Phaeohyphomycosis due to Pyrenophora phaeocomes and Drechslera nobleae in an Appaloosa mare

Abstract

A 21-year-old Appaloosa mare was presented with a pigmented cutaneous mass at the base of the right side of the neck. The diagnosis of phaeohyphomycosis due to pigmented fungi, known as Pyrenophora phaeocomes and Drechslera nobleae, was made based on a histopathology report followed by polymerase chain reaction (PCR) and 18S rRNA gene sequencing. The mass was surgically excised with clean margins, which is usually curative.

A 21-year-old, 518 kg, Appaloosa mare was presented to Blue Ridge Veterinary Services in Mount Airy, North Carolina for a dermatologic evaluation. The mare had a cutaneous mass on the lateral aspect of the right side of the caudal neck.

On gross examination, the mass was darkly pigmented, slightly raised, alopecic, circular, and approximately 8 mm in diameter. The owners had not reported any change in the size or appearance of the lesion since it was first noticed 6 wk earlier. The owners also reported that the lesion did not cause any apparent discomfort to the mare. No other skin lesions were identified, and the horse was otherwise healthy.

Surgical excision and histopathology were recommended and pursued. An adequate level of sedation was achieved using a combination of injectable detomidine hydrochloride (Dormosodan; Zoetis, Kalamazoo, Michigan, USA), 0.01 mg/kg body weight (BW), IV and injectable butorphanol tartrate (Torbugesic; Zoetis), 0.01 mg/kg BW, IV. An incisional block was performed at the surgical site using 10 mL injectable lidocaine (Lidocaine HCl 2%; Phoenix Pharmaceuticals, St. Joseph, Missouri, USA). An elliptical incision was made around the mass, and a relatively deep margin was taken, down to the muscular fascia. The subcutaneous tissues were closed in 2 layers using 2-0 polydioxanone suture (PDS; Ethicon, Markham, Ontario) in a simple continuous pattern. The skin was then closed using 0 nylon suture (Ethilon; Ethicon) in a cruciate pattern.

The excised mass was submitted to the Virginia Department of Agriculture and Consumer Services (VDACS) for histopathology. It was determined that the mass contained intralesional brown pigmented spherical fungal elements (Figure 1). There was severe pyogranulomatous dermatitis, which was multifocal and focally extensive to coalescing. The overlying epidermis was intact and excision was complete. The histological diagnosis was non-progressive cutaneous mycosis caused by a pigmented fungus, referred to as phaeohyphomycosis. Further diagnostics were pursued to reveal the specific type of fungus involved.

Figure 1.

Photomicrographs show the nodular mass composed of multiple granulomas, each with a periphery of lymphocytes, a layer of macrophages, and a central area that has many neutrophils, some multinucleate giant cells, and the spherical brown fungal elements. a — 40×; arrows show the centers of 3 of the granulomas. b — 400×; arrowheads show the pigmented fungal hyphae. Arrows indicate giant cells. “N” indicates an area with many neutrophils.

A fixed tissue block was sent to the Animal Health Laboratory at the University of Guelph for fungal identification. Using18S rRNA gene sequencing the fungus in the tissue was identified as 99.7% (358/359 bp) identical with Pyrenophora phaeocomes and Drechslera nobleae.

Discussion

Phaeohyphomycosis is a non-progressive cutaneous mycosis caused by a wide variety of pigmented fungi. The lesions are thought to be caused by ubiquitous saprophytic environmental fungi, which contaminate wounds due to penetrating plant material or via secondary infection of a small skin wound (1). The name phaeohyphomycosis is applied to diseases caused by various species of dematiaceous (brown pigmented) fungi whose tissue form is made up of septate, dark-walled hyphae. This differs from mycetomas because the filaments are not organized into granules, but rather are dispersed singly or in small groups within the lesions (2). Most infections with dematiaceous fungi involve the dermis and subcutaneous tissues. The fungal agents do not usually invade blood vessels (1).

In one study, cutaneous fungal infections were reported to be rare in horses and to comprise less than 0.5% of skin biopsy submissions (3). However, another study, which evaluated non-neoplastic skin lesions in horses, found that up to 5.9% of submissions included fungal elements which were divided evenly between fungal granulomas and cases of follicular dermatophytosis (4). The incidence of fungal granulomas seems to be dependent on environment. More temperate and rainy areas have an increased incidence of cutaneous fungal infections. A study at Oregon State University showed that occurrence of fungal granulomas is often seasonal, with peak months being May and August (1). This could be due to the increased frequency of biopsies done during the spring, summer, and fall months. The present case is consistent with the reported seasonality of fungal granulomas as the lesion was observed and biopsied during the summer. Although most horses affected by phaeohyphomycosis are less than 10 y old (1), the horse in this case was much older.

Pyrenophora phaeocomes is a widely distributed fungus which occurs on grasses and has known conidial states that are species of the hyphomycete genus Drechslera (5). Drechslera nobleae has been described as a plant pathogen (6). These environmental fungal species likely entered the skin via penetration by a plant or another wound, although there was no history of wounds or traumatized skin prior to granuloma formation in this horse.

Some fungal agents can cause systemic phaeohyphomycosis which is rare but fatal. Predisposing factors associated with systemic phaeohyophomycosis in both animals and humans include debilitating chronic diseases as well as treatment with immunosuppressants and broad-spectrum antibiotics. The route of entry for systemic infection is thought to be inhalation (7).

The cutaneous fungal granuloma from this mare is unlikely to be related to a systemic infection. Given that the lesion was completely excised and the prognosis was considered excellent, no further treatment was pursued. However, some sources recommend both medical and surgical treatment (8). Aside from surgery, treatment may consist of immunotherapy and/or anti-fungal chemotherapy. Common medical treatments which may reduce the size of fungal lesions include fluconazole and potassium iodide. Voriconazole is another antifungal drug that may be useful due to its excellent bioavailability with oral administration, although it is very expensive. Ketoconazole is not usually recommended in these cases due to poor bioavailability after oral administration (2). Long-term medical treatment is considered in some cases in which surgery is not recommended, but the most consistently successful treatment for fungal granulomas is surgical excision (9).