Bovine cutaneous melanocytic tumours

Amanda Mansz

Animal Health Laboratory, University of Guelph, Guelph, ON. 

AHL Newsletter 2025;29(1):12.

Following the identification of a large, soft to gelatinous, diffusely black, cutaneous/subcutaneous, freely mobile mandibular mass in a mature black Angus bovid at slaughter, the mass was submitted to the AHL for histologic evaluation. On gross examination, the mass measured 14.0 cm x 11.0 cm x 7.0 cm, was covered by black-haired skin, and extended from the dermis to the deep subcutaneous adipose tissue (Figs. 1a, 1b). On histologic examination, the neoplastic mass was comprised of heavily melanized, plump round to spindle-shaped cells arranged in sheets, nests and packets, with some streams and whorls. Fewer than 1 mitotic figure was observed per 2.37mm2 microscopic field, and lymphovascular invasion was not seen. The benign features of the submitted mass, typified by the absence of lymph node metastasis, minimal cellular pleomorphism, and rare mitoses, warranted a diagnosis of cutaneous melanocytoma.

A retrospective publication on 10 melanocytic tumours in young cattle describes bovine melanocytic tumours as uncommon, but not rare, with an age between 2 months to 2 years at diagnosis, and Angus/Angus crossbred cattle as being over-represented. A distribution over the trunk was most often seen, but one was located on the jaw. In those animals available for post-surgical removal follow-up, no tumours had recurred.

In a search of the AHL database for melanocytic tumours in Bos spp. since January 2015, a total of 2 malignant melanomas, 5 benign melanocytomas, and 3 “melanocytic tumours” were diagnosed. Tumours had a generalized anatomical distribution with locations described on the maxilla, fetlock, flank, abdomen, tail, elbow and carpus in young bovids (less than 2 years of age). Criteria of malignancy for melanocytic tumours is not well-defined in cattle; however, the two diagnosed malignant melanomas did display lymph node metastasis. Scott’s Color Atlas of Farm Animal Dermatology suggests 80-90% of these melanocytic tumours are benign, but may grow to a large size (50cm) and become necrotic and ulcerated, thereby justifying early removal.

Figure 1. Cutaneous melanocytic tumour in a mature bovid. 1a. A large black melanocytic mass expanding the dermis extending to the deep subcutaneous adipose tissue. 1b. A cut surface of the                 mass highlights the black pigmentation of the tumour.

References

1. Scott, DW. Bovine neoplastic and non-neoplastic growths. In: Color Atlas of Farm Animal Dermatology, Scott DW, ed. Blackwell Pub, 2007:98-100.

2. Miller MA, et al. Cutaneous melanocytomas in 10 young cattle. Vet Pathol 1995;32(5):479-84.