Heindrich Snyman, Emily Brouwer-Drebit, Margret Stalker, Jaden Dales
Animal Health Laboratory, University of Guelph, Guelph, ON (Snyman, Brouwer-Drebit, Stalker), Dales Exotic Mobile Vet Services (Dales)
AHL Newsletter 2022;26(2):18
A formalin-fixed excisional skin biopsy from a cutaneous mass obtained from a 7-year-old male South African white lion (Panthera leo melanochaita) was received at the Animal Health Laboratory for histopathological evaluation. The mass presented as a 1.0 × 1.0 cm ulcerated region along the dorsal mid-spine. Following surgical excision, a diagnosis of a spindle cell neoplasm with extensive epidermal ulceration was made on histopathology. In addition, a number of unusual perifollicular changes (follicular atrophy and destruction, perifollicular fibrosis, epithelial dysplasia and melanin dispersal) were also noted within the sample, and along with the ulcerated changes, was thought to possibly represent an unusual reaction to excessive self-trauma and habitual licking around the primary tumour site. Although not validated in this species, a number of immunohistochemical (IHC) cell markers (Melan A/PNL2, CD3, CD20, and CD18) were attempted, along with a toluidine blue stain. None of the neoplastic cells and only a few perifollicular mast cells and lymphocytes showed any positive immunoreactivity or staining.
The central mass appeared to be completely excised, however, regrowth was noted at the same site approximately 2 years later. This time, the mass was a darker, black pigmented region with a raised cobblestone surface (Fig. 1A) that appeared to be changing more aggressively over the preceding couple of weeks. The mass was again surgically excised and submitted to the AHL for histopathology.
In contrast to the original small, central well-demarcated nodule, multiple occasionally coalescing nodules of short, interlacing streams and bundles of elongate to more plump, almost round spindle-shaped cells were now more extensively distributed throughout the dermis (Fig. 1B). Nodules also often crowded around and variably effaced hair bulbs and adnexal structures, and exhibited faint fibrovascular packeting/nesting (Fig. 1C). Rare individual deep dermal nodules, as well as some of the smaller and poorly-demarcated peri-follicular aggregates occasionally contained dark brown to black granular pigment (melanin) that obscured the nucleus. Mitotic activity was rare.
Given the rounder and more packeted morphology of tumour cells, as well as the presence of cytoplasmic melanin pigment, a tentative diagnosis of cutaneous melanoma was made. Melan A/PNL2 IHC was performed for further confirmation. Surprisingly, but consistent with that of the initially-excised mass, IHC staining for both markers again proved negative. Distant dermal melanocytes also did not show any immunoreactivity, suggesting that the Melan A/PNL2 antibodies may not cross-react in this specific felid species. As such, another common neural crest origin and melanocyte IHC cell marker, S100, was performed instead, with the diffuse positive immunoreactivity of the neoplastic cells ultimately supporting a diagnosis of cutaneous melanoma in this lion (Fig. 1D).
Although melanoma is a fairly well-known entity in domestic cats, it has only rarely been reported in large non-domestic felids, with only four such reports being present in the primary literature (one Siberian tiger and three African lions). In the African lion cases, one mass was located intra-ocular, another within the haired skin along the mandibular lip, and the third along the pinna. Metastatic disease was reported in two of these cases and as such, could also represent a possible concern for this lion. Interestingly, one of these case reports also involved a white lion. This does raise the question of whether or not leucism (which is due to partial loss of pigmentation rather than complete absence of melanin in the case of albinism) in conjunction with environmental UV light exposure could represent a possible predisposing factor for the development of cutaneous melanoma in this subspecies of lion. Although the sample size of white lion cases here is far too low to make any significant conclusions, similar associations have been recently reported in the paler-coated golden and king colour variant wildebeest (Connochaetes taurinus) (n=4).
This lion did very well throughout recovery and the excision site healed uneventfully during the subsequent weeks. To the authors’ knowledge, the lion continues to do well, and there have not been any signs of reoccurrence at the excision site, metastasis or additional lesions arising elsewhere. Consistent and diligent monitoring is ongoing to provide the best quality of life possible. AHL
References
1. Van der Weyden L, et al. Metastatic cutaneous melanoma in a white African lioness (Panthera leo). Vet Sci 2021;8(8):154.
2. Cagnini, DQ, et al. Ocular melanoma and mammary mucinous carcinoma in an African lion. BMC Vet Res 2012; 25:176.
3. Steeil, JC, et al. Diagnosis and treatment of a dermal malignant melanoma in an African lion (Panthera leo). J Zoo Wildl Med 2013;44:721–727.
4. Eckstein C, et al. Cutaneous metastatic melanoma in a Siberian tiger (Panthera tigris altaica)-case report. Arq Bras Med Vet Zootec 2020;72:921–925.
5. Adetunji SA, et al. Melanoma in golden and king wildebeests (Connochaetes taurinus). J Zoo Wildl Med 2018;49:134–142.
Figure 1. Cutaneous melanoma in a male South African white lion. A. Regrowth of the mass at the original excision site showing dark black pigmentation and a raised cobblestone surface. B. Skin, H&E stain 4x. The dermis contains multiple coalescing nodules that often crowded around and variably effaced hair bulbs and adnexal structures. C. Skin, H&E stain 40x. Neoplastic cells are composed of short, interlacing streams and bundles of elongate to more plump, almost round spindle-shaped cells that occasionally exhibited faint fibrovascular packeting/nesting and contain variable amounts of dark black cytoplasmic melanin pigment. D. Skin, S100 IHC stain 40x. Neoplastic cells exhibit strong positive nuclear and cytoplasmic immunoreactivity, consistent with a melanocytic origin.