Like Zebras for Horses

By Anna O'Brien, DVM on Mar. 8, 2013

There’s a saying vet students hear repetitively when learning the art of creating a list of differential diagnoses for a patient: "When you hear hoof beats, think horses, not zebras." This quote is meant to remind students that usually the most common diseases are the culprit of clinical signs, and not the weirdo exotic stuff. Which is too bad, since we’re taught the weirdo exotic stuff, are fascinated by it, and desperately want to diagnose it.

The hoof beat quote is good to keep in mind once you’ve graduated, as well. It helps keep you grounded and reminds you that, no, your practice is not like Dr. House’s on TV, where he gets all the cool stuff. That equine lameness really is only a hoof abscess and not a fractured navicular bone, and that case of doggie diarrhea really is only dietary indiscretion and not caused by a parasite seen only in Ethiopia. But that’s not to say that every once in a while, you do get a doozie (my technical term for a real head-scratcher).

I have one such "hoof beat" case that comes to mind that was definitely zebras and not horses. A few springs ago, a client called about her draft horse seeming "off," Upon examination, the handsome giant with long white feathers down his legs appeared to have sort of a stiff neck, sort of seemed painful all over, and was sort of reluctant to walk; all clinical signs were vague.

A picky eater normally, his appetite was off and he had a low-grade fever. Thinking infectious causes because of the fever, the first thought that came to my mind was Lyme disease, which, like in dogs and humans, can cause generalized myalgia (muscle pain) and joint pain in horses.

Drawing blood for further diagnostics, we started him on antibiotics for suspected Lyme disease. I told the owner I would call back in a few days for an update. Most cases of equine Lyme disease respond quickly to antibiotic therapy, so much so that this rapid response can be used as a diagnostic before we even get the blood results back.

However, after a few days, the horse was not better. He was, in fact, worse. Drastically losing weight and muscle mass, he was now visibly favoring his front left leg, even while standing still. Blood work did not support Lyme disease and didn’t show much else, either.

Further lameness examination suggested the lameness was up high, somewhere near the shoulder. But his feet also were hot and sore, indicating the start of laminitis, a painful inflammatory condition of the hooves.

Another lesson taught in vet school is: Don’t give the patient multiple problems. This means, usually, a patient has one thing wrong and it’s manifested in many ways. Don’t complicate things by trying to diagnose multiple problems to explain each clinical sign. This case, however, did seem to have multiple problems now: laminitis in the feet, something potentially in the shoulder, and this pesky fever and weight loss.

Sure, trauma could explain a shoulder injury, and stress from the pain could cause weight loss, but severe muscle wasting over the course of days?

Unfortunately, this story has a sad ending. The laminitis was so painful I couldn’t keep the horse comfortable and the owner elected euthanasia. However, a necropsy was performed at the diagnostic lab, giving the owner and me some closure. On necropsy, the pathologist found a tumor (a melanoma) in the area of the shoulder blade that was pressing on a large nerve. The tumor was spreading along this nerve in the early stages of an aggressive malignancy.

So there we had it: a reason for the shoulder lameness, the general achiness, the muscle wasting, and yes, even the fever — sometimes pesky, sneaky tumors can cause a low-grade fever. My "zebra" was a tumor. Not an especially rare tumor, mind you, since horses do get melanomas with some regularity, but the location and the resulting clinical signs were highly uncommon, at least in my experience. The laminitis was a secondary issue stemming from being stall-bound and bearing excessively on the right front foot, a common and unfortunate complication in horses that are sick and confined.

This case was a reminder for me that the practice of medicine is always humbling. Just when you think you know stuff, you are reminded that biology will throw you for a loop when you least expect it. And although most of the time you should think horses when you hear hoof beats, it doesn’t hurt to entertain the thought of a zebra every now and then.