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blood (Stage V) (Lana et al. 2006). As mentioned, PARR testing can assist in differentiating between reactive lymphocytes (polyclonal) and neoplastic lymphoid tissue (monoclonal), and may be helpful in supporting a diagnosis of lymphoma in horses with an unclear diagnosis. Clonality results should, however, be examined alongside cytology and histology, and PARR should not be relied upon to definitively phenotype lymphoma as clonal rearrangement of both B and T cell receptors has been demonstrated in human, canine and feline lymphomas (Weiss and Spagnolo 1993; Strauchen 2004; Wilkerson et al. 2005; Avery 2009; Sato et al. 2011; Keller et al. 2016). In addition, recent advances in dogs using real time PARR testing for clonality (utilising melting curve analysis) demonstrates promise in making this testing more readily accessible for clinical use (Langner et al. 2014). While PARR testing is sensitive, it is not infallible, and false


negatives or positives can occur (Keller et al. 2016). As mentioned, PARR testing should be examined alongside additional diagnostics, as was performed in the current case. PARR testing was used to verify that the B cell population was indeed the neoplastic population and that the T cell population was the infiltrate, thus verifying the diagnosis as a TCRBCL. As equine lymphoma is frequently a more insidious disease, often poorly exfoliative into body cavity fluids and typically comprised of a heterogeneous population of lymphoid cells, initial diagnosis by cytology can be difficult. PARR testing was successful in this case of extranodal lymphoma, and could be a useful tool for equine practitioners. While this particular tumour was accessible, the enhanced sensitivity of PARR testing could potentially play a role in detecting hard to find tumours within the thoracic cavity, abdominal cavity or blood. This enhanced sensitivity could potentially aid in detection of lymphoma within body cavity fluid when biopsy is contraindicated or difficult to achieve. Additional research is warranted to assess the use of PARR testing in horses with suspected but undiagnosed or inaccessible tumours. Due to the severe respiratory distress, the unknown and


potentially poor prognosis, the possibility of unrecognised multicentric disease, and the owner’s concern for the horse’s well-being, humane euthanasia was elected in this case. Because of the close proximity of the mass to important neurovascular structures, complete surgical resection with appropriate margins was not considered possible, and necropsy findings confirmed this. Potential treatment plans were discussed, including chemotherapy or radiation to decrease the size of the mass, possibly allowing for adequate surgical excision, which could have been followed by other adjunct therapies. With the exception of cutaneous lymphoma, prognosis and treatment success is generally viewed as poor in equine lymphoma (Weaver et al. 1996; Rhind and Dixon 1999; Montgomery et al. 2009), but is largely unknown (Taintor and Schleis 2011). In cases of localised disease, most reports describe that, due to poor prognosis, treatment was not attempted (Lane 1985; Lester et al. 1992; Rhind and Dixon 1999; Montgomery et al. 2009; Marques et al. 2012) or was unsuccessful (Burba et al. 1991), although surgical excision (Rebhun and Del Piero 1998), radiotherapy (Weaver et al. 1996), and systemic chemotherapy (Saulez et al. 2004) have had occasional success. An early and better characterised diagnosis, in combination with treatment earlier in the course of disease, may result in improved success with treatment.


Although thoracic and laryngeal lymphoma has been


documented to cause respiratory distress (Burba et al. 1991; Lawn 2005; Marques et al. 2012), to the authors’ knowledge, focal, solitary, extranodal retropharyngeal lymphoma causing respiratory distress in a horse has not been reported. Lymphoma should be considered in horses with focal masses of the retropharyngeal region. Although treatment was not pursued, PARR testing was successful in this case and may be helpful for accurate characterisation of lymphoma in horses to more precisely determine prognosis and the most effective treatment plans, as it has been in human species and small animals.


Authors’ declaration of interests No conflicts of interest have been declared.


Ethical animal research


The hospital from which material were derived for this study consistently uses an admission/consent form that includes an option for owners to opt out of research studies. Approval was given by this owner for use of photos, video or tissue samples to be used for research purposes.


Source of funding None.


Authorship


E. Collar was involved in all aspects of horse care, tissue sampling, data analysis and interpretation, and major preparation of the manuscript. J. Parker was involved in all aspects of horse care, tissue sampling, data analysis and interpretation. E. Gorman assisted with cytology and histology analysis and interpretation, diagnostic recommendations and manuscript preparation. D. Russell assisted with histology analysis and interpretation, and manuscript preparation. B. Valentine performed the post-mortem examination. All authors contributed to critical manuscript review and approved the manuscript.


Manufacturers' addresses


1Amneal Pharmaceuticals, Glasgow, Kentucky, USA. 2VetOne, Norbrook Laboratories Ltd, Newry, Northern Ireland, UK. 3Leukocyte Antigen Biology Laboratory, UC Davis, Davis, California, USA. 4Qiagen QIAxcel capillary electrophoresis system, Qiagen, Valencia,


California, USA. 5BioCalculator software, Qiagen, Valencia, California, USA.


References


Aleman, M. (2009) Lymphoma in horses. In: Large Animal Internal Medicine, 4th edn., Ed: B.P. Smith, Mosby, St. Louis, Missouri. pp 1776-1779.


Avery, A. (2009) Molecular diagnostics of hematologic malignancies. Top Companion Anim. Med. 24, 144-150.


Badial, P.R., Tallmadge, R.L., Miller, S., Stokol, T., Richards, K., Borges, A.S. and Felippe, M.J. (2015) Applied protein and molecular techniques for characterization of B cell neoplasms in horses. Clin. Vaccine Immunol. 22, 1133-1145.


Burba, D., Jann, H. and Confer, A. (1991) Surgical reduction of a laryngeal lymphosarcoma mass causing dyspnea in a horse. Equine Pract. 13, 14-18.


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