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Pathogenes Testing Options

Updated: Oct 26, 2023

If you are looking at our site you probably have a horse with a possible diagnosis of EPM, equine protozoal myeloencephalitis.

We know from our more than twenty years of experience with EPM the best course of action is to make decisions based on examination and testing.

A good outcome starts with examination and testing

A decision tree is binary decision, a pathway for assessment, and useful for assessing a neurologic horse. Our decision tree starts with a clinical exam and then test selection. The test results can determine what is causing disease.

Both SAG 1, 5, 6 ELISA and CRP are recommended to rule EPM in or out as a possible cause of disease. If the SAG's are negative, another diagnosis should be considered! Horses with antibodies are exposed to S. neurona, but don't necessarily have disease. Horses with S. neurona infections in the intestine are at higher risk for EPM, but don't necessarily have protozoa in the central nervous system. Antibodies protect the horse, especially it there is re-exposure in the environment. We showed that by experiment. Low levels of exposure keep the immune system engaged and this immunity protects the horse against further disease. It is good to know the serotypes of S. neurona that are present in the environment, this is determined by the SAG 1, 5, 6 ELISA. Testing normal horses in the herd can give you an idea of the serotypes present on a farm.

Testing for diseases that look like EPM

Other diseases look like EPM. Polyneuritis (PNE) is such a disease. Your testing options include the myelin protein antibody test, MP2/MPP, also called the sidewinder test. This test detects antibodies to myelin proteins in the serum. Seropositive horses are diseased. If myelin proteins antibodies are detected, the condition is called a demyelinating polyneuropathy. These horses look like they are sidewinding because they have severe weakness, paresis, on one side of the body.

Polyneuritis horses generally relapse, they get better and then they relapse with similar signs. The disease is progressive, getting worse with each relapse. Relapsing disease was once considered an indication of EPM, we disagree! Relapses are an indication of a dysregulated immune reaction and can be treated, but not with anti-protozoal agents. Some horses that don't fully recover from "EPM" are really untreated PNE horses.

Once diagnosed with demyelinating polyneuropathy, the CRP test is valuable to monitor the health of the horse. The CRP will change when the inflammation is addressed, this value can take 14 days to change. It can take 5 months or more for the myelin antibodies to decrease. Other causes of inflammation can keep the CRP elevated. Encysted parasites and hind gut ulcers are foremost on the list for chronic inflammation in horses.

Another useful test for horses with polyneuritis is the neurofilament light test, NfL. This test detects proteins released from diseased and broken axons. Any positive value is abnormal. It is possible to have a positive NfL test and a negative MP2/MPP. The NfL values will change quickly, within 10 days of proper treatment.

The other Sarcocystis that infects horses, commonly

S. fayeri strains that cause clinical signs are detected with our test because we look for the toxin produced by this organism. Most horses, up to 85% of horses in the United States, are infected with S. fayeri, but few, estimated at 6%, are infected with the toxin producing strains. Horses infected with S. fayeri don't show disease. Horses with neurological signs and positive for S. fayeri toxin benefit from treatment.

Lyme disease can be ruled out by screening for antibodies against Borrelia in areas of the country that have Lyme carrying ticks. Horses with Lyme disease have a different clinical presentation that EPM horses, but the signs are close enough that testing can be useful.

In the long run testing saves money

It is important to treat the correct condition. Before a diagnosis of EPM is considered, trauma, osteoarthritis, laminitis, and cervical stenosis should be ruled out. Sometimes an endoscopic exam is useful. Radiographs of the head and neck can be insightful. A good physical and neurological exam are important and selecting the right tests before treating can save money and return the horse to use.


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