Title

Exophiala Infection in Aquaria: Identification, Environmental Study and Challenge Experiments

Date of Completion

January 2010

Keywords

Biology, Microbiology|Health Sciences, Pathology|Biology, Veterinary Science

Degree

Ph.D.

Abstract

Exophiala species are melanized fungi considered widely distributed in the environment, particularly in soil, decaying wood, polluted water and sewage; however, these fungi are recovered at low frequency from low-nutrient or hydrocarbon-polluted environments. Naturally occurring Exophiala species infection has been reported in both captive marine and freshwater teleosts, and in cartilaginous fish, with high mortality. E. salmonis and E. pisciphila are the most commonly isolated species, although more recently, novel species have been associated with infection in multiple fish species. ^ In two geographically distant commercial aquaria, ulcerative cutaneous lesions and disseminated fungal infections were identified in two separate captive populations of weedy and leafy seadragons. Culture of the lesions and molecular analyses of the sequences generated identified a novel Exophiala species, Exophiala sp. nov. Characteristic microscopic features of lesions in these fish were ulcerative skin lesions, renal necrosis and fungal vascular invasion. Exophiala sp. nov. isolates, that had greater than 99% nucleotide sequence identity to that isolated from seadragons, were later identified from similar ulcerative skin lesions and visceral lesions in a captive population of sandlance. While the source of the infection in these two separate populations of fish was thought to be environmental, culture of multiple tank environment sites and the water column during the phaeohyphomycotic outbreak in sandlance did not yield any Exophiala spp. isolates. ^ Experimental infections of chinook salmon in saltwater aquaria were conducted using Exophiala sp. nov. isolated from lesions in sandlance. Confirmation of melanized fungus in ulcerative skin lesions and in foci of renal necrosis, recovery of the fungus by culture from lesions, and identification of Exophiala species nov. by molecular testing of isolates fulfilled Koch's postulates for associating a pathogen with disease and indicated that Exophiala sp. nov. can be a primary pathogen of salmon. In a separate pilot study, we confirmed that salmon in freshwater aquaria were susceptible to Exophiala sp. nov. infection by intraperitoneal inoculation and produced lesions in all the exposed fish. Experimental infection studies and taxonomic classification of Exophiala sp. nov. are useful in understanding the pathogenesis, epidemiology and treatment options for aquarium fish. ^