Metastatic Melanoma in Horses

Abstract

The clinical and pathologic findings are reviewed for 14 horses with metastatic melanoma. All were older gray horses, with an average age of 16 years. The most common sites of primary tumors were the ventral tail, perineum, and parotid salivary gland. Metastases were found in multiple locations and caused a variety of clinical syndromes. The most common sites for metastases were the lymph nodes, liver, spleen, skeletal muscle, lungs, and surrounding or within blood vessels throughout the body. Many of the horses had dermal melanomas for years (range 1-6 years) before succumbing to metastatic disease. Histologic characteristic of dermal masses was not predictive of malignancy in the majority of cases. Treatment consisting of surgical debulking and administration of cimetidine, an autogenous vaccine, or both was attempted in 4 horses with no effect on outcome.Key words: Dermal melanocytosis; Horse; Neoplasia; Perineum; Tumor.M elanomas, with a prevalence of 4-15%, are common skin tumors of horses. 1 The true incidence may be higher because these tumors are easily identified and infrequently submitted for histologic examination. Many reports have estimated that as many as 80% of older gray horses will develop dermal melanomas, with the majority of these masses being benign.1,2 The statement also has been made that many of these histologically benign dermal melanomas may eventually metastasize and thus should be considered potentially malignant.1,2 The true incidence of metastatic melanoma in horses is unknown. Over the time period of the study (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001), 101 horses were diagnosed with cutaneous melanomas at our hospital. The prevalence of metastatic melanoma in this population was approximately 14%. However, given the infrequency with which these tumors are submitted for a histologic diagnosis, the true prevalence of metastatic melanomas may be less. The purpose of this study was to describe the clinical and histologic characteristics of metastatic melanoma in 14 horses. Materials and MethodsMedical records for horses admitted to the teaching hospital and to the necropsy service were examined over an 11-year period. Horses with a histologically confirmed melanoma present in a location other than the epidermis were included in the study. Patient records were reviewed and signalment, presenting complaint, clinical signs, previous treatment, response to treatment, and pathologic findings were recorded. The age distribution for cases was compared to the age distribution of the general hospital population by using chi-squared analysis with a significance level of P Յ .05.A gross postmortem examination was performed on all 14 horses. The brain and spinal cord were evaluated in 11 of the 14 horses. For histopathology, tissue samples not thicker than 1 cm were fixed for 4-10 days in 10% buffered formalin. Tissue samples were reduced, placed in histology cassettes, and progressively dehydrated in alcohol From the Department of Clinical Studies (MacGillivray, Swe...

Malignant melanoma in a grey horse: case presentation and review of equine melanoma treatment options

“…The pleural fluid likely accumulated due to a combination of malignant exudation and the space-occupying effect of the mediastinal melanoma. However despite the lungs being the most common internal organ system affected [8], thoracic melanoma is rare and pleural effusion secondary to melanoma has only once been reported to the authors’ knowledge [4]. There was no evidence of melanocytes on pleural fluid cytology; highlighting the importance of additional testing to determine the underlying cause in cases presenting with pleural effusion.…”

“…MacGillivray [4] reported that 14% of dermal melanomas may become malignant, but stated that this figure was likely an overestimate as benign melanomas are infrequently submitted. A later study investigating a group of 296 Lipizzaners reported a 50% incidence of melanomas and no clinical evidence of malignancy [5].…”

IL-12 based gene therapy in veterinary medicine

“…Grey horses develop metastatic melanoma spontaneously and are especially suitable as a large animal model for melanoma since a genetic predisposition for this disease is recognized [43] and distant metastases occur spontaneously to similar organs as in the human disease. Conventional oncologic therapies, such as surgery, cryosurgery or local chemotherapy, have only a limited antitumor effect [44]. In horses, gene therapy of this disease has been attempted employing suicide gene therapy [45] or therapeutic genes encoding IL-12 and IL-18 [46-48].…”

RACK1, a clue to the diagnosis of cutaneous melanomas in horses

“…Up to 18% of all skin tumors are melanocytic [1]. The true incidence may be even higher since a number of epidemiological studies do not include a histological report.…”

“…The term “melanoma” is used for malignant melanocytic tumors, whereas “melanocytoma” refers to the benign forms, with the corresponding restrictions. Indeed morphological criteria are not always predictive of clinical features [1]. In human melanoma, morphology-based melanoma classification has presented limited clinical relevance.…”