Introduction
The terms Mass, Growth and Tumour simply describe an abnormal localised tissue growth. Most equine growths, for example, polyps, gingival hyperplasia and cysts are non-neoplastic, as they do not have autonomous (independent) growth (Bienert-Zeit et al. 2022). The cells of true neoplasms independently grow and divide more than they should and do not die when they should, e.g., by programmed cell death such as (apoptosis) and malignant tumours show aggressive behaviour. There is very limited factual data on the prevalence of equine oral neoplasia, but the literature indicates that neoplasia is less common in horses than other species, and that less than 1.1% of all equine neoplasms are reported to develop directly in the mouth. In contrast to other sites in the body, the majority of reported equine oral neoplasms are malignant, mainly squamous cell carcinomas, and also adenocarcinomas, undifferentiated carcinomas, ameloblastic carcinomas and fibrosarcomas. Oral tumours can of course develop from any tissue in the oral cavity, including connective tissue, muscles, nerves and blood vessels. Only a small number of benign neoplasms such as cementoblastoma, fibroma and osteoma have been reported, as compared to oral tumours in small animals. [16]

If the growth has been detected by the owner, such as growth of the incisor region or those that cause external facial swelling, its duration and progression can then be evaluated. Information on whether the growth is affecting horse’s prehension, mastication, work performance, or quality of life can also be assessed. As well as a general clinical examination, and examination of the maxillary and mandibular areas for swellings or painful areas, a detailed intra-oral examination is next performed, to assess the nature and size of the lesion, and the possible presence of additional lesions. If the lesions appear to involve the mandibular bone or the nasal or maxillary areas, radiographs are indicated and whenever possible, computed tomography of the head should be obtained. A most important part of the investigation is obtaining a biopsy of the mass, that should be a deep sample, avoiding necrotic areas and preferably including some of the transition zone that has normal tissue. Due to the rarity of equine oral neoplasms, it is helpful if the histology can be read by an experienced equine pathologist. Calcified growths often need surgically-obtained biopsies and then long delays for decalcification and histological processing. It is often more pragmatic to initially perform a complete excision of the calcified mass.

Clinical signs with many oral tumours; include: Ptyalism (excessive salivation), Halitosis, Quidding, Dysphagia, Anorexia, Weight loss, facial swellings and bitting/performance problems.

Squamous Cell Carcinoma is the most commonly reported malignant neoplasm of the equine oral cavity or pharynx. They can arise on mucosal surfaces including the gingiva, tongue, hard or soft palate. They have a high malignancy and usually cause local invasion and bone destruction. Infection by Equus caballus papillomavirus type 2 (EcPV2) has been implicated as a contributing factor in oral SCC development, similar to HPV-related human cancers. Less common mucosal tumours in horses are adenocarcinomas and undifferentiated (poorly differentiated) carcinomas. SCCs are red, irregular fleshy growths, that may have necrotic areas and inevitably grow fast, and cause marked local destruction and swellings. Clinical signs include ptyalism, breath malodour, Quidding, Dysphagia (difficulty eating), Weight loss, Unilateral nasal discharge (due to sinonasal invasion), nasal obstruction and facial swellingif extension into sinuses occurs. Less common epithelial oral tumours include adenocarcinomas and undifferentiated (poorly differentiated) carcinomas. Unless detected and treated early, the prognosis is very poor.

Ameloblastomas/Ameloblastic Carcinomas. Ameloblastomas and their malignant variant, ameloblastic carcinomas are dental tumours but are a non-calcified epithelial tumour that arise adjacent to teeth, especially incisors from the epithelial precursors of enamel. They cause localised often rounded, radiolucent growths that cause variable local tooth and bone resorption, depending on their duration and malignancy. Surgical excision of early cases is usually successful.

Fibrosarcoma are a malignant mesenchymal tumour arising from subgingival connective tissue. They are less common than SCC and are often more firm and fleshy with a lower predisposition for local or metastatic invasion. Fibrosarcomas are a malignant mesenchymal tumour arising from the subgingival connective tissue. They are less common than SCC and clinically are often more firm and fleshy than SCC that can be soft and necrotic due to their rapid growth. Fibrosarcomas appear to have a lower predisposition for local or metastatic invasion than SCC. They have been recorded to respond to local excision and intralesional therapy.

Equine juvenile mandibular ossifying fibroma. In contrast to most other oral swellings, these very firm swellings that develop on the incisor areas of very young horses can be considered pathognomonic. They are benign but if not treated can enlarge quickly and cause resorption of the adjacent bones and teeth and at that stage, may require resection of the rostral mandible or of the premaxilla.

Calcified dental tumours. These include compound and complex odontomas, and cementoblastomas/cementomas (to be differentiated from reactive nodular hypercementosis – “cement pearls”), osteomas, osteosarcomas. These all need radiography or CT imaging for evaluation and imaging may show large homogenous calcified rounded masses possibly osteomas or cementomas or well differentiated dental structures such as a compound odontoma or a less structured dental appearance such as complex odontoma. These lesions need surgical removal that may involve extensive maxillary sinus or mandibular bone surgery. If associated teeth need to extracted, there is increased risk of post-operative oro-maxilalry fistula formation.

Reading
Astrid Bienert-Zeit, Jennifer Rawlinson and Cynthia Bell. Oral, nasal and sinus masses in: Equine Dentistry and Maxillofacial Surgery, Eds JK Easley. PM Dixon and N du Toit, ist edition 2022, pp 273-286