CHAPTER 77 Oral Cavity Masses
Oral cavity masses in the horse are not commonly recognized until they cause clinical signs because visibility and examination of the oral cavity are often limited without chemical restraint and use of a full-mouth speculum and bright light source. Routine, thorough, oral cavity examination as part of a horse’s semiannual wellness program is recommended to increase the opportunity for detection of subclinical disease at a treatable stage.
Tumors of the oral cavity of horses are uncommon. Clinical signs include ptyalism, halitosis, quidding, tongue protrusion, nasal discharge containing food material, dysphagia, inappetence, and weight loss. Tumor invasion of local bony and soft tissues is often advanced, and regional metastasis may have already occurred before clinical signs become apparent and prompt veterinary attention. Direct and thorough visualization of the oral cavity will reveal most tumors. Radiography can be used to determine bony involvement, and nasal endoscopy can reveal soft tissue masses on the tongue base and displacing the soft palate dorsal to the epiglottis. Definitive diagnosis requires biopsy and histologic examination. In horses with oral cavity neoplasia, submandibular lymphadenopathy may develop secondary to reactive inflammation rather than metastatic disease, so biopsy of these lymph nodes may not reveal neoplasia. Successful treatment is dependent on the type, size, and accessibility of the tumor.
Soft Tissue Neoplasia
The most common primary soft tissue neoplasm affecting the oral cavity is squamous cell carcinoma, which can involve any of the mucosal surfaces. Other primary or metastatic tumors are rarely reported but include melanoma, fibrosarcoma, hemangiosarcoma, lymphosarcoma, rhabdomyoma and rhabdomyosarcoma of the tongue, and myxomatous tumors.
Tumors of the lips and rostral portion of the tongue are most readily resolved because they are noticed earlier and can be adequately excised or are approachable for intralesional chemotherapy and radiotherapy. Invasive squamous cell carcinomas have a high recurrence rate after surgical excision and have often metastasized by the time they are diagnosed. Complementary radiotherapy after radical surgical excision of squamous cell carcinoma may prevent or prolong time to recurrence. Fibrosarcomas also tend to recur after excision and are less responsive to radiation therapy. The prognosis for resolution of oral squamous cell carcinoma and fibrosarcoma is generally poor. Lingual rhabdomyosarcoma can be managed by surgical excision, but the long-term prognosis is unclear.
Bone and Dental Neoplasia
Juvenile mandibular ossifying fibroma is the most common bony tumor of the mouth of horses, and it affects the rostral portion of the mandible in horses 2 to 14 months of age. Only rarely is the incisive bone (premaxilla) involved. The tumors are mostly composed of dense fibroblastic stroma with osteoblastic conversion. The mass is typically identified bulging from the mandible below the incisor arcade and is subject to trauma and subsequent ulceration and bleeding.
The definitive treatment of choice is surgical excision by partial mandibulectomy to remove the affected portion of jaw. Debulking of the mass only to the level of the mucosal surface of the mandible is associated with recurrence. However, repeated partial resections performed in this manner have been observed to cause cessation of tumor growth and to allow remodeling of the tissues as the animal matures. Other rarely reported bony tumors of the horse’s head include osteoma and osteosarcoma. Radical and complete surgical excision may yield a good prognosis with these tumors, depending on location.