Source: Penn Vet New Bolton Center

Cancer isn’t as common for horses as it is for humans and dogs and cats. And because equine cancer symptoms — weight loss, nausea, lethargy, loss of appetite, lameness, skin and coat conditions, among others — often don’t start appearing until the cancer has advanced, it can be hard to reverse its progression. For years, chemotherapy has been veterinarians’ go-to treatment for fighting the disease.

But is chemotherapy effective and should doctors recommend it?

“Chemo for horses can be quite expensive,” says Dr. Daniela Luethy, an Internal Medicine lecturer at New Bolton Center, who, along with her Penn Vet colleagues might see as many as 50 equine lymphoma cases a year. “We don’t want to recommend a costly but ineffective treatment.”

Recently, Luethy tackled the chemo effectiveness question in a retrospective study published in March 2019 in the Journal of Veterinary Internal Medicine. Looking at cases from 1991 to 2017, the research documents long-term outcomes for horses with lymphoma who were treated with chemo.

“Until now, we didn’t have research supporting the idea that chemo works for equines,” says Luethy. “There were studies on specific chemo drugs but nothing covering a range of chemotherapeutic protocols. The goal of our study was to understand whether chemo prolongs life for horses with the disease.”

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Luethy’s research focused on 15 horses with different types of lymphoma at different stages of progression. Some of the horses were former or current New Bolton Center patients and others were treated at outside practices or veterinary schools. All were given chemotherapy orally, intravenously, or subcutaneously. “First, we wanted to know whether horses responded to the drugs,” explained Luethy. “We found approximately 90 percent did.”

Five of the study’s horses achieved complete remission and nine partial remission. The disease stabilized in one horse — it didn’t get worse or spread.

Eight months was the average survival time for the animals. Says Luethy, “This may not seem like a long time, but the range of years was pretty remarkable — one horse lived only one month but others are still alive today.”

The study also looked at the side effects of chemo. “Did the treatments compromise the horses’ comfort or quality of life enough that we should avoid prescribing it,” Luethy asks in the research.  

The researchers were pleased to find side effects overall were mild and easily treated, or they went away without any medical intervention.

“The results are encouraging,” Luethy says. She added that the retrospective study poses opportunities for further research with more case control, including comparisons of mixes of multiple chemotherapy drugs and tracking responses when chemotherapy is used in conjunction with a steroid.

For the immediate future, however, Luethy is reassured by the results presented in her 2019 paper: “For me, the biggest outcome of this research is that I now feel comfortable recommending chemo for my patients knowing the side effects are minimal and the treatment can prolong lives.”

Photo: Shutterstock/SpeedKingz