacterium bovis (BCG) stain may help identify organisms
Solitary (or multiple) ulcerated and fistulated dermal lesions in cats and dogs:
· Opportunistic or systemic fungal infections (cryptococcosis, histoplasmosis, blastomycosis), especially when cigar-shaped forms are absent
· Bacterial abscesses/severe bacterial pyoderma
· Leishmaniasis (see I-P15, H-P07, U-P03)
· Foreign body reactions
· Sterile granuloma and pyogranuloma syndrome
· Feline leprosy (Mycobacterium lepraemurium, see I-B03)
Cutaneous equine lymphangitis with draining tracts:
· Epizootic lymphangitis/pseudoglanders (Histoplasma capsulatum var. farciminosum): Ulcerative, nodular pyogranulomatous lymphangitis usually of hind limbs and neck
· Glanders/Farcy (Burkholderia mallei): Neutrophilic to pyogranulomatous ulcerative lymphangitis of the head and neck
· Melioidosis (Burkholderia pseudomallei): Wide host range; often causes an ascending lymphangitis of the distal extremities
· Ulcerative lymphangitis (Corynebacterium pseudotuberculosis): Ascending neutrophilic to granulomatous ulcerative lymphangitis of the lower limbs (especially fetlock) with intracellular and extracellular diphtheroid bacteria
· Equine strangles (Streptococcus equi ssp. equi): Regional lymph node abscesses most severely affecting the mandibular and retropharyngeal lymph nodes, which often rupture and contain purulent exudate
Histologic differentials (especially when cigar form is absent):
· Histoplasma capsulatum: 2‑4 µm diameter, intrahistiocytic yeast, with single narrow‑based budding (hourglass appearance)
· Cryptococcus neoformans: 2-20 µm diameter yeast with thick mucin-positive 2 µm thick capsule; forms single or multiple narrow‑based buds; stains well with mucicarmine
· Blastomyces dermatitidis: 8‑25 µm diameter, round, multinucleated yeast with double contoured, thick walls, with single broad‑based budding
· Cutaneous leishmaniasis: Chronic ulcerative dermatitis with 2 µm intrahistiocytic protozoal amastigotes, with a vesiculate nucleus and kinetoplast
· Reported in cattle, mules, donkeys, goats, swine, rats, mice, hamsters, domestic fowl, camels, dolphins, armadillos and chimpanzees
· Occupational hazard for people working closely with vegetation or soil; risk factors include rose gardening, topiary production, hay baling and Christmas tree farming
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