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.