Some practitioners try to diagnose EPM with a visual neurological exam. They skip the test indicating the results are not accurate. Don't take this shortcut of a proper diagnosis. The steps taken to properly diagnose the horse will save time and money in the future. Using response to treatment as a diagnosis has several variables such as stall rest, anti-inflammatory use, and length of time before improvement is seen. These variables can confound the diagnosis. In the mean time, you may be delaying necessary treatment for a different disease. Small quantities of anti-protozoal drugs can affect the test results - TEST THE HORSE BEFORE TREATMENT!
|CC Mike Baird Morro Bay|
One problem with many of the tests is that they employ Surface AntiGens (SAG) 2, 3, and 4. These three surface proteins of the protozoa are common to all of the Sarcocysts in the horse - the one that causes EPM, and the many that don't. SAG1 and SAG5 are specific to Sn, the cause of EPM, and do not occur on other Sarcocysts. The author recommends one of the two tests based on SAG1,5, and 6.
When a horse is in the very early stage of disease, its immune system may not yet produce IgG (specific) antibodies to show as a positive on any test. Performing two tests 2 to 4 weeks apart, and showing a 2-fold to 4-fold rise in titer is conclusive of active disease. This is called serial testing.
Administering anti-protozoals before infection (as often happens in sale houses and training programs) will result in delayed antibody response if the horse later contracts EPM. This means the horse has lowered resistance to the pathogen, and will contract the disease easier if exposed to it. When there is already an active infection, anti-protozoals will cause some death of protozoa in the blood stream. The dead protozoa will trigger the immune system to form antibodies to Sn. As the infection ends the titer starts to drop. It is critical to accurate diagnosis to test the horse BEFORE giving anti-protozoal substances.
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 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. There is debate between researchers on the validity of
anti-bodies in the CNS.
One concern is that a CSF tap containing contamination with just 8
red blood cells
negates the results. Another is the belief that anti-bodies
may cross the blood-brain-barrier giving false positives.
Even a well-performed CSF tap may make the ataxia worse.
A December 2011 estimate from
Blood for tests can be collected by your veterinarian at the barn. The samples can be tested immediately with the Multiplex strip, or are shipped to labs. Lab results should be available within one week. Blood tests will show the presence of antibodies in the blood. One test does not conclusively indicate an active infection in the CNS, but serial testing 2-4 weeks apart with a rise in titer means active infection. A previous vaccination with the Fort Dodge product will not produce a positive result. Future vaccines will carry a marker to distinguish the vaccine titer from infection.
This test is generally run on serum, but can be run on CSF for additional cost. It looks for the presence of antibodies specific to three surface antigens (SAG1, 5, and 6) created by the protozoa. Each of these SAG are exclusive to a phenotype, and the test will measure 100% of the strains of Sn. It does not use SAG2, 3, or 4, and will not detect antibodies to other Sarcocystis through these SAG's. There are no false positives with this test. 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 machine read, and is objective in the titer. The results are read as a dilution, not as a negative or positive. The use of two serial tests is helpful to measure the change in titer. The change in titer can also indicate if treatment is working, or if the horse is relapsing. Knowing the strain that infects the horse is important to prognosis and treatment.
This test is a major leap forward in Sn antibody testing. The Peptide ELISA is available for $38 from Pathogenes. Blood must be shipped 2nd day air, and does not need ice. The SAG1 ELISA available from Antech is a valid test, but why not go for more complete information?
This 20-minute stall-side test for serum is new in August 2011. The test is based on Sn SAG1, 5, and 6 so it is specific to Sn, and will not detect antibodies to other Sarcocysts. The test uses high-tech human diagnostics on a strip. It is calibrated to the SAG1 ELISA, and indicates a positive at SIGNIFICANT levels of antibodies. The test will be negative for background exposure to Sn. The test gives a single point value for SAG1 and 5 and 6 , so it does not produce a titer. It is useful in diagnostics, producing serial tests, end-of-treatment decisions, and keeping a tab on the titer for relapses. The test is excellent for use by the ambulatory vet. The test is available from Prota in a 2-test pack for $60. A positive Sn strip is now accepted for entry into the Oroquin-10 drug trial. Fill out the Peptide ELISA submission form, attach a dried strip, and place in regular mail. The author recommends a test based on SAG1,5,6.
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 relies on SAG2, 3, and 4, and will measure antibodies to Sarcocysts other than Sn, leading to false positives. It is not a quantitative test, and will only give a negative, positive, or high positive result. It is read by humans, and is subjective. This test was used in the 1980's to test the US horse population's exposure to Sn. Because of the cross reaction with other protozoa, the values of 50%-80% of horses being exposed to Sn is highly suspect. The exposure rates for S. fayerii in the horse population are known to be about 80%. This test is best used as a diagnostic when a negative result is obtained, meaning the horse does not have EPM. The test is available from several different labs for about $45, and the blood must be shipped overnight on ice.
This test can be run on blood or CSF. It looks for the presence of antibodies to Sn SAG2, 3, and 4. Unfortunately, these same three surface proteins also are created by species of Sarcocystis that do not cause EPM. This means the test can't tell the difference between EPM-causing and non-EPM-causing protozoa. The test is read by humans, and is subjective. It can be run separately to identify antibodies to N. hughesi. It is a quantitative test, using percentages to predict the percentage chance of Sn infection. It is less specific than the Peptide ELISA, and will produce false positive results. This test is only available through UC Davis, and the June 2010 lab fee including both protozoa was $86. The lab will run a full panel using both IFAT and Western BLOT for $136. The test requires shipping blood overnight on ice, which adds to the cost. CAUTION! If your vet does not specifically opt out of the WB, you will be charged for it.
An interesting note on the IFAT... There are no USDA licensed tests for antibodies to Sn, and NO USDA TEST for EPM. EPM is a clinical diagnosis, not a laboratory diagnosis. Even though UC Davis advertises the IFAT as an EPM test, there is no such thing.
This test can be run on blood or CSF. It looks for the presence of the protozoa DNA, but cannot tell the difference between live and dead 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. Improvements are being developed for the test, and it may offer better diagnostics in the future.
New research is suggesting that the protozoa is killed fairly quickly, and the resulting neurological deficits are reactions of the CNS to cell wall toxins and inflammation. If this is correct, the PCR test would have a very limited window to identify protozoa.
An article is now available indicating the close genetics and limited variants of Sn. The implications of this study are that the protozoa has a form similar to T. gondii, and questions the use of SAG2, 3, and 4 for diagnostics. The latest research is showing that crystalline structure of the surface antigen is very important to the virulence factor. Sn SAG1 looks like it plays a very large role in the infection of horses.
Wendte, J.M., et al., Limited genetic diversity among Sarcocystis neurona strains infecting southern sea otters precludes distinction between marine and terrestrial isolates. Vet. Parasitol. (2010), doi:10.1016/j.vetpar.2009.12.020
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