10 Things You Might Not Know About Equine Cancer

Equine cancer is discussed less than cancer in dogs, cats, and people. However, increased longevity in horses and advanced diagnostics and treatments have stimulated more conversations. We worked with Dr. Emily Berryhill of the UC Davis veterinary hospital’s Equine Internal Medicine Service to bring you the latest on equine oncology.

1. Horses may have a lower predisposition for cancer than other species. Tumor incidence is lower in horses compared to other long-lived species. It is unknown if this represents true resistance or if it takes longer for cancer to develop in horses and is more difficult to detect due to their large body size. Since horses are more likely to die from causes such as colic, laminitis, or catastrophic injuries, the true incidence of cancer in horses is undetermined.

2. Bovine papillomaviruses (BPVs) are linked to equine sarcoids. Cattle have a natural immune response to BPVs that leads to regression of warts, but immune response is likely impaired in sarcoid-affected horses. Studies have identified BPVs in over 70% of sarcoid cases. 

3. Some horses are prone to sarcoids, but the reason is unclear. Environmental and genetic factors affect sarcoid development. Research has shown increased sarcoid prevalence in families and across breeds. Associations between sarcoid occurrence and variants in the major histocompatibility complex, a set of genes essential to the immune system, have been identified. Data suggests that sarcoid susceptibility is likely influenced by multiple genes rather than a single gene. 

4. A genetic test can identify horses at risk for ocular squamous cell carcinoma (SCC). Squamous cell carcinoma is the most diagnosed tumor of the horse's eye. SCC risk factors include UV exposure, pigmentation, and genetics. A variant in the damage-specific DNA binding protein 2 (DDB2) gene, discovered at the UC Davis Veterinary Genetics Laboratory with funding from CEH, results in increased risk in Haflingers, Belgians, Rocky Mountain Horses, Connemara Ponies and crosses. Horses with two copies of the variant (homozygous) should undergo routine examinations for early detection and better prognosis and wear a UV protective fly mask. 

5. Up to 80% of gray horses have at least one melanoma by age 15. Melanomas are common in older gray horses. In a classification system from grades 1 to 5, with 5 being the most severe, melanomas are expected to increase by 0.3 grades each year. Gray is caused by a copy number variation in the syntaxin 17 gene and horses with three copies (G3) show faster graying and high incidence of melanoma. 

6. Melanomas in non-gray horses are usually aggressive and malignant. Melanomas are associated with gray horses. However, non-grays, such as bays and chestnuts, can develop melanomas, and they are likely to be malignant and metastasize early. 

7. Mares with extreme behavioral changes RARELY have granulosa cell tumors (GCTs). These common tumors of the reproductive tract are often benign but have historically been associated with issues such as aggression and stallion-like behavior. However, a recent UC Davis study showed that only stallion-like behavior was associated with GCTs. Other abnormal behaviors, such as aggression and estrous-cycle behaviors, were not associated with GCTs (Huggins et al. 2023). Thus, undesirable behaviors or abdominal discomfort are unlikely to be associated with ovarian hormones. 

8. Lymphoma is the most diagnosed malignant cancer in horses. It affects tissues that contain lymphocytes, white blood cells that help the body fight disease and infection. This includes lymph nodes, thymus, spleen, and linings of the digestive, respiratory, and urogenital tracts. Horses 4 to 10 years old are more commonly affected, but it can occur at any age. No specific risk factors have been identified. Clinical signs are non-specific, so affected horses are typically diagnosed in the late stages of the disease. Horses that are only affected in their skin (cutaneous lymphoma) tend to have a better prognosis than those with lymphoma affecting internal organs. Treatment options include surgery, radiotherapy, and chemotherapy, with response dependent on type and extent of disease. 

9. The cause of most equine cancers is unknown, but there are ways to lower cancer risk. Regular sheath cleaning to reduce smegma accumulation, reducing exposure to excessive sunlight (especially in light-colored horses by providing shade, utilizing sunblock and UV blocking fly masks), and general cleanliness are important for prevention of tumors. 

10. The key to successful cancer treatment is early diagnosis. Regular grooming is a good time to look for small lumps and bumps. Treat these seriously and have them examined by a veterinarian. Seek early veterinary assistance for skin abnormalities that persist for a few weeks, or if a new lesion develops around the original lesion or in a new location on the body.