ISSN 2398-2977      

Parotid / guttural pouch: melanoma

pequis

Synonym(s): Melanocytoma, melanosarcoma, malignant melanoma, melanosis


Introduction

  • An uncommon, sometimes alarming neoplastic condition affecting the parotid region of gray horses.
  • Cause: unknown etiology but possible neoplastic or hyperplastic condition may be involved. Most common in gray horses and may be seen in conjunction with few or many melanomas at other sites such as the perineum, lip, eyelid, etc. 
  • Signs: firm, well defined nodular expansion of the parotid region. Extent varies markedly.
  • Diagnosis: history, characteristic clinical appearance, biopsy. 
  • Treatment: benign neglect is the only option, although a commercially available canine melanoma vaccine may be helpful in some cases.   
  • Prognosis: guarded, although most cause no significant clinical signs. Massive expansion may limit neck movement but only very rarely any pharyngeal obstruction. A few cases are severe and involve the guttural pouch and then can cause secondary effects including neurologic deficits in the vagus, hypoglossal and glossopharyngeal nerves (causing pharyngeal/laryngeal dysfunction), epistaxis, and in the sympathetic trunk (causing Horner's syndrome Neurology: Horner's syndrome). Obstructions of the internal auditive tube can occur with consequent vestibular syndromes.

Pathogenesis

Etiology

  • Widely viewed as a neoplastic disease and variously referred to as melanocytoma, melanosarcoma, malignant melanoma, melanosis.
  • Although tumors typically appear the same grossly, they can be histopathologically divided into melanocytic nevi, anaplastic malignant melanoma, dermal melanoma, and dermal melanomatosis.
  • Possibly arise as a result of disturbed melanocyte function with consequent neoplastic cell transformation. This condition occurs in horses that are born pigmented and fade to gray over time, not in those who are born white or albino. This observation has led to a theory that tumors arise as melanocytes migrate from hair follicles and form congregates of cells which may then undergo neoplastic change.

Predisposing factors

General

  • Exclusively restricted to horses that are born pigmented and then fade to gray or flea bitten gray as they age.
  • Most cases are encountered in older horses (>8-10 years).
  • Contrary to the human situation, equine parotid (and other) melanoma is not thought to be associated with exposure to UV light.

Pathophysiology

  • Benign hyperplasia of melanocytes that have migrated from hair follicles is usually blamed for initial changes. Subsequent more aggressive neoplastic transformation of cells results in rapid or slow (or occasionally static) expansion of the tumor in nodular forms.
  • Some cases ulcerate resulting in discharging of a thick tarry black material within the guttural pouches. Very rare for external skin ulceration to occur.
  • Physical incorporation of nerves within the guttural pouch may cause neurologic problems.
  • No evidence of any infectious or toxic involvement.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • McGillivray K C et al (2002) Metastatic melanoma in horses. J Vet Intern Med 16, 452-456 PubMed.
  • Goetz T E et al (1990) Cimetidine for treatment of melanomas in three horses. JAVMA 219 (6), 449-455 PubMed.
  • Goldberg S A (1919) The differential features between melanosis and melanosarcoma. JAVMA 56, 140-143.

Other sources of information

  • Scott D W & Miller W H (2003) Equine Dermatology. W B Saunders, USA. pp 767-773.
  • Pascoe R R & Knottenbelt D C (1999) Manual of Equine Dermatology. W B Saunders, UK.
  • Scott D W (1988) Large Animal Dermatology. W B Saunders, USA.

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