Review of Oral Neoplasia in the Horse

Take Home Message—Equine oral neoplasia is overall uncommon. When oral masses are found upon examination, a proper diagnosis is imperative as not all oral masses carry poor prognoses.

I. Introduction

Literature published regarding equine oral oncology is limited and usually involves only a series of case reports or broad non-specific reviews or book additions. The most comprehensive review of the literature can be found in the third edition of Equine Dentistry.¹ Neoplasia within the oral cavity can be characterized either by tissue of origin (odontogenic, osteogenic, or soft-tissue) or by clinical behavior and pathologic features (benign vs. malignant, invasive vs. localized, proliferative vs. ulcerative). There is value to recognizing all these qualities within a mass as all components help to identify the neoplasm and determine an appropriate treatment plan.

Swellings either within the oral cavity or associated with surrounding mandibular/maxillofacial structures warrants close examination as these swellings could be inflammatory, neoplastic, cystic, nutritional, or developmental in origin. Clinical signs that can accompany oral neoplasia are: distortion of normal anatomic features, oral bleeding, dysprehension, dysmasesis, dysphagia, weight loss, recurrent fever, depression, halitosis, nasal discharge, and airway restriction. With sedated oral examination, intraoral and extraoral radiography, and biopsy, a diagnosis if not at least a category of disease can be reached. As treatment planning and prognosis for oral masses varies dramatically, narrowing of the diagnosis to at least a general characterization of benign vs. malignant is helpful.

As with long bones and bony imaging elsewhere on the body, a clinician can start to get a feel for the aggressiveness of tumors by their appearance on radiographs. The involvement of teeth in the region of a neoplastic mass can help further clarify the nature of a tumor. The following chart represents a simplified version of this differeniation.²

II. Discussion

Cystic Masses

Oral masses that are characterized as cystic in nature include abscesses, true cysts, temporal teratomas and odontomas. Temporal teratomas and odontomas will be discussed later under odontogenic tumors as they are not truly cysts. They mistakenly fall into the cystic category on initial diagnoses periodically as significant amounts of fluid can accumulate around unerupted dental tissue. Abscesses are a localized collection of infected purulent fluid formed by the disintegration of tissue. Most abscesses are formed by invasion of tissues by bacteria, but some are caused by fungi, protozoa or even helminthes, and some are sterile. Common oral causes for formation of abscesses include fractured tooth with pulp exposure, non-vital intact teeth, dental malformation, foreign body, retained tooth roots (either traumatic or post-extraction), bony sequestration, osteomyelitis and severe periodontal disease. Treatment will vary depending on the etiology, abscess location and size, and radiographic findings.

A true cyst is a pathologic cavity filled with fluid, lined by epithelium, and surrounded by a definitive connective tissue wall. The cystic fluid is secreted by the cells lining the cavity.³ Cysts usually form within the jaw of an animal because most cysts originate from the numerous rests of odontogenic epithelium that remains after tooth formation. Clinical presentation includes a firm non-painful swelling that appears radiolucent on intraoral images. These masses though benign in nature can be very destructive due to their expansile growth, and large regions of normal anatomy can be destroyed prior to their discovery. Unerupted teeth and/or association with dental structures are common though not necessary. Although rare, the two most common forms of dental cysts seen in domestic animals are radicular and dentigerous cysts. More common non-dental cysts located in the maxillofacial region of the horse are maxillary sinus cysts and pharyngeal cysts. Some non-cystic lesions that may present similarly in the horse are progressive ethmoidal hematoma, nasal polyps, heterotopic polyodontia, and fibrous osteodystrophy.

Radicular cysts are very rare in the veterinary field. Radicular cysts most likely result when inflammatory products from vital or non-vital teeth stimulate epithelial cell rests within the periodontal ligament to proliferate. This results in a non-draining fluid-filled swelling associated with the root or reserve crown of one or more teeth. To date, no reports of radicular cyst have been presented in the equine literature. Under reporting due to lack of recognition and/or the dynamic nature of the equine periodontal ligament, cementum, and alveolar bone, may be the reason(s) for this species apparent avoidance of radicular cysts. A dentigerous cyst is a cyst that forms around the crown of an unerupted tooth. Fluid accumulates in the layers of reduced enamel epithelium and/or between the epithelium and the unerupted tooth crown.¹ In horses, the term dentigerous cyst has been used inappropriately to describe temporal teratomas. True dentigerous cysts are very rare in the horse, but when reported they have been associated with either unerupted canine or first premolar teeth. In veterinary medicine, the most common treatment for both radicular and dentigerous cysts is extraction of the associated tooth/teeth with complete debridement of the cystic region. A radiographic recheck is usually recommended at 6 months to one year post-operatively to ensure complete resolution of the cyst.⁴

Odontogenic Tumors

Odontogenic tumors in horses are uncommon. Unfortunately, the low incidence rate of these tumors has led to a generalized poor understanding of their nature, progression, and treatment course among veterinarians. Cystic masses and odontogenic tumors can have similar clinical presentation, and misdiagnosis can occur between the two groups. Odontogenic tumors in the horse are sometimes incidently found on oral exam, but more commonly, tumor growth becomes quite large prior to owner recognition and the involvement of a veterinarian. Tumors that have become large enough to be traumatized by opposing teeth usually present with oral bleeding. Quidding, oral bleeding and airway restriction are the most common complaint in the horse. Odontogenic tumors that are locally invasive (e.g. ameloblastoma) may cause disfigurement from invasion of the maxilla or mandible with displaced but often firmly-seated teeth. These may cause large regional swellings, exophthalmus, epiphora, and blepharospasm. Mandibular lymph nodes often palpate within normal limits unless secondary infection is present. Appetite and activity level are usually unaffected unless the tumor obstructs mastication or moderate to severe airway restriction is present.

A basic knowledge of dental development is helpful in understanding the pathogenesis and presentation of many odontogenic tumors.^5,6 A quick review of literature demonstrates the lack of clarity in the reporting of odontogenic tumors described in the equine. Tumors described 10-20 years ago may fall under a completely different category/name than in more recent literature. Therefore, a review of the literature can be very frustrating and confusing. In general, odontogenic tumors of all species are characterized as epithelial, mesenchymal, or mixed epithelial-mesenchymal tumors. Tumors can be either benign or malignant, but in domestic animals most tumors are benign in nature though some may be very aggressive regionally. An older form of classification is based on the inductive nature of a tumor; tumors would be classified as inductive or non-inductive. At this point, a review of dental development is necessary.  [...]