Retrospective study of phaeohyphomycosis in aquarium‐housed fish, with first descriptions of Exophiala lecanii‐corni and Neodevriesia cladophorae in fish

Abstract

A broadening fish host range is affected by novel and known pigmented fungal pathogens. A review of 2,250 piscine submissions received by the Aquatic Pathology Service, University of Georgia, revealed 47 phaeohyphomycosis cases (2.1%), representing 34 bony and cartilaginous fish species. The majority involved bony fish (45/47, 95.7%) and were predominantly marine (41/47, 87.2%), with only a few freshwater species (4/47, 8.5%). Cartilaginous fish cases included two zebra sharks (Stegostoma fasciatum) (2/47, 4.3%). Northern seahorses (Hippocampus erectus) had the highest incidence overall (7/47, 14.9%). Culture and sequencing of the internal-transcribed spacer region of the rDNA (ITS), large ribosomal subunit gene D1/D2 domains (LSU) and the DNA polymerase II gene (RPB2) were performed for fungal identification when fresh tissue was obtainable. Exophiala, Ochroconis and Neodevriesia spp. were identified, with Exophiala as the most common fungal genus (8/11, 72.7%). Exophiala lecanii-corni and Neodevriesia cladophorae were described for the first time from fish.Microscopically, lesions were characterized by necrosis, granulomatous inflammation and angioinvasion most frequently affecting the skin/fin, skeletal muscle and kidneys. In this study of diverse aquarium-housed fish species, phaeohyphomycosis cases occurred sporadically and in rare outbreaks with variable pathologic presentations, tissue distributions and severities.

Disseminated phaeohyphomycosis in a beluga sturgeon (Huso huso)

“…However, additional novel and known pigmented fungi are increasingly identified as pathogens of bony and cartilaginous fish. Commonly implicated genera include Exophiala, Ochrochonis, and Phoma (Armwood et al 2021). Significantly, fungal identification based on morphologic features, particularly in histologic sections, is unreliable and molecular methods should be employed when fresh or frozen tissue is available.…”

Phaeohyphomycosis due to Exophiala in Aquarium-Housed Lumpfish (Cyclopterus lumpus): Clinical Diagnosis and Description

“…Of the pathogenic melanized fungi, Exophiala spp. were the most commonly identified, although culture and molecular identification of fungal species were not available in most cases [8]. Waterborne Exophiala spp., order Chaetothyriales, have been known to cause cutaneous and disseminated infections in fish, crabs, amphibians, and turtles.…”

“…Microscopic lesions consist of necrosis with infiltrates of inflammatory cells centered on slender, variably brown, septate fungal hyphae. Macrophages are the predominant cell in these infiltrates, but lymphocytes and granulocytes may also be present [8]. Depending on the fish species, inflammatory infiltrates may not form granulomas but instead may be loosely organized in the necrotic tissue and admixed with hyphae.…”

“…Depending on the fish species, inflammatory infiltrates may not form granulomas but instead may be loosely organized in the necrotic tissue and admixed with hyphae. A common feature of these infections is fungal angioinvasion with hyphae traversing the endothelium and walls of affected blood vessels and being present in vascular lumina [8,10]. Initial infection is suspected to occur across the gills or skin via inoculation or through preexisting lesions [10].…”

“…Although there is evidence for in vitro efficacy of antifungal medications against Exophiala spp. [28], phaeohyphomycosis carries a guarded prognosis in fish [8,29]. This is attributed to a combination of virulence factors in melanized fungi, the progressive nature of the disease, and a general lack of veterinary antifungal options in the treatment of systemic fungal infections [8,9,29].…”