Photo by Dr. Rhea Morgan, copyright 2008

Zebras are obnoxious and hard to catch. I know this because I once watched one dash out into the field, halter dangling around its neck while my syringe was falling from its body. Vaccination was unsuccessful, needless to say. At that moment, I decided that never would be too soon to ever deal with another zebra.

Most of the time in medicine, the zebra is a metaphor – a symbol of the uncommon condition or disease, the animal spirit of Occam’s razor. if you will. Veterinarians and physicians are taught as students that when presented with a set of signs or symptoms, it’s best to pursue the most likely cause of those symptoms rather than assume that the signs point toward some exotic disease and the zebra is that exotic disease.

The problem is that the zebras get all the attention. When was the last time you saw a brown horse trending on Twitter? Nope, everyone wants to chase the zebra. Zebras are flashy and black-and-white. They may be strong and scary, but in a way they are uncomplicated. Medicine is often complex and sort of ambiguous, so an exotic, headline-grabbing disease is easy for the mind to snatch up.

Exam room conversations go something like this:

Client: “My dog threw up last night and again this morning. I think he’s got that Martian Jabberwock Fever that’s going around. I read a whole thing about it on Facebook.”

Veterinarian: “Has he been to Mars lately? Or has there been any household exposure to Jabberwocks?”

Client: “Well, no, but my cousin in Vermont went to Mars last month, and his sister’s aunt’s dog was visiting my mother last week.”

Veterinarian: “Any changes in food recently?”

Client: “Only half a rotting narwhal carcass he found on the beach yesterday. Can you just run the test for Martian Jabberwock Fever?”

Veterinarian: “I think it might be best to start with a stool sample, some bloodwork, and X-rays to make sure he didn’t swallow narwhal horn or get a parasite or bacterial infection, or just eat the trash or a chocolate cake.”

Client: “Aren’t you listening? My boss’s cousin’s pen-pal’s dog DIED of Martian Jabberwock fever and he was vomiting too.”

Part of the problem is that a known zebra makes a far more appealing target than trying to pick the right horse out of the herd of unknown possibilities. Even if the zebra is a horrifying, fire-breathing, demon-spitting, apocalypse-inducing monster, there is a comfort in knowing that the hoofbeats come from one source.

Unfortunately, the medical zebras, like the real ones, are rare and hard to manage. Sometimes horses and zebras can look alike from a distance – vomiting could be a sign of Martian Jabberwock Fever, but it is more likely to be the result of narwhal horn obstruction, a virus, bacteria, or indigestion.

The only way to really identify the source (or sources) of the hoofbeats is to run the appropriate diagnostic tests.

This method is far more accurate at distinguishing horses from zebras than Dr. Google, who is really good at calling everything a zebra.

With a few exceptions, these tests don’t so much tell you which horse is creating the hoofbeats as they do which horses aren’t making the noise. This is why in medicine we more often rule something out, to determine what is NOT ailing the patient, almost as often as we rule something  in. There are very few tests that point to a specific disease. Most of our diagnostics help show what kind of disease we are looking for and narrow down the source of the signs. And yes, the tests will help rule out the zebra, just in case he’s lurking in the grass.