Lymphoma in horses

Cancer might be rare in horses compared to humans and small animals, but it occurs and requires treatment nonetheless, often with chemotherapy. It hasn’t always been clear, however, just how effective chemotherapy is at fighting equine cancer. But recent research suggests that, for equine lymphoma at least, chemotherapy can help horses achieve remission and extend their lifespans for several months or years.

At the 2018 American College of Veterinary Internal Medicine Forum, held June 14-16 in Seattle, Washington, Daniela Luethy, DVM, Dipl. ACVIM, a large animal internal medicine lecturer at the University of Pennsylvania School of Veterinary Medicine, in Kennett Square, shared the results of a study she and colleagues conducted to evaluate the long-term outcomes of treated equine lymphoma cases.

Clinical signs of cancer in horses can be vague, nonspecific (such as weight loss, failure to gain weight, or fever), and unapparent until the disease is in an advanced state, making it challenging to diagnose. Lymphoma is a type of hematologic (blood) cancer in which the tumor cells arise from lymphocytes, a type of white blood cell. Multicentric lymphoma can, essentially, affect the entire body, whereas cutaneous lymphoma affects the skin.

Treatment typically doesn’t cure lymphoma, although veterinarians can use it, in some cases, to slow or reverse clinical sign progression and extend the horse’s lifespan. Because few studies evaluating long-term lymphoma treatment outcomes exist, Luethy and colleagues set out to do just that.

They included 11 horses in their retrospective study, based on medical record searches and recruitment on veterinary e-mail lists. Eight horses had multicentric lymphoma, while three had cutaneous lymphoma.

Luethy said the most common lymphoma immunohistochemical classification (essentially, the type of lymphoma) was T-cell rich large B-cell, found in six study horses. Further, biopsies from three horses’ tumors tested positive for the gamma herpesvirus equine herpesvirus-5 (EHV-5). Studies have shown that, in humans, gamma herpesviruses are associated with lymphoproliferative diseases, while equine studies have revealed more frequent EHV-5 detection in horses with lymphoma than healthy controls.

Veterinarians treated horses with a variety of chemotherapeutic medications, including:

  • Cyclophosphamide (9 horses);
  • Vincristine (9 horses);
  • Lomustine (8 horses);
  • L-asparaginase (7 horses);
  • Doxorubicin (6 horses);
  • Cytosine arabinoside (2 horses);
  • Chlorambucil (1 horse); and
  • Intralesional cisplatin (1 horse).

In addition, Luethy said, nine horses received corticosteroids and three received the antiviral valacyclovir.

Eight horses (73%) responded to treatment: five achieved complete remission (including the three EHV-5 horses), and three responded partially. One horse’s disease remained stable, and two horses died due to lymphoma complications during treatment. Two pregnant mares were among the study horses; one survived to foaling, and one died during treatment.

Overall, treatment resulted in a mean survival time of 13 months, with a range of one to 41 months, Luethy said. Horses with multicentric lymphoma had a shorter median survival (7.5 months, with a range of one to 28 months) than did horses with cutaneous lymphoma (13 months, with a range of 16 to 41 months).

Seven horses had adverse effects directly attributable to the chemotherapy, Luethy said. The team classified the reactions based on the Veterinary Cooperative Oncology Group’s Common Terminology Criteria for Adverse Events Grading Scale (see sidebar). Adverse events included:

  • Grade 1 alopecia, or hair loss (2 horses);
  • Grade 1 combined neutropenia and lymphopenia—abnormally low numbers of two white blood cell types in the blood (2 horses);
  • Grade 1 lymphopenia (1 horse);
  • Grade 1 lethargy (1 horse);
  • Grade 2 gastrointestinal signs (1 horse);
  • Grade 2 injection site reaction (1 horse);
  • Grade 2 hypersensitivity (1 horse);
  • Grade 4 hypersensitivity (1 horse); and
  • Grade 5 hypersensitivity (1 horse).

“The adverse events were mostly mild and certainly not a reason to avoid chemotherapy if an owner is willing to pursue chemotherapy,” Luethy said.

She noted that limitations included the study’s retrospective nature, particularly with regard to varying treatment protocols.

“However, these findings show that chemotherapy can be used successfully for treating equine lymphoma,” she said.