Over the paste decade, Equine Protozoal Myeloencephalitis has been recognized as one of the most prevalent and uncontrollable diseases confronting the horse industry. One of the most troublesome aspects of EPM is often the difficulty horsemen and veterinarian have in accurately diagnosing the disease.
Identifying the EPM protozoa and treating our horse for the disease
might soon become easier. Under the guidance of David S. Lindsay PH.D, the Virginia-Maryland Regional College of Vet Medicine in Blacksburg VA is in the process of developing an improved test for EPM.
Lindsay, a professional at Virginia Tech's department of Biomedical sciences and pathoiology, along with his research team, hopes his study will perfect a direct
agglutination test for the presence of the parasite that could propose improved specificity and improved sensitivity for EPM testing.
According to Lindsay, EPM is considered one of the most significant parasite diseases affecting horses in the
Americas.
EPM is a neurological disorder caused by the parasite Sarcocystis neurona. The disease causes gait abnormalities, loss of balance, muscle wasting, weakness, paralysis, seizures and even behavior disorders in its victims. The attention that EPM has received over the past 10 years has propelled equine practitioners to be more sensitive to test for the disease whenever confronted with a horse that hosts unidentified abnormalities in its behavior.
However, timely and accurate diagnosis of the disease can still depend on the owners awareness of changes in ths horse's behavior, if the horse displays clinical signs of the neurological disease, as well as the vet's ability to accurately detect and diagnose the disease.
Initially discovered 35 years ago, EPM has caught the attention of virtually every facet of the horse-health field since a test for detecting the s. neurona parasite was developed about a decade ago.
However, there are many questions regarding the validity and procedure of the current testing methods for detecting EPM.
How it works:
The western blot test is the current test used to detect EPM. This test uses a single dilution of cerebral spinal fluid (CSF). The parasite is homogenized in a buffer, then filtered through a gel to separate the antigens by molecular weight. The separated antigens are then transferred to a piece of
nitrocellulose paper. If the sample tests positive for EPM, the antibodies will react to a band on the paper.
Lindsay said most equine practitioners are capable of accurately conducting the test, but the single dilution aspect of the test makes it difficult to obtain an estimate of the amount of antibody present. And that's the key difference between the western blot and the new test under
development.
In Lindsay's proposed test, the CSF is obtained via a spinal tap like before. The extracted CSF is diluted in a buffer and reacted with the antigen. If the EPM antibodies are present, they fall to the bottom of the plate. Then, a dilution test is conducted to detect the presence and amount of EPM antibodies in the sample.
The more you can dilute the sample and still obtain a positive reaction the more antibodies are present in the sample. The
agglutination test Lindsay's developing actually quantifies the level of antibodies present as they clump around the a foreign agent. This foreign agent is the EPM parasite.
"Our test hasn't been compared to the CSF western blot, he said, Technically, the new test will be easier to do because it requires fewer reagents. It still requires a moderate amount of interpretation however.
Some horse owners believe that EPM is over diagnosed. And Lindsay noted that EPM is a difficult disease to diagnose. It should never be done solely on the results of a one time
serological test, whether it be the western blot or Lindsay's new test under
development.
Serological tests can provide an indication of exposure. Exposure does not always correlate with disease. In the case of EPM, many horses are serologically positive for exposure.
Lindsay doesn't believe that the seroprevalence, or the presence of antibodies, is over estimated because there can be a presence of antibodies, but not the actual disease. Horses that develop EPM are the ones with parasites in the central nervous systems . That's why equine practitioners must use CSF in the western blot test.
When the CSF is contaminated with blood, problems can occur in accurate diagnosis. The technicalities presented with testing for EPM make diagnosing the disease a very meticulous process.
A common concern among horse owners that many horses may register positive for EPM with blood testing, but not show any symptoms. Some seroprevalence testing has suggest that as many as 30 to 60 percent of horses have been exposed to the parasite, yet researched
do not fully understand why some develop signs of EPM and others do not.
Lindsay explained that there are many factors that govern resistance to infectious agents.
"Its not surprising that some horses are more susceptible than others", do we know why? No! The answer is probably related to genetics and how the horses produce a cell mediated immune response to the infection. Serological tests that measure antibodies can be used only for predictive purposes. In the future, we may have a test that measures the cell-mediated immune response to EPM and it might be more predictive if a horse will or will not develop the disease.
That's a long way off".
Statistically, most EPM cases are diagnosed in the summer and fall.
According to Lindsay, there is no known reason as to why these seasons produce the most EPM discoveries. A
possibility is the increased opossum activity and more potential transmission as opossums are the
definitive host for s.neurona.
The parasite is ingested as horses graze on infected pastures, and the parasite can survive for years in the soil once excreted by opossums. If you eradicated all the opossums on the farm you still might have the parasite in teh field for years. The ingested parasite
eventually moves to the horse's central nervous system.
Another possible reason for the number of EPM discoveries being highest in the summer and falls seasons, is the typically owners spend more time with their horses and are more apt to pay attention to their horses behavior and vital signs.
Aid in Early Diagnosis:
There are many proposed treatments for EPM, however, the disease is best treated with a combination of anti
parasitic and anti inflammatory medications. Nutritional supplements are often used as SUPPORTIVE THERAPY TO REINFORCE THE
MEDICATIONS. Ultimately EPM can cause lingering problems that can impair athletic performance and can affect a horse so
severally it must be euthanized.
If existing diagnostic tests are improved, veterinarians can begin treatment earlier and
receive help in analyzing the progress of their therapeutic interventions. Lindsay isn't certain that his developing test can
provide an earlier diagnosis-but hopes it will.
"Diagnosis is difficult and depends on more than a serological test, it may help monitor the course of infection by giving an antibody titer".
A titer is a measure of the amount of antibodies present. The higher a titer the more antibodies that are present. If an animal has a low titer and then develops a high titer it's indicative of active infection in most diseases and probably EPM too.
Lindsay hopes his test will be beneficial in finding recent infections. In the future, he looks toward taking a sample and then taking another sample two weeks later. Then, both samples will be tested at the same time. If the second test is two to four time higher, it indicates an active infection. It may also be useful in monitoring response to treatment. The titer should drop after the parasite is killed: it may never go to zero but it should drop.
Lindsay's validation testing among horses is being conducted with Drs.
Anne Cook, Sharon Witonsky and Virginia Buechner Maxwell, all with the department of large animal clinical sciences at the
Virginia Maryland Regional College of Veterinary Medicine.
The research in Lindsay's study so far has only been looking at the blood serum. The research team needs to compare the serum to the CSF to obtain any preliminary results. The problem is that they can't find enough horses that are truly negative (via blood
samples) for EPM in the united states. They have looked in France, where it is believed that the disease doesn't exist in order to find horses that are negative for EPM.
Lindsay hopes his future test will be more accurate and less expensive. Lindsay said that in an ideal world, a vet would be able to come to the barn and conduct a simple blood test in order to detect EPM. However,
technology isn't that advanced at this time and vets still must obtain CSF in order to fully diagnose the disease.
