The Comparative Pathology of Equine Cutaneous Phycomycosis

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Abstract
Pythiosis (204 cases, 77%), basidiobolomycosis (48 cases, 18%), and conidiobolomycosis (14 cases, 5%) were diagnosed morphologically from 266 horses with phycomycosis. All lesions were cutaneous ulcerative granulomas and three horses with pythiosis had metastatic lesions in regional lymph nodes. Lesions of pythiosis contained characteristic yellow, coral-like coagula and had a fibrotic surface containing sinus tract openings. Basidiobolomycosis was characterized by infrequent small yellow coagula and a yellow line of fungal invasion beneath an edematous surface. Lesions of conidiobolomycosis had numerous small coagula and a nasal location. There were minor differences in inflammatory cell populations within the granulomatous lesions. Most differences were associated with coagula size and fungal morphology. Coagula were composed of collagen, fibrin, cellular debris, degranulated and whole eosinophils and hyphae. Histochemistry revealed no major differences among the three diseases. Pythium sp hyphae were 2.6 to 6.4 μm in diameter, had thick walls, and occasionally were septate. Basidiobolus haptosporus hyphae were 5.1 to 20.5 μm in diameter, had thin walls, and commonly were septate. Conidiobolus coronatus hyphae were 5.1 to 12.8 μm in diameter, had thin walls, and commonly were septate. A perihyphal eosinophilic cuff (Splendore-Hoeppli phenomenon) with a radius of up to 20 μm was associated with the latter two fungi. Ultrastructurally, Pythium sp was composed of a thick, single density cell wall while B. haptosporus and C. coronatus had thin, double-layered cell walls.