Novel Exophiala Infection Involving Ulcerative Skin Lesions in Japanese Flounder Paralichthys olivaceus

Abstract

A dematiaceous fungus was isolated from Japanese flounder Paralichthys olivaceus with ulceration and erosion of the skin surface. The fungus was identified as an Exophiala species, with different morphological, biological and molecular characteristics from three previously described pathogenic Exophiala species. Fungal hyphae extended laterally in the dermis, and were absent from the epidermis and musculature of the skin lesions and kidneys of the diseased fish. An inflammatory response with granuloma occurred in the dermis involving accumulations of epithelioid cells around the hyphae. The granulomas were surrounded by lymphocyte-like cells. Epidermal degeneration was observed above the inflamed dermis, suggesting that the inflammatory response caused epidermal damage. Experimental infection reproduced hyphal extension and infiltration of inflammatory cells in the dermis of the flounder, confirming the pathogenicity of the fungus.