Neuromuscular diseases can be imperceptible and progress until the animal is unable to walk. Sometimes these diseases suddenly show up, seemingly out of nowhere. In any case, it is a scary scenario for the horse owner.
Equine protozoal myeloencephalitis, EPM, polyneuritis equi, PNE, equine muscular sarcocystis, EMS, Lyme disease, equine herpes virus, EHV-1, and rabies can have similar clinical signs and vary from acute to chronic.
The signs of these diseases can be weakness, unusual or atypical lameness, behavior changes, seizures, and recurring signs after EPM or Lyme treatment. The veterinarian will use a decision tree to figure out a diagnostic plan, often vets have decision trees that are a mental exercise, after they complete a neurologic examination. The exam incudes a history of when the signs were noticed and what treatments were used. The gait is often assessed at a walk and trot. Don't forget behavior changes, that is an important sign! There are some diagnostic aids, radiographs, ultrasound, complete blood count and chemistry analysis that help with a differential diagnosis. If PNE is suspected the veterinarian may run a urinanalysis.
The clinical history may reveal multiple treatments for EPM. Multiple EPM treatmens are consistent with relapsing disease due to polyneuritis equi and often, not EPM. Serologic testing is helpful to rule out Sarcocystis neurona and rule in PNE or EMS. We save your serum sample for six months in case additional testing is needed for a diagnosis, especially if the horse doesn't recover completely.
It is important to have a follow up examination a week to 10 days after initiating treatment. The horse should be improving. At this time antibody tests aren't repeated, the antibodies will take months to go down, sometimes up to 10 months. Antibody tests include the SAG's (1, 5, 6) S. fayeri anti-toxin, the Sidewinder (MP2, MPP), and Lyme. Useful tests are the C-reactive protein and neurofilament light (NfL). These tests measure inflammation and axon damage, respectively, and can be repeated in 10-30 days. These values will decrease with an appropriate therapy.
If there is anything to emphasize, it is communication! Communication with the veterinarian. We are always available for consulting and we are firm that a veterinarian must be involved with the case. We want you to use our tools in the case analysis to get your horse back to heath.