Discussion
Dispite the patient being repeatedly examined in order to determine the exact diagnosis, it took four weeks from the onset of the first clinical signs of generalised pruritus and alopecia until the patient started to develop additional clinical signs indicating the presence of a malignant tumour. Knottenbelt et al. (2015) reminds us that pruritus can be the only sign of the presence or progression of carcinoma.
Since the skin was altered, due to self-mutilation rather than the primary skin disease, a skin biopsy was not immediately performed. At the same time, the horse was on a high dose of corticosteroids, and consequently a skin biopsy was not performed until a few weeks after they were discontinued. As the use of corticoids resulted in a deterioration of the secondary skin infection, administration of antibiotic therapy was indicated.
As is also described in Knottenbelt et al. 2015, whereby horses with pruritis do not suffer from paraneoplastic syndrome, this horse also started to loose condition during hospitalization, despite maintaining to preserve an appetite. However this patient was never in satisfactory condition due to being on prolonged box-rest for four weeks, and consequently this symptom and pruritus alone were not in themselves an indicator of a fatal disease.
After there was no response to various therapies, paraneoplastic pruritus was considered, but the examinations undertaken did not confirm the occurrence of a tumour until three weeks after admittion to the hospital where there was a rapid deterioration of the the horse’s condition.
The pattern of moth-eaten alopecia is important to notice and differentiate from other skin conditions that cause pruritus and alopecia. When comparing the photos of our patient with the patients in the article by Combarros et al. (2020) and Curnow (2020), the pattern of alopecia looks identical. It’s unique presentation is not particularly simmilar to any other skin disease, so it could potentially bepathognomonic and help assist with ascertaining future and early correct diagnosis.
Serum was collected for examination of the proliferation marker, thymidine kinase, which can be significantly increased in the presence of lymphomas (Larsdotter et al. 2015), leukemia or multiple myeloma. This result was however negative, although this does not reliably exclude malignant tumour (Moore et al. 2021).
As it is unusual for small lymphocytes to be predominate in pleural effusion, small round cell lymphoma was suspected, but the tumour cells were large round cells, and so they were not perfectly compatible with the cells seen on the pleural effusion. Thus the cells in the effusion might have represented T-lymphocytes reacting to the neoplasia.
Histology confirmed a malignant round cell neoplasm which had infiltrated all of the submitted lymph nodes samples (mediastinum, spleen, stomach, kidney) as well as being present in the thymus and in the adipose tissue around the nodes. This location of the tumour and the changes in lymph nodes in the area of ​​the digestive tract and kidneys would indicate mediastinal, thymic or thoracic lymphosarcoma, which most often affects geldings and stallions younger than 10 years of age (Fukunaga et al. 1993). Lymphosarcoma is the most common thoracic neoplasia in horses, occurring in 54-74% of cases (Sweeney and Gillette 1989, Mair and Brown 1993). Thoracic lymphosarcomas usually have a rapid course and the average time from the appearance of the first clinical signs to euthanasia is approximately four weeks (Mair and Hillyer 1991). These masses can be as large as 15-36 cm in diameter, and are often associated with large volumes of pleural effusion, which can dislocate the lungs dorsally, leading to compressive atelectasis of the lungs (Mair et al.1985, Sweeney and Gillette 1989, Meyer et al. 2006). Pleural effusion is caused by neoplastic infiltration of the mediastinal lymph nodes with reduced pleural lymphatic drainage (Sweeney 1992). Drainage of this fluid can sometimes be achieved with the placement of a permanently inserted catheter, however often without successas due to the rapid increase in the volume of fluid produced. The tumour in this patient was nodular, occured in the mediastinum, and had enlarged adjacent nodes which were adhesed. Subsequently it was impossible to precisely circumscribe the tumour margins. Knottenbelt et at. (2015) describes this pattern as being typical of lymphosarcomas involving the mediastinum, but less so the case in thymic lymphosarcomas. However, the absence of immuno-positive staining for CD3 and CD79a did not support a diagnosis of T-or B-cell lymphoma. The second assumed possibility was histiocytic sarcoma, which albeit rare in horses, does metastasise quickly, affects the skin and subcutaneous tissue, parenchymatous organs, bone marrow and surrounding soft tissues. It can also metastasize to the lymph nodes of the mediastinum and also carries a poor prognosis (Beusker et al. 2010, Knottenbelt et al. 2015). However, this type of tumour did not completely match the description in this patient.
The exact identity of the tumour cells was ambiguous. They could be large lymphocytes (poorly differentiated, high-grade) or they could be histiocytes (disseminated histiocytic sarcoma). Other round cell tumour types and other cell lineages were possible but appeared less likely. The second examination confirmed the presence of lymphocytes.
But whatever the true identity of the cells, the lesion had disseminated widely and was also present in the blood vessels, suggesting that further metastasis was likely and the prognosis would have been poor. Immunostaining was performed to try to distinguish B-cell lymphoma, T-cell lymphoma and histiocytic sarcoma. But poorly differentiated tumour cells do not react appropriately to immunostains, and additionally immunostains are not available for every possible tumour cell lineage. So while a definitive diagnosis remained unclear the second histological examination did indicate incidence of lymphoma. When we consider the case history, the comparrission with previously peered reviewed studies of similar cases, the clinical findings and also the confirmed presence of large amount of lymphocytes on histological and cytological samples, these findings confirm a diagnosis of an aggressive type of equine lymphoma.
The presentation of pruritus, self-mutilation and generalised moth-eaten alopecia could be the first clinical signs to suggest a diagnosis of neoplasia of the lymphatic system in the horse which is commonly only confirmed after a deterioration in the horse’s general status with follow up cytology of the pleural or peritoneal fluid or upon taking a biopsy of a mass for histology and / or imunohistopatology or euthanasia.