Cutaneous habronemiasis superimposed on a squamous cell carcinoma (SCC) in a horse
By Andrés de la Concha-Bermejillo
In the summer of 2018, four partial biopsies of an infected hot brand skin lesion from a Quarter Horse stallion were submitted in 10% buffered formalin to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) for histologic diagnosis. No clinical history was submitted.
Histologically, the biopsies consisted of haired skin including epidermis, dermis, and subcutis. Two different lesions were identified in the biopsies. One of them consisted of eosinophilic and granulomatous dermatitis characterized by numerous discrete areas of inflammation in the dermis containing central areas of necrosis filled with eosinophilic material, degenerating leukocytes and cell debris surrounded by a thick rim of macrophages. A cross section of a nematode larvae was embedded in one of the areas of granulomatous inflammation. Large numbers of eosinophils were found in the adjacent connective tissue. Special stains for the presence of Pythium sp. fungal hyphae were negative. The second lesion consisted of a non-encapsulated squamous cell carcinoma (SCC) emerging from the epidermis and infiltrating the underlying epidermis. The SCC had poorly defined borders and was composed of neoplastic epithelial cells arranged in sheets, nests, and trabeculae separated by a moderate to large amount of connective tissue stroma with a prominent desmoplastic response. Neoplastic epithelial cells were round to polyhedral with a moderate amount of amphophilic cytoplasm and round, vesiculated nuclei with prominent nucleoli. Fourteen mitotic figures were found in 10 random high powered fields examined. Some of the neoplastic cells exhibited karyomegaly. The morphologic diagnosis of cutaneous habronemiasis superimposed on a squamous cell carcinoma was established.
Cutaneous habronemiasis is a parasitic disease of horses and other equids which is caused by the invasion of Draschia megastoma (formerly H. megastoma), Habronema majus (previously known as H. microstoma), and H. muscae nematodes and is transmitted by houseflies, face flies, and stable flies. Cutaneous habronemiasis is a frequent cause of ulcerative cutaneous and ocular granulomas in horses throughout the world.
Adult nematodes inhabit the stomach of horses and eggs and larvae (L1) are passed in the feces and subsequently ingested by fly maggots where they mature to L3. The larvae are deposited by adult flies in the skin of horses. If the larvae are deposited near the mouth, they are swallowed and in the stomach, they complete their life cycle.
Fly larvae of H. muscae, H. and D.megastoma more frequently are deposited in the medial canthus of the eye, the glans penis, and prepuce. Lesions are also common on the distal extremities because this is a common site for lacerations.
Habronema has also been reported as a secondary infection of cutaneous lesions induced by pythiosis, Corynebacterium pseudotuberculosis infection, and other skin lesions.
Squamous cell carcinoma is the second most common cutaneous neoplasm in horses and more often affects mucocutaneous junctions or the skin of external genitalia. Ocular and periocular lesions are also common. Chronic exposure to sunlight is considered a risk. Burn scars and chronic non-healing wounds also may be predisposed to SCC. Approximately 20% of SCC in the horse result in metastases. SCC in horses has been associated with Equuscaballus papillomavirus-2 and -8 (EcPV-2 and EcPV-8).
Differential diagnoses of skin masses in horses in addition to SCC and habronemiasis include equine sarcoids, pythiosis, fungal and bacterial granulomas, botryomycosis, nodular collagenolytic granuloma and exuberant granulation tissue.
Submission of formalin fixed and chilled unfixed biopsies of the lesions for histopathology and culture is important to establish the diagnosis.
For more information about this case, contact Veterinary Pathologist Dr. Andrés de la Concha-Bermejillo. To learn more about TVMDL’s testing services visit tvmdl.tamu.edu or call 1.888.646.5623.
Knottenbelt DCand JCroft JS. Cutaneous squamous cell carcinoma (SCC):“What’s the problem?” Equine Vet. Educ.(2018)
Pugh DG, Hu XP, Blagburn B. Habronemiasis: Biology, Signs, and Diagnosis, and Treatment and Prevention of the Nematodes and Vector Flies. J Eq Vet Sci 34:241-248, 2014.
For questions about this article, please contact Dr. Andres de la Concha at (979) 845-3414.