Pheohyphomycotic fungoma in the tail of a big-belly seahorse
Heindrich Snydman, Véronique LePage
Animal Health Laboratory, University of Guelph, Guelph, ON (Snyman), Ripley's Aquarium of Canada (LePage)
AHL Newsletter 2025;29(4):17.
A single 7 year-old female big-belly seahorse (Hippocampus abdominalis) (Fig. 1) died following a 2-day period of anorexia. An in-clinic postmortem exam was conducted, and the only obvious gross finding was that of regional subcuticular swelling along the ventral peduncular base of the tail just caudal to the anal pore (Fig. 2A, 2B). A small overlying defect was present along the region of swelling with the underlying dermal tissue being discoloured dark grey-black. Light digital compression of the site resulted in protrusion of this dermal tissue through the defect as multiple, contiguous, firm, dark grey-black nodular masses. Neoplasia was considered at this time, and the extruded masses were fixed in 10% neutral buffered formalin and submitted to the Animal Health Laboratory for further histological analysis.
On histological examination, the sampled masses all appeared identical, being composed of numerous individual and occasionally coalescing dense clumps of fine granular eosinophilic debris that were widely permeated by a dense mats of intertwined dark yellow-brown pigmented fungal hyphae with moderate numbers of infiltrating macrophages and fewer neutrophils (Fig. 3A, 3B). Hyphae were slender in morphology, measuring ~ 1.5-2.0 µm wide with parallel sides, regular septation and occasional dichotomous branching. A histological diagnosis of fungal hyphal mats with associated necrotic debris and macrophage infiltration was made with the localized nodular presentation being consistent with a so-called fungoma. The darkly pigmented characteristics of the hyphae were further consistent with a fungal infection caused by a dematiaceous fungus (phaeohyphomycosis).
Pigmented fungal hyphae are common in the environment and infection in fish is usually associated with some or other predisposing condition (e.g., regional trauma/penetrating injuries, other infections, or immunocompromised state). In this case, it is possible that a primary ulcerative skin lesion, e.g., along the tail, may have acted as an initiating cause for the observed pigmented fungal infection. This is particularly significant in this case, as the tail plays an important role in prehension in sygnathids, and as such, can often be exposed to excessive abrasive injury from environmental substrates.
Exophiala sp. probably represents the most common pigmented fungus isolated in fish, but various species can be implicated. This has become particularly important in aquaculture-reared cleaner lumpfish with four main species being identified - E. angulospora, E. psychrophile, E. salmonis, and E. aquamarina. Cultures were not performed on the current case, but various Exophiala sp., as well as Cyphellophora olivacea (syn. Phialophora olivacea) have been identified in other fish cases submitted to the lab.
Figure 1. Adult big-belly seahorse (Hippocampus abdominalis) in holding tank.
Figure 2. Postmortem findings in a captive big-belly seahorse with tail swelling. A. There is a regional defect within the ventral surface of the tail base with protruding dark grey-black pigmented dermal tissue. B. Gentle digital pressure results in protrusion of the masses through the opening
Figure 3. Histological findings of extruded tail base nodules in a captive big-belly seahorse. A. Nodules are composed of numerous individual and occasionally coalescing indiscriminate dense clumps of fine granular eosinophilic debris. B. Clumps were widely permeated by a dense mats of intertwined dark yellow-brown pigmented fungal hyphae with associated infiltrating macrophages and neutrophils.
References
1. LePage V, et al. Diseases of of captive yellow seahorse Hippocampus kuda Bleeker, pot‐bellied seahorse Hippocampus abdominalis Lesson and weedy seadragon Phyllopteryx taeniolatus (Lacépède). Journal of Fish Diseases 2015;38(5):439-450.
2. Scholz F, et al. Systemic mycoses in lumpfish (Cyclopterus lumpus L.) in Ireland: Aetiology and clinical presentation. Bulletin of the European Association of Fish Pathologists 2018;38:202–212.
3. Erkinharju T, et al. Cleaner fish in aquaculture: Review on diseases and vaccination. Reviews in Aquaculture 2021;13-1:189-237