by Dr. Paul Morley
Equine protozoal myeloencephalitis is the tongue-twisting name of one of the most prevalent and puzzling neurological diseases found in horses. The disease began to capture headlines about two years ago when it was first "discovered," although it was almost surely present prior to that time. Since then, its causes and cure, like many of those diseases affecting the human, have been the source of controversy among veterinary researchers and practitioners. Even more perplexing, it seems, is which horses have it, which don't and what to do to manage what can be a life-threatening situation.Recent investigations suggest that a protozoal parasite called Sarcocystis neurona, which infects the nervous system of the horse, is the agent that causes EPM. Apparently, the carrier is the opossum and horses are commonly exposed to the parasite in regions the opossum is found. As a matter of fact, prevalence for the disease among horses living in those regions is often 50 percent or higher.
When testing for the disease, veterinarians often do so with baited breath. Certainly, there is no definitive test for the disease; meaning there is no test which can conclusively determine that a horse has EPM. Rather, there is a combination of symptoms and tests which assist a veterinarian in declaring that a horse has the disease. However, no diagnostic test is perfect and it is critical to understand how sensitive or specific a test might be to better understand when a test result is true or false.
When testing for EPM, your veterinarian will take a sample of your horse's cerebrospinal fluid (CSF) so that a western blot assay can be performed. Preliminary research suggests that this particular test is 89 percent sensitive and 89 percent specific for the diagnosis of EPM. But again, it's not perfect, and veterinarians are often as frustrated as their clients when it comes to determining when a "false positive" or "false negative" might be indicated rather than having an accurate result.
It's also important to understand the difference between sensitive and specific. Sensitivity and specificity describe the rate of false-positive and false-negative test results. Sensitivity is the proportion of truly diseased animals that test positive, whereas specificity is the proportion of non-diseased animals which test negative. In other words, the western blot assay can determine 89 percent of the time which animals are truly diseased and which animals are not.
False test results, whether positive or negative, can occur for a number of reasons which can be broken down into four broad categories: problems with the biological marker that is measured by the test, differences among individual patients, problems with sample collection or storage and problems with the testing procedure.
Diagnostic tests are meant to identify some characteristic or marker of a disease that is not found in healthy animals. An ideal marker of a disease would be found in every affected animal early in the disease process as well as throughout its entire course, and it would never be found in unaffected animals. However, no marker of disease is perfect and some tests assay (or identify on) general markers, whereas others are designed to detect markers of a very specific disease.
For example, you may have the flu, but it would take a specific test to determine what kind of flu you had, whether it be Asian, Hong Kong, Russian, etc. Markers which identify on generalized disease state may not be helpful when symptoms can be caused by many different conditions.
In addition, patients differ and some healthy animals may express markers found in diseased animals, while some diseased animals may not express markers other diseased animals do. Plus, depending on the stage of the disease, markers will or will not be expressed. For example, clinicians have reported that horses in the very early stages of EPM may not have detectable concentrations of antibody in the CSF, but they will be positive if retested a week or two later.
Another problem is that occasionally, the CSF sample, which is a watery substance, may be contaminated with blood and this can cause a falsely elevated S. Neurona antibody concentration.
And finally, it takes some skill to perform these tests accurately and the variability in skill and diligence can easily affect results, as is true of any technical task.
While it is not possible to be 100 percent confident of the disease status of any animal, it is possible to compare tests results with that of a population of animals which truly have the disease. That, combined with the symptoms the horse might exhibit, usually gives the veterinarian a high rate of accuracy when diagnosing the disease.
Further research is needed to determine the accuracy of specificity and sensitivity for the western blot assay, however, this test has been used extensively in the past five years for the diagnosis of EPM and available information suggests that it is highly sensitive and highly specific. It will not, however, be useful when used in horses in which the disease prevalence is very low, and therefore, should not be used in horses which appear normal.
Dr. Paul Morley is an assistant professor in the Department of Preventive Medicine at The Ohio State University in Columbus, Ohio. He presented in findings during the 1997 AAEP Convention in Phoenix, Arizona. Further information on EPM can be obtained from Ohio State University.
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