HORSES

Eumycotic mycetoma in a horse

Josepha DeLay, Marc Desjardins, Graeme Doodnaught

An excisional cutaneous biopsy from a 9 year old Thoroughbred-cross mare was submitted to the AHL for histologic evaluation. The 3 by 4 cm ulcerated mass was located at the tailhead (Figure 1) and had recurred following surgical excision 2 years prior to the current surgery. Histologically, dermis and subcutis were replaced and expanded by numerous discrete 200 - 300 µm diameter aggregates of pale eosinophilic debris containing vague spherical structures and surrounded by wide collars of neutrophils, macrophages, and a few multinucleated cells (Figure 2). In biopsy sections stained with PAS and methenamine silver stains, clusters of fungal hyphae with frequent bulbous dilations were identified among the eosinophilic debris (Figure 3), confirming a diagnosis of eumycotic mycetoma.

Mycetomas develop in skin and subcutis, forming mass lesions often with draining tracts and granular debris among the discharge. Although the prefix ‘mycet-’ is synonymous with ‘myco-‘, referring to fungal etiology, mycetomas may be caused by either fungal or bacterial (Actinomyces spp.) infection. Mycetomas definitively associated with fungi are termed eumycotic mycetomas, whereas those caused by Actinomyces spp. are classified as actinomycotic mycetomas. In both conditions, microorganisms are arranged in discrete aggregates or granules giving the lesions a distinctive (and quite striking) histologic appearance.

Fungi contributing to the lesions in horses originate in soil or from plant material, and infection generally occurs by contamination of a pre-existing wound or by direct implantation of contaminated plant material, such as thorns. Lesions are typically limited to skin, and complete surgical excision can be a challenge in long-standing cases with fistulation.

Ulcerated mycetoma at dorsal aspect of tailhead (clipped).

Mycetoma, hematoxylin and eosin stain. Multiple discrete foci of eosinophilic debris are surrounded by mixed inflammatory cells and fibrous stroma.

Mycetoma, methenamine silver stain. Fungal aggregates stain black, and correspond to discrete islands of eosinophilic debris in H and E-stained sections.

Figure 1. Ulcerated mycetoma at dorsal aspect of tailhead (clipped).

Figure 2. Mycetoma, hematoxylin and eosin stain. Multiple discrete foci of eosinophilic debris are surrounded by mixed inflammatory cells and fibrous stroma.

Figure 3. Mycetoma, methenamine silver stain. Fungal aggregates stain black, and correspond to discrete islands of eosinophilic debris in H and E-stained sections.