August 2019

Signalment (JPC# 21474-7,-8):  1-year-old, breed not specified, dog

HISTORY:  This dog from Texas had a severe, ulcerative dermatitis involving the legs.

HISTOPATHOLOGIC DESCRIPTION:  Haired skin and subcutis:  Multifocally obscuring the dermis, separating and surrounding collagen fibers and adnexal structures, infiltrating the subcutis, and extending to all borders are coalescing, disorganized nodules composed of a core of cellular debris, fibrin, and drop-out (lytic necrosis) centered on faint, negative images of 6-10µm diameter hyphae with nonparallel walls and poorly discernible, irregularly angled, non-dichotomous branching.  Hyphae are surrounded by numerous degenerate and fewer viable neutrophils, epithelioid macrophages, fewer multinucleated giant cells (foreign body and Langhans’ types), and low numbers of eosinophils, plasma cells, and lymphocytes.  Inflammatory nodules are admixed with abundant hemorrhage, fibrin, hemosiderin-laden macrophages that often exhibit erythrophagocytosis, and colonies of 1-2um basophilic cocci.  Nodules of inflammation are separated by numerous reactive fibroblasts and loose, fibrous connective tissue with perpendicularly arranged small caliber blood vessels (granulation tissue), that progresses to more mature fibrosis.  The wall of a large blood vessel is focally discontinuous with loss of endothelial cells and replacement by eosinophilic cellular and karyorrhectic debris and few previously described inflammatory cells (vascular necrosis), and the lumen is partially occluded by a large fibrin thrombus.  Small vessels are often lined by reactive endothelial cells and are variably occluded by moderate amounts of fine to coarse fibrin with enmeshed erythrocytes and few inflammatory cells (fibrin thrombi). 

Gridley’s stain:  Hyphae are 6-10µm (occasionally up to 15µm) wide, thick walled, and rarely septate, with irregularly angled, non-dichotomous branching and thick, unevenly stained, non-parallel walls.

MORPHOLOGIC DIAGNOSIS:  Haired skin and subcutis:  Dermatitis and panniculitis, pyogranulomatous and eosinophilic, multifocal to coalescing, severe, with hyphae, vascular necrosis, hemorrhage, fibrin thrombi, and fibrosis, breed not specified, canine.

ETIOLOGIC DIAGNOSIS:  Cutaneous pythiosis

CAUSE:  Pythium insidiosum

CONDITION:  Oomycosis (formerly phycomycosis)











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