Polyps of the upper respiratory tract are uncommon in horses, comprising 8% of horses with space- occupying masses of the paranasal sinuses and account for approximately 2.5% of all cases of sino-nasal disease in the horse, but can become extensive and result in partial or complete obstruction of nasal air flow (Tremaine and Dixon, 2001; Crecan et al., 2021). Similar to humans, polyps appear to result from chronic inflammation, including chronic sinusitis or rhinitis, and typically consist of a fibrovascular core with granulomatous inflammatory changes and overlying mucosal epithelium (Knottenbelt, Patterson-Kane and Snalune, 2015). There are no current, comprehensive reports of a inflammatory polyp apparently originating from the apex of a developing cheek tooth, nor any case reports detailing the radiographic, computed tomographic, gross pathological, cytological, bacteriological, and histopathological findings of nasal polyps in horse. This report outlines these findings, in addition to the clinical history and multidisciplinary diagnostic approach towards the case.

A three-year-old Connemara filly was referred to a UK equine hospital for further investigation of unilateral nasal discharge and facial swelling. Diagnostic imaging revealed the presence of a heterogenous, soft tissue opacity with areas of focal mineralisation within the right paranasal sinuses, completely obliterating the nasal cavity, in addition to endodontic disease and significant periapical pathology associated with the right, permanent fourth premolar (Triadan 108). Subsequent histopathology confirmed the mass as a fibro-inflammatory polyp. In a horse of this age, the mixed heterogenous structure visible on computed tomography in apparent close association with a dental structure (cheek tooth 108) was suspicious of an odontogenic tumour with mixed odontogenic epithelial and mesenchymal origin (such as ameloblastoma or complex odontoma). Mineralisation is uncommon with sino-nasal neoplasia and typically accounts for less than 25% of the tumour volume (Cissell et al., 2011). Previous studies have described mild, focal areas of increased, mineral-attenuation in cases of sino-nasal myxoma (n=1) and undifferentiated carcinoma (n=1), with more widespread and/or multifocal mineralisation in osteosarcoma, nasal adenocarcinoma, and ossifying fibroma (Cissell et al., 2011). Histopathological findings were consistent with a giant fibro-inflammatory polyp associated with osteomyelitis of the alveolar bone, likely as a sequela to endodontic disease and chronic periapical infection of Triadan 108. This case highlights the poor sensitivity of diagnostic imaging in the investigation of equine sino-nasal masses and the importance of a multi-disciplinary approach to cases. Both polyp development and dystrophic mineralisation appear to result from chronic inflammatory processes, and the histopathological findings would support this. Hyperplastic pulpitis is one possibility, but this has not yet been reported in horses. Endodontic disease and periapical infection is not an uncommon cause of sinusitis in horses, hence it is more likely the fibro-inflammatory polyp in this instance developed as a sequalae to chronic odontogenic sinusitis.

References
- Crecan, C.M. et al. (2021) ‘Removal of an Extensive Nasal Polyp in a Standing Horse, Using a Direct Approach to the Nasal Cavity Through a Bone Flap’, Bulletin of University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca. Veterinary Medicine, 78(1), pp. 135–139. Available at: https://doi.org/10.15835/buasvmcn-vm:2020.0049.
- Knottenbelt, D.C., Patterson-Kane, J.C. and Snalune, K.L. (2015) ‘Other epithelial neoplasms’, in Clinical Equine Oncology. Elsevier, pp. 247–280. Available at: https://doi.org/10.1016/B978-0-7020-4266-9.00014-3.
- Tremaine, W.H. and Dixon, P.M. (2001) ‘A long-term study of 277 cases of equine sinonasal disease. Part 1: Details of horses, historical, clinical and ancillary diagnostic findings’, Equine Veterinary Journal, 33(3), pp. 274–282. Available at: https://doi.org/10.2746/042516401776249615