Abstract

Dematiaceous fungi in the genus Exophiala are commonly isolated from plant materials, decaying wood, sewage sludge, and wood pulp samples.¹¹ The genus contains several species acting as opportunistic pathogens, causing cutaneous, subcutaneous and systemic clinical diseases in man.⁵ In fish, naturally occurring disease with high mortality has been reported predominantly in intensively farmed salmonids including Atlantic salmon (Salmo salar L.),^6,8-10 cutthroat trout (Salmo clarkii),² and lake trout (Salvelinus namaycush).² Reports in non-salmonid fish are limited to a single epizootic in farmed channel catfish (Ictalurus punctatus Rafinesque)^3,7 and reports in individual captive scup (Stenotomus versicolor),¹ Atlantic cod (Gadus morhua),¹ common seahorse (Hippocampus hudsonius),¹ Sargassum triggerfish (Xanthichthys ringens),¹ brown clownfish (Amphiprion sebae),¹ and smooth dogfish (Mustelus canis Mitchill).⁴ This is a report of a cluster of deaths, occurring over a period of 4 years at the Shedd Aquarium in three species of captive coldwater marine fish, attributed to cutaneous and disseminated infections with Exophiala-like fungi.

Affected fish included four lumpfish (Cyclopterus lumpus) an Atlantic wolffish (Anarhichas lupus) and a thornback ray (Platyrhinoidis triseriata). All fish were wild caught and maintained in the same water system, with biologic filtration and UV sterilization of water between tanks. The lumpfish were housed together in the same tank as juveniles, but different species never occupied the same tank simultaneously. Enclosure substrate was minimal consisting of concrete and crushed coral.

Fish were in captivity for at least 1 year prior to observation of clinical signs. Signs were nonspecific and included anorexia and muscle wasting of between 1 month and 1.5 years' duration. All four lumpfish (designated A, B, C, D) developed single to numerous crateriform skin ulcerations predominantly on the head. The ray had a single cutaneous ulceration. No cutaneous lesions were noted in the wolffish. Clinical decline resulted in death in lumpfish A and B, the ray, and wolffish, and euthanasia in lumpfish C and D. Necropsies were performed by zoo pathology staff (lumpfish B, and D) and Shedd Aquarium staff (lumpfish A, C, wolffish and ray) with submission of formalin-fixed tissues.

Gross lesions recorded in the lumpfish were multifocal 1–4-mm diameter areas of branchial necrosis and multifocal to coalescing, slightly raised black foci in heart, spleen, kidney, liver, mesentery, skeletal muscle, gastrointestinal tract, peritoneum, and eye (B); discrete granuloma-like lesions in heart, spleen, and kidney (A). Wet mounts of cutaneous, branchial, and internal lesions in lumpfish B, revealed large numbers of 2–3-µm diameter, right-angle branching fungal hyphae. The wolffish had numerous 2–3-mm diameter white foci in the epicardium and a single 3-mm diameter aggregate of soft, friable, yellow material in the spleen. No internal lesions were noted in the ray.

The distribution of histologic lesions is presented in Table 1. Cutaneous lesions in lumpfish B, C, and D, consisted of severe necrotizing, granulomatous and ulcerative dermatitis with large numbers of intralesional fungal hyphae. Often immediately subjacent skeletal muscle was similarly affected. In lumpfish B and C vascular invasion was evident, with fungal hyphae visible in vascular lumens often incorporated within thrombi composed of fibrin, erythrocytes, and eosinophilic granular cells. In H&E-stained sections, hyphae were light brown, parallel-walled, 2.5-µm diameter, septate, and exhibited occasional to frequent right-angle branching. All visceral lesions listed in Table 1 were similar, characterized by necrosis with admixed and/or surrounding small to moderate numbers of macrophages and lymphocytes, rare eosinophilic granular cells and moderate to large numbers of intralesional similar fungal hyphae. Lumpfish C had multifocal areas of hepatic necrosis with intralesional fungal hyphae and no evidence of an inflammatory response.

Table 1. Distribution of histologic lesions

^aN/A: lesion not submitted for histology.
^bFungal hyphae and necrosis present but no evidence of an inflammatory response.

Fungal cultures of lumpfish B kidney, and wolffish spleen, each yielded Exophiala-like fungi, which were not identical to each other or to any of the established species of Exophiala. Further morphologic and DNA analyses of the isolates are currently underway in order to achieve more definitive identification.

The similar histologic appearance of fungal hyphae and lesions in all fish in this series, whilst not definitive, made it highly likely all were infected by an Exophiala-like fungus. Hence, exophialosis may be a more common cause of death in captive coldwater marine fish than previously suspected, and should be considered a differential diagnosis in fish with granulomatous disease, both with or without cutaneous lesions.

The fish all occupied the same water system and were in captivity for a prolonged period of time prior to clinical decline, suggesting persistence of the fungus somewhere in the aquarium environment. Given the pattern of cutaneous ulceration in multiple lumpfish, colonization of skin or opportunistic infection of clinically inapparent cutaneous abrasions, followed by vascular dissemination to visceral organs seemed the most probable pathogenesis. The predominance of lumpfish in this case series suggested a species susceptibility. Possibly their bottom-dwelling lifestyle resulted in protracted contact with substrate containing the organism or predisposed them to cutaneous abrasions and subsequent fungal colonization.

Acknowledgments

We would like to thank William Hana and the Shedd Aquarium laboratory staff for providing information on fish management, the University of Illinois Veterinary Diagnostic Laboratory histopathology staff, and the Loyola University Medical Center clinical microbiology staff.

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