Chemotherapy can be used successfully to treat horses with lymphoma, a study has found.
Adverse effects were found to be mostly mild, affecting about two‐thirds of treated horses.
Lymphoma, or lymphatic cancer, is the most common malignancy seen in horses, representing 1% to 14% of tumors seen in horses.
Clinical presentation varies depending on the sites affected.
The prognosis is variable and not well described in the scientific literature.
Daniela Luethy and her colleagues, writing in the Journal of Veterinary Internal Medicine, note that treatment for horses is often palliative. It may include surgical excision, corticosteroids, chemotherapy, or a combination of treatments.
Descriptions of chemotherapy for treatment of horses with the condition have been limited to case reports. No study has evaluated a large number of horses treated this way to determine its effectiveness and long‐term outcomes.
The researchers set out to learn more about the long‐term outcome of chemotherapy use in horses with lymphoma.
They found 15 cases with enough data for inclusion in their study.
Complete remission was achieved in five of the horses and a partial response was achieved in nine others.
Stable disease was achieved in one of the horses.
The overall response rate was found to be 93.3% (14 of the 15 horses).
The median survival time was eight months, ranging from one month to 46 months.
Nine horses experienced a total of 14 adverse effects arising from chemotherapy, although most were mild. They included hair loss, low neutrophil levels, lethargy, neurotoxicity, mild colic, and various levels of hypersensitivity.
The worst adverse effects most commonly were linked with the use of the drug doxorubicin, including one horse that died 18 hours after receiving it.
Discussing their findings, the study team said horses with cutaneous lymphoma and those that achieved complete remission tended to have longer median survival times than those with alimentary or multicentric lymphoma, and those that achieved partial remission.
While adverse effects occurred in two‐thirds of cases, they were usually mild and self‐limiting, although one horse died as a consequence of adverse effects attributed to doxorubicin administration.
Their findings, they said, supported the use of chemotherapy for treating lymphoma in horses.
“Because of the retrospective nature of our study, the optimal chemotherapeutic approach for horses with lymphoma cannot be determined because of the lack of randomization and insufficient number of cases for statistical evaluation.
“However, when examining median duration of survival for patients that received different chemotherapy regimens, the horses that received 5 and 4‐drug protocols had longer median survival time than did patients that received all other regimens except the 1‐drug (lomustine) protocol.”
This finding is in line with results in dogs with lymphoma, they noted.
Corticosteroids alone may result in remission in some lymphomas, and the corticosteroid administration in 12 of the 15 horses in the study had complicated interpretation of the benefits of chemotherapy.
The authors say many barriers exist to implementing chemotherapy in horses. These include financial constraints, availability of the drugs used, and personnel exposure and environmental contamination with chemotherapeutic drug residue.
In addition, owner concerns exist regarding performance of horses while receiving chemotherapy as well as the impact on fertility.
Several of the horses in the study continued to be exercised during their chemotherapy, including at least two who successfully competed during treatment. This suggests carefully planned chemotherapy need not necessarily impact performance beyond any effects related to the disease process itself.
“In conclusion, chemotherapy may be used successfully for the treatment of horses with lymphoma to achieve remission and potentially increase survival time,” the study team reported.
Results will likely be dependent on anatomic distribution and the stage of the disease.
“Adverse effects associated with chemotherapy are common but usually mild and self‐limiting.”
Retrospective evaluation of clinical outcome after chemotherapy for lymphoma in 15 equids (1991‐2017)
Daniela Luethy, Angela E. Frimberger, Daniela Bedenice, Barbara S. Byrne, Erin S. Groover, Rachel B. Gardner, Trisha Lewis, Valerie S. MacDonald, Lauren Proctor‐Brown, Joy E. Tomlinson, Kenneth M. Rassnick, Amy L. Johnson.
Journal of Veterinary International Medicine, 12 January 2019, https://doi.org/10.1111/jvim.15411