Question: I’m familiar with equine skin cancers, like melanoma and squamous cell carcinoma, but I don’t think I’ve ever encountered a horse with the kinds of internal cancer seen in so many people.    Do horses get things like lung cancer, uterine cancer or lymphoma? If not, do we know why? Could we learn anything from their biology that might help prevent these diseases in people?

Answer: Yes, horses do get internal cancers. Your awareness of melanoma and squamous cell carcinoma is not surprising. Together with sarcoids, current data indicates they are the most common types of equine cancer.

Skin cancer is more noticeable

There are a few things to consider. Firstly, for any species, including humans, cancers on the skin are more readily noticeable. And given that melanoma is associated with graying hair color, squamous cell carcinoma with ultraviolet light exposure, and sarcoids with bovine papillomavirus infection (and genetics), it isn’t surprising they occur so frequently.

Diagnosing internal disease in horses is challenging

Secondly, techniques for diagnosing internal diseases in horse-sized animals have lagged behind those used in human medicine but have been improving. We can now use endoscopes to look inside less invasively and take samples for pathology.

As a pathologist, I have seen more samples of internal cancers from horses being submitted for examination over the past 20 years. At the same time, imaging capabilities have also expanded.

Susceptibily to cancers varies between species

Finally, susceptibility to different cancers varies between species for various reasons, including genetics. Pancreatic cancer is very common in people but rare in animals.

Most cancer types have been reported in horses. Lymphoma, a cancer that originates in the cells of the lymph system, is not as common in horses as in dogs or cats but could account for up to 5 percent of diagnosed tumors.

We understand more about equine lymphoma now but still have quite limited treatment options. Lung and uterine cancers appear to be rare.

Comparing differences and similarities between cancers in humans and other animals is extremely valuable. Of our domesticated species, the dog has been most often studied in this way. But we have a lot to learn from other animals if the cancers occur in sufficient numbers for study.

How viral infection and genetics influence sarcoids and interact with the immune system can teach us a lot about basic cell biology. Honestly, we need more such work!

Janet Patterson-Kane, BVSc,
PhD, Dip ACVP, FRCVS
Author of Clinical Equine Oncology