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Few equine diseases are as frightening--or confounding--as equine protozoal myeloencephalitis (EPM). A neurological condition caused by protozoal parasites, EPM often produces subtle, easy-to-miss signs early on. Yet, left untreated, the disease can cause irreversible damage to the spinal cord and brain. Although great strides have been made in understanding and treating EPM, the average horse owner can be forgiven for spending a bit more time worrying about this disease than most others.
It's little comfort that some of the leading researchers in the field readily admit that they are uncertain as to why some horses contract EPM or which treatments are consistently effective. Even determining whether a particular horse has the disease is difficult, because the standard tests frequently produce inaccurate results.
The diagnostic challenges posed by EPM reflect the complex nature of the disease itself. EPM is caused by one of two organisms, Sarcocystis neurona or Neospora flughesi. As with most protozoal diseases, the horse is not the main host for these parasites but is instead an accidental and rare victim of an agent whose natural host is common in the environment. S. neurona's "home base" is the opossum, which is found widely in the Americas. Horses ingest the protozoa when they consume forage or water contaminated with opossum droppings. Infection with these protozoa does not mean disease, however. In fact, most horses who are exposed to S. neurona or N. hughesi develop antibodies to the organisms without ever becoming ill. Only when the protozoa somehow pass from the bloodstream to the central nervous system and undergo rapid multiplication does the horse develop clinical signs of EPM. Affected horses become increasingly uncoordinated and, in the worst cases, unable even to stand.
There are three basic diagnostic tests for EPM, all of which seek to detect S. neurona infection. (A test for N. hughesi is not yet available.)
The blood test detects antibodies to S. neurona. A negative blood test can virtually rule out EPM, missing only those horses who have so recently (one to two weeks) been exposed to the protozoa that they have not yet produced specific antibodies. A positive test, however, does not necessarily mean a horse has the disease-EPM occurs only when protozoa cross the blood-brain barrier, multiply and damage the nervous system. Experts estimate that in some areas of Kentucky as many as 90 percent of horses carry blood antibodies to S. neurona--and the vast majority show no signs of disease. In addition, some surveys suggest that, nationwide, some 5O percent of all horses carry antibodies to S. neurona. As a result, the blood test alone is generally considered useless in diagnosing EPM, and some veterinarians prefer to by- pass it altogether.
The other two EPM tests require a spinal tap, the withdrawal of fluid from the spinal cord, which is a demanding and slightly risky procedure. The easiest test on the spinal fluid looks for antibodies, a sign that the protozoa have passed from the bloodstream to the nervous system. The presence of antibodies in spinal fluid can be an indicator of disease but only hen a "clean" sample has been obtained-if even a minute amount of blood carrying antibodies contaminates the sample, a false positive reading will result. And getting a clean spinal tap isn't easy, even for experienced veterinarians. A study at the University of Pennsylvania's New Bolton Center found that as few as eight red blood cells per microliter--about one millionth of the concentration present in the bloodstream and far too few to stain a sample pink--was enough to trigger a positive reading. As with the blood test, a negative spinal tap result can pretty well be trusted, but a positive one is not necessarily conclusive evidence of disease. An extremely low level of antibodies in the spinal fluid is more likely to indicate a sample's contamination than a horse's illness.
The other diagnostic analysis done on spinal fluid is the polymerase chain reaction (PCR) test, which uses gene amplification technology to detect the presence of the S. neurona organism itself in spinal fluid. When PCR tests were first put into use a few years ago, expectations were high that they would revolutionize EPM testing. In practice, however, the tests have proved to be unreliable.
In light of the uncertainties surrounding current EPM testing techniques, it's clear that the notion "better safe than sorry" does not apply to EPM testing. A horse who stumbles occasionally or takes a few more missteps than usual is not a prime candidate for testing. Nor is it wise to see darker implications in minor lamenesses and niggling training problems that might once have automatically been attributed to overwork or rider error. But in the face of borderline signs of uncoordination, the question of whether to test for EPM is rarely simple. Choose wisely and you can save your horse's life; take the wrong course and you may be treating a nonexistent condition or ignoring another serious problem.
