Q: When I had my horse euthanized, you told me he had cancer through-out his body. I am curious as to how common this type of cancer is in horses and how long they can live with it. Can cancer be detected in the blood work?

A: Horses, like all species, suffer from cancer. Some cancers are relatively benign and never cause the horse a problem, others are very aggressive and can cause death in a few short months, and others take years or decades to become fatal. The most commonly diagnosed forms are those visible from the outside; tumors of the skin, eye, etc. Some forms of cancer can be detected in the blood (lymphoma, leukemia), but usually any blood abnormalities are too nonspecific to diagnose cancer. Beyond that, equine veterinarians are at a great disadvantage for detecting tumors. In small animals (and humans), the primary diagnostic tool for detecting tumors is radiography. Unfortunately, an adult horse is just too large to get radiographs of the chest or abdomen. There are no x-ray machines powerful enough to penetrate the great mass of a full grown horse, making early detection of tumors difficult. The only way to diagnose these is by cytology, and this is often a hit-or-miss proposition. With cytology, samples of body fluid are collected and examined in the hope that cancerous cells shed by the tumor will be found. But the body has many compartments and each one (chest, abdomen, lungs, spinal cord) has to be sampled. Additionally, if the tumor is buried deep in a large organ like the liver, there will be no cells shed and the abdominal fluid will not indicate a tumor.

Although radiography is not very helpful, we have found ultrasound to be useful in many cases, and ultrasound guided biopsies are an excellent diagnostic tool. There is also work being done with identifying protein markers in the blood, but a functional test is still years away. We are just grateful that cancers are fairly rare in horses and look forward to the day when better and less invasive tests are available.