Cancer in Horses

By Lucia Unger, DrMedVet, Dipl. ECEIM, of the Swiss Institute of Equine Medicine (ISME); and Vince Gerber, DVM, PhD, Dipl. ACVIM, ECEIM, FVH, of ISME, WEVA treasurer and junior vice-president

Aside from skin tumors, cancer is a rare diagnosis in horses compared to humans and small animals. However, the true prevalence of many types of cancers in horses might be underestimated, because definite antemortem (in the live horse) diagnosis is challenging and often requires referral to a clinic.

In horses, signs of cancer can be vague and nonspecific, such as weight loss or failure to gain weight, exercise intolerance, fever, and apathy. Veterinarians should suspect neoplasia (tumors) in cases where they’re ruled out common differential diagnoses, such as severe parasite burden, dental or digestive problems, or infection, or in horses that do not respond to common therapeutic measures.

Any horse with suspected neoplastic disease should undergo a complete diagnostic workup, which can be quite extensive and, thus, expensive. Depending on the horse’s history and clinical presentation, these workups might include blood work, rectal exam, abdominal and thoracic ultrasound, thoracic radiographs, abdominal and thoracic tap, fine needle aspirates, or biopsies from internal organs or lymph nodes. Some of these examinations are only possible in a referral clinic setting and require a veterinary specialist’s expertise.

In short, it can be challenging to properly work up a horse suspected of having cancer. As such, new methods to facilitate and improve cancer diagnosis in horses as early, simply, and accurately as possible are needed.

In equine medicine noninvasive tumor markers that can be easily detected in blood are not yet routinely available, but increasing research efforts focus on this topic: A recent study from Sweden introduced a novel, promising biomarker for equine lymphoma, one of the most common generalized neoplasias in horses¹​. Furthermore, our research team is currently investigating the diagnostic and prognostic value of microRNA blood profiles for equine sarcoids, squamous cell carcinoma, and other cancers. The use of such tumor markers to diagnosis equine cancer could help avoid more invasive workup for the patient and allow early diagnosis.

Unfortunately, due to these diagnostic challenges, some horses only receive a cancer diagnosis at an advanced stage of disease, rendering treatment options unrewarding. Some tumors can be fully excised, whereas others have already metastasized or are inoperable due to their location or extensive tissue involvement. Veterinarians can use chemotherapy as adjunct or palliative therapy, but it remains an expensive treatment with potentially significant side effects. Immunotherapy in addition to chemotherapy is gaining importance in human and small animal oncology and is already used in the horse to treat skin tumors such as sarcoids and melanoma.

Novel blood-borne tumor markers should render the diagnostic workup easier, allowing earlier diagnosis and therapy. This will, hopefully, increase affected horses’ chances of survival in the future, particularly if combined with immunotherapy and chemotherapies.

¹-Larsdotter S, Nostell K, von Euler H. Serum thymidine kinase activity in clinically healthy and diseased horses: a potential marker for lymphoma. Vet J 2015 205(2), 313-316)