What Is Western Blot in Horses?

A western blot test is a chemical test with many applications, but it's particularly well-known to horse owners as the standard test for the common but dangerous and debilitating disease equine protozoal myeloencephalitis, or EPM.

  1. EPM

    • Equine protozoal myeloencephalitis is a neurological disease caused by parasitic protozoa inhabiting the brain of a horse. These protozoa have actually evolved for dependence not on horses but on opossums, and infect horses only when they ingest traces of the feces of infected opossums. states the EMP Horse website. The disease causes lesions in the horse's brain and nervous system, sometimes resulting in permanent nerve damage even after the death of the parasite.

    Tests

    • Several tests exist for EPM, all of which involve analysis of either blood or cerebrospinal fluid. Since exposure to the EPM parasite is extremely common--over half of horses in the American Northeast have ingested it at some point--but does not always result in infection or disease, accurate testing is very important.

    The Western Blot Test

    • Western blotting exploits the tendency of particular proteins to bond with particular antibodies. By introducing a protein or antibody to a sample, western blotting can test that sample for the presence or absence of that substance's partner, according to Dr. Paul Morley on the NetPets' EPM page. In horses, western blotting tests cerebrospinal fluid for the antibody to the parasite--if present, the horse has been exposed.

    Limitations

    • Because western blotting tests merely for presence or absence, it's impossible to know from such a test just how many of the parasites the horse has ingested, whether it has contracted EPM from them, or even whether they're alive in the horse at the time of the test--and since these protozoa are so common in certain regions, western blotting is often only a preliminary, basic test. And horse owners should note that the test itself is, like most such tests, not 100 percent accurate, especially if the cerebrospinal fluid becomes contaminated with blood or other substances.

    Uses

    • Because of its limitations and the increasing availability of more versatile tests, only the negative results of a western-blot test are now used diagnostically with horses. That is, if testing finds no trace of the antibody in a horse's CSF, it's a good bet the horse has never been exposed to the EPM protozoa, notes the EPM Horse site. This is a more useful and reliable diagnosis than can be made from a positive result.