Squamous cell carcinoma in the hoof of a horse
Rebecca Egan, Amy Gaw
Animal Health Laboratory, University of Guelph, Guelph, ON (Egan), Temiskaming Veterinary Services, Temiskaming, ON (Gaw).
AHL Newsletter 2021;25(2):26.
A 20-year-old Quarter Horse presented to the veterinarian with a protracted history of non-weight bearing lameness in the left hind limb. The hoof was unbalanced with more substantial overgrowth noted on the lateral aspect of the foot. Substantial pain with application of hoof testers was mitigated with local anesthesia. Radiographs revealed a space occupying lesion associated with regionally extensive lysis of the lateral aspect of the third phalanx (Fig. 1). Euthanasia was elected, and the affected limb was disarticulated at the fetlock so that the hoof could be submitted to the AHL for gross examination and histopathology.
Figure 1. Radiographs of the left hind hoof, with asterisks (*) denoting a soft tissue opacity associated with regionally extensive lysis of the lateral aspect of P3.
Gross examination confirmed the presence of an ill-defined mass arising from the latero-plantar aspect of the hoof laminae/corium causing regionally extensive lysis and effacement of the distal phalanx. Parasagittal sectioning showed mild congestion surrounding the tip of P3, which was thinned and displaced dorsally by the irregularly thickened sole. Next, both halves of P3 were removed from the hoof, and dissection confirmed the presence of a mass arising from the hoof laminae/corium locally infiltrating and replacing the lateral and plantar aspects of P3. Grossly, the mass was comprised of slightly nodular, firm, tan-white tissue, and there were occasional short tan-white streaks (compatible with keratin) within the tissue adjacent to the plantar aspect of P3 (Fig. 2).
Histologic examination revealed squamous cell carcinoma, which occurs fairly commonly in the skin of horses, but reports of this tumor being identified in the equine hoof are sparse (1, 2). In this case, the distal phalanx was effaced by a mass of squamous epithelial cells arranged in variably thick trabeculae and islands supported by abundant collagenous stroma (desmoplasia) (Fig. 3). The neoplastic cells displayed disorderly maturation with frequent central keratinization, forming either a small central keratin pearl or a large central lake of keratin. Occasionally the thinner trabeculae were slightly reminiscent of hoof laminae. Overall, the neoplastic cells displayed up to threefold anisocytosis and anisokaryosis, frequent single cell death, patchy intracellular and intercellular edema, and infrequent mitotic figures. AHL
Figure 2. Gross images of the lateral half of P3, with asterisks (*) denoting an infiltrative mass effacing bone.
Figure 3. Histologic images of the infiltrative mass effacing the bone of the distal phalanx, with asterisks (*) denoting neoplastic epithelial cells and arrowhead (^) indicating the supporting fibrous stroma. H&E stain
References
1. Spugnini EP, et al. Isolated limb perfusion electrochemotherapy for the treatment of an advanced squamous cell carcinoma of the hoof in a mare. Open Vet J 2017;7(2):192-196.
2. Berry CR, et al. Squamous cell carcinoma of the hoof wall in a stallion. J Am Vet Med Assoc 1991;199(1):90-2. PMID: 1885337.