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Case 1: A horse with nasal bleeding
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Vetscope Pathologie received three endoscopic biopsies composed of beige-brown friable tissue. The biopsies were sampled during rhinoscopy from an older Lusitano gelding, which showed intermittent unilateral nasal bleeding for several days. Rhinoscopic examination revealed a relatively well demarcated mucosal swelling and ulceration with a silvery surface (circle) at the level of the ethmoid. There was marked bleeding upon sampling.
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In histology, the samples comprised immature and ulcerated granulation tissue (1) covered with serocellular exsudate and blood (2). On the surface of two samples, there was a layer with long and slender basophilic to eosinophilic structures (3).
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In higher magnification, the superficial structures present as occasionally branching, septate fungal hyphae of up to 25 um length and of variable diameter. Occasional hyphae are pigmented (arrow heads). Rare conidia are present (arrows).
With the Grocott silver stain, fungal structures stain black, while tissues, exsudate and blood have a green color. This special stain is particularly useful, if there are only few fungal structures present in a biopsy.
Diagnosis: nasal mycosis with presence of pigmented hyphae (pheohyphomycosis)
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Comment: In histology, an unambiguous species identification of hyphal fungi is not possible. For species identification, a fungal culture is necessary, which can only be performed from native sampling material, however. Due to the presence of pigmented hyphae, this fungal infection is called pheohyphomycosis. In Middle Europe, infections with pigmented hyphal fungi are most commonly caused by the saprophytic fungi Alternaria sp. and Curvularia sp., which are facultative pathogens. A common fungal pathogen of upper airways is Aspergillus sp., the common causative agent of equine guttural pouch mycosis and of nasal mycoses of dogs and cats.
In this present case, the endoscopic findings and the very superfical layer of fungal structures would be compatible with aspergillosis. However, most Aspergillus species have non-pigmented, parallel-walled hyphae of 3-5 um diameter, which is not completely true here. The literature describes cases of mixed fungal infections in equine nasal mycoses, which is most likely true in this case as well. The fungal infection is likely caused by inhalation of dead and contaminated plant material, which hooked into the nasal mucosa, as there was little plant material in the biopsies present (not shown on photos).
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Important to know: In particular in cases with Aspergillus sp. infections, fungal hyphae are located very superficially, and can detach and get lost in the course of sampling. Thus, there is an increased risk that sampled biopsies comprise only fibrino-purulent exsudate and granulation tissue without any fungal structures present. If aspergillosis is suspected, Vetscope Pathologie recommends the submission of multiple biopsies, deriving from both superficial diphteroid material and deeper tissues. For an additional fungal culture, the submission of native tissue in sterile NaCl is recommended.
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Literature:
- Jubb, Kennedy and Palmer's Pathology of Domestic Animals, 6th ed.
- Nivrutti More, et al., Vet Pathol 2019: Mycotic Rhinits and Sinusitis in Florida Horses.
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