EPM Test Summary

There have been several improvements in the available EPM testing in the last five years.  Research is ongoing to find a quick, portable, and accurate test for EPM. Tests are still not seen as definitive, if there are no visible symptoms of EPM.  Because the test alone, without symptoms present, is not conclusive evidence of EPM, the tests are not now a routine part of pre-purchase exams.

CC Mike Baird   Morro Bay

Tests are carried out on either blood (serum) or cerebrospinal fluid (CSF).  There are currently four main types of tests that can be carried out on either the blood or CSF, and these tests can produce different results.  Tests on CSF are seen as more reliable at screening for an active infection in the central nervous system (CNS).  It is possible for both false positive and false negative test results on blood and CSF.  One challenge is that each horse responds differently to the antibody test.  Lower dilutions may be positive results for some horses.  The author recommends the SAG1 ELISA.

CSF and Blood Tests

Tests of CSF usually require trailering the horse to a larger veterinary hospital for a spinal tap.  These are performed as an outpatient procedure, with the horse standing and heavily sedated.  An 8” needle is inserted into the spinal column to draw out fluid for testing.  This is an invasive procedure, but the small risks can be minimized by taking the horse to a veterinary hospital that performs this procedure daily.  Tests on the CSF will measure the presence of anti-bodies in the CNS, an important distinction from testing the blood.  A January 2009 estimate from New Bolton Center said the cost for a neuro exam, sedation, spinal tap, and lab testing of the CSF was $600.

Blood for tests can be collected by your veterinarian at the barn.  The samples are sent overnight to labs, and results should be available within one week.  Blood tests will show the presence of antibodies in the blood.  It does not conclusively indicate an active infection in the CNS.  If the horse was vaccinated for EPM, it will carry antibodies to the protozoa, and blood tests will be positive for exposure.  Blood tests run in January 2009, in PA were $150 and $230, including the farm call, overnight shipping and lab fees, but not an exam.

Western BLOT Test

This is the original test available for EPM.  It measures the presence or absence of the antibody to the protozoa in the blood or CSF.  It is generally not a quantitative test, and will only give a negative, positive, or high positive result.  Approximately 50% of horses in the northeast have been exposed to S. neurona.  If your horse ingested these protozoa, and then cleared the protozoa from the blood by itself, it will carry the antibody.  The horse may not have an active infection, but in this case the horse’s exposure will still result in a positive test.  This test is only seen as diagnostic when the results are negative, meaning that the horse has never been exposed to the protozoa, and does not have EPM.

SAG1 ELISA Test

This test can be run on blood or CSF.  It looks for the presence of antibodies specific to one surface antigen (SAG1) created by the protozoa.  It is a quantitative test, and the results will show the level of the antibodies, helping to sort out exposure from active infection.  This test is very specific to one protein found only on S. neurona, and will give fewer false positive results.  In a study, 24 of 26 known positive horses had antibodies to S. nuerona with SAG1.  The other two specimens lacked SAG1, but carried SAG5 or SAG6.  The latest research shows S. neurona from different areas to be very close in DNA, including marine species.  In artificially infected horses, symptoms were visible at a dilution of 1:32.  Field cases are showing neurologic symptoms at a dilution of 1:16.  The results are read as a dilution, not as a negative or positive.  The use of multiple tests run in sequence is helpful to measure the change in titer.  The change in titer can indicate if the medication is working, or if the horse is relapsing.

There have been questions about the validity of the SAG1 ELISA after a study was published showing the test gave false negatives for 30% of positive horses.  However, the flawed study used a 1:200 dilution as a basis for positive results - more than 10 times the dilution where clinical symptoms are being observed in the field.  This measurement would indeed exclude many positive horses. 

The SAG1 ELISA is available for $20 from Pathogenes.  A new stall-side rapid test utilizing SAG1 exists, but is not yet available.

IFAT Test

This test can be run on blood or CSF.  It looks for the presence of antibodies to four surface proteins created by S. neurona.  Unfortunately, these same four surface proteins also are created by different species of protozoa that do not cause EPM.  This means the test can't tell the difference between S. neurona and other protozoa.  It can be run separately to identify antibodies to N. hughesi.  It is a quantitative test, using mathematical formulas to predict the percentage chance of active infection. The results will help distinguish between exposure and active infection.  It is less specific than the SAG1 ELISA, and will produce more false positive results.  This test is only available through UC Davis, and the February 2009 lab fee including both protozoa was $84.  The lab will run a full panel using both IFAT and Western BLOT for $134.  the results are read as a percent chance of an active infection, not as negative or positive.

Form for sample submission to UC Davis

PCR Test

This test can be run on blood or CSF.  It looks for the presence of the actual protozoa.  The protozoa often move within the body, and samples could be taken from an area where they do not exist.  The test returns a higher percentage of false negative results than other tests, and is not recommended.  UC Davis has a PCR laboratory, but does not use this test for diagnosis of EPM.  Improvements are being developed for the test, and it may offer better diagnostics in the future.

Recent EPM Research

The adage that it takes both a positive EPM test, and clinical symptoms to make a positive diagnosis of EPM, is based on the history of imperfect tests, and hard to define symptoms.  Research is continuing to create an accurate, stall-side test for EPM.  When this becomes available, there should be discussion on the use of the test during a pre-purchase exam.

July 2010

References:

International Journal for Parasitology 38 (2008): 623-631

IFAT test study   http://jvdi.org/cgi/reprint/15/1/8.pdf

IFAT CSF blood contamination study   http://jvdi.org/cgi/content/full/19/3/286