Technician Update

Progressive Skin Lesions in a 30-year-old Gelding

By Tammy Treitline, LVT, VTS-EVN

A 30-year-old Quarter Horse gelding was presented for a 3-year history of progressive skin lesions over the left side of the neck and a small black mass under the throat latch, slightly right of the midline.

The client had administered several topical ointments, systemic antifungals and antibacterial treatments with either no lasting improvement or no response at all. The gelding was housed on a cattle farm and used for light riding around the farm. He was not receiving any medications or supplements, and he did not have a history of chronic diseases such as pituitary pars intermedia dysfunction (PPID). He was up-to-date on core vaccinations and deworming. His diet consisted of grass hay and pasture, with occasional pelleted complete feed.

The gelding appeared bright, alert and responsive; body condition score was 7 of 10. His eyes appeared normal and his nose was clear of any discharge. Physical examination showed alopecia extending along the entire left side of the neck with a few spots starting on the right side of the neck and in the gaskin area of both hindlimbs. The skin appeared dry, scaly, leathery, markedly friable and tender to the touch (Figure 1A and B). The small black mass was approximately 1 cm by 1 cm on a small stalk, appeared smooth with no ulceration, was firm to the touch and dark black in color (Figure 3C).

Palpation of the lymph nodes and thoracic auscultation disclosed no abnormalities. Normal borborygmi were present in all abdominal quadrants. The musculoskeletal system appeared normal with good muscle mass and no evidence of lameness.

After the physical examination, the team believed the case fit more into a non-
infectious rather than an infectious dermatology category based on the history and clinical presentation. However, we wanted to rule out more common infectious causes, so skin scraping samples, impression smears, and biopsy specimens were planned. The gelding was sensitive to touch on the neck, especially on the left side, so detomidine hydrochloride 10 mg IV was administered into the right jugular vein to provide sedation and analgesia prior to obtaining the samples.

Microscopic examination of a skin scraping sample was negative for external parasites and fungal hyphae. The impression smear revealed cocci bacteria, inflammatory cells, several red cells and a few epithelial cells. These results could indicate chronic inflammation with a secondary bacterial infection. Biopsy specimens were obtained from 3 different areas of the patient and sent to a referral laboratory for histopathologic examination.

Histopathologic results of the excisional biopsy specimen of the 1 cm by 1 cm black mass in the throat latch showed a fibropapilloma (sarcoid) resulting from bovine papillomavirus. Although complete excision can be curative in these cases, lesions may develop around the head, lips, legs and ventral trunk because of the bovine papillomavirus.

Results of the second biopsy specimen, obtained from the upper one-third of the neck on the left side, showed concurrent furunculosis and pyogranulomatous dermatitis. Furunculosis is defined as a bacterial infection occurring in the hair follicles, and pyogranulomatous dermatitis is defined as a type of chronic inflammation characterized by the presence of both neutrophils and macrophages, with other inflammatory cells present.

Furunculosis can be caused by bacterial infection, dermal inflammation, deep skin lesions, intradermal edema and trauma. Pyogranulomatous dermatitis can be caused by parasites, bacteria, fungi, and viruses. The non-infectious causes of pyogranulomatous dermatitis could be eosinophilic granuloma and idiopathic systemic granulomas. In this case, furunculosis and pyogranulomatous dermatitis could be a result of a bacterial or viral infection, be secondary to trauma, or be the result of an autoimmune disease.

Since the skin scraping was negative for external parasites or fungal organisms and the impression smear showed cocci bacteria and inflammatory cells, the patient’s skin lesion could potentially be due to a virus, such as bovine papillomavirus, or an autoimmune disease, such as sarcoidosis or cutaneous lupus.

Results of the third biopsy specimen, which was obtained from the mid-section on the left side of the patient’s neck, showed orthokeratotic hyperkeratosis—a thickening of the outer layer of the skin caused by overproduction of keratin. Orthokeratotic hyperkeratosis can be a result of a mineral or vitamin deficiency, a hormonal imbalance, or an autoimmune disorder. In the early stages of sarcoidosis and cutaneous lupus, orthokeratotic hyperkeratosis is sometimes seen on histopathology reports.

The veterinary team initiated treatment with dexamethasone 5 mg IV once a day for 4 days, switching to an oral steroid (prednisolone) at 500 mg once a day to reduce the inflammation and pain associated with the skin lesions while waiting for the histopathology reports. The skin lesions improved, and the horse was less sensitive to touch after the intravenous injections of dexamethasone.

Based on the patient’s clinical presentation, history, lack of response to previous treatments, and histopathology reports, the veterinary team suspected an underlying autoimmune disease as the cause of the skin disorder. A definitive diagnosis for an autoimmune disease, such as sarcoidosis or cutaneous lupus, would require more testing.

Additional testing for cutaneous lupus includes complete blood count (CBC), serum biochemistry profile, urinalysis, direct immunofluorescent testing (DIF), and antinuclear antibody testing. The complete blood count, serum biochemistry, and urinalysis would assess for evidence of inflammation and anemia, in addition to evaluating the systemic function of other organs of the body, such as the kidneys. DIF testing would evaluate the basement membrane layers of the tissue for deposits of immunoglobulins, while the antinuclear antibody test would identify specific autoantibodies associated with equine cutaneous lupus.

Additional testing for sarcoidosis includes PCR assays to detect genetic material associated with bovine papillomavirus.

Overall, this case provided an interesting dermatologic presentation, with a chronic history and continued progression of the skin lesions that allowed for evaluation with skin diagnostics and biopsy specimens. These cases can be challenging to diagnose definitively. Sometimes, the response to treatment is the most valuable information that is acquired. MeV

Teaching Points
The purpose of the skin—the largest organ of the horse—is to protect the internal organs from external threats, recognize sensations, provide immunity, regulate the horse’s temperature, and synthesize vitamin D. Horses are susceptible to skin diseases if the skin becomes compromised. Typically, equine dermatology cases are straight forward, with diagnosis possible from clinical presentation, thorough history collection and some testing.

The complete history provides valuable information that allows the veterinary team to establish a timeline and to identify different treatments that have been previously used for the patient.

Dermatology cases are separated into 2 categories: infectious and non-infectious. Infectious dermatology cases include fungal infections (eg, ringworm), bacterial infections (eg, dermatophilosis) and external parasite infestations (eg, lice). In the upper Midwest, we see infectious cases like these—often in the spring and fall—with seasonal weather patterns and housing.

Non-infectious dermatology cases include trauma, allergies, and nutritional and immune-mediated problems.