Skin cancers of various types are the most common cancers I encounter in horses. This is most likely due to the fact that I can see these types of tumors. Unlike in small animal medicine, where the patient is small enough to take X-rays or abdominal ultrasounds when a mass is palpated in the belly, it is impossible to obtain a meaningful X-ray on a horse abdomen due to their size. Also, abdominal masses in horses can only be palpated via rectal palpation and one’s arm can only reach so far.
Treatment of equine skin cancer depends on the type of cancer. Sarcoids, a type of local but sometimes locally invasive skin cancer, are frequently left alone — a term called “benign neglect.” This is because any type of trauma, surgical or otherwise, often aggravates these tumors, stimulating them to grow larger. Squamous cell carcinoma, on the other hand, can be surgically removed and this is often curative.
Melanoma is seen most frequently in gray horses. Depending on how extensive the mass is, melanoma may be left alone and monitored, or it may be removed. Cryotherapy is sometimes the treatment of choice for this cancer.
Aside from various skin cancers, the other fairly common cancer in equines, much like in cattle, is lymphosarcoma. However, unlike cattle, where a majority of lymphosarcoma cases are caused by bovine leukemia virus, equine lymphosarcoma is a spontaneous event, meaning not caused by an infectious agent.
Equine lymphosarcoma, like in bovines and any other species, is tricky. Originating in lymph tissue that can be located in any part of the body, this cancer is sometimes not obvious to find. Sure, an animal with huge lymph nodes is fairly straightforward, but more often, a horse with lymphosarcoma may only be diagnosed on the necropsy floor — obviously too late to help the animal. At necropsy, we will sometimes find an intestine totally coated with lymphosarcoma, explaining the profuse diarrhea the horse was suffering from. Or we’ll find a mass impinging on a nerve, explaining the lameness we were seeing. Lymphosarcoma can be a jack-of-all-trades, and unfortunately a master in lethality because of it.
Treating internal cancer like lymphosarcoma in large animals is different than in small animals. The chemotherapeutic agents used in small animals (and in humans) are extremely expensive and dangerous to those administering them. Add in the size of a horse and the amount of chemotherapy required for treatment, plus the containment issues, and you have an expensive, logistically challenging treatment regimen. This isn’t to say that a horse diagnosed with cancer doesn’t have options. If the owner has the financial abilities, referral to an equine clinic, usually associated with a vet school, may offer hope of remission, depending on what type of cancer it is.
I would wager that many equine cancers are under-represented because they are simply not diagnosed. This being said, I always have to remind myself when I encounter a mystifying case to include cancer on my list of differential diagnoses. It’s easy to forget about when we’re not looking.
Dr. Anna O’Brien