With sick animals, people often think negative diagnostic results are "bad" results since the cause of disease is still unknown. However, we would do well to remember Thomas Edison’s mindset: "I have not failed. I have just found 10,000 ways that don’t work." For example, if blood test results indicate that a horse has normal bloodwork (including kidney and liver function tests) and no evidence of anemia or infectious disease, much information can be gained from these "negative" results.

If basic test results are all normal, it tells the veterinarian which organ systems are functioning within normal limits. Far from being unnecessary expenses, these so-called "negative" test results can point the way to the next steps in diagnosing the real problem. But as Neil Williams, DVM, PhD, points out, despite a complete postmortem and multiple diagnostic tests for infectious diseases, the cause of equine abortions sometimes cannot be determined.

In 1977 several Thoroughbred mares in Newmarket, England, developed an unusual grayish vaginal discharge after being bred, and their failure to conceive caused significant concern. Standard aerobic culturing failed to detect conventional bacterial causes for endometritis, and examination of cells showed a white blood cell response. Since all routine diagnostic methods had shown negative results, other diagnostic laboratories were consulted for further assistance. Eventually a vaginal swab from an affected mare was taken to the Public Health Laboratories in Cambridge for testing using methods to detect human gonorrhea. This testing showed the growth of a new, slow-growing Gram-negative coccobacillus. The equine disease is now known as contagious equine metritis (fortunately not horse gonorrhea!); the bacterial cause is Taylorella equigenitalis.

Who would have thought to take a mare’s vaginal swab to a public health laboratory? By ruling out other causes of equine vaginal discharge and still running into negative results, veterinarians and scientists were forced to think outside the box