How was your horse diagnosed? (blood or cerebrospinal fluid? Western Blot or IFAT? Neck x-rays?) What symptoms are you seeing in your horse now and have the gotten progressively worse?
My concern is that with the history you've given above--that this has been ongoing for 2 years---it's more likely that you are dealing with something else and not EPM. EPM tends to get progressively worse in a shorter time period than that. When considering EPM as a possible cause of neurological deficits, it is very important to get an accurate diagnosis. There are many other diseases/causes that present with similar symptoms to EPM so a full physical examination and appropriate diagnostic testing including blood tests for exposure to S. Neurona, x-rays of the head/neck, testing for other diseases such as West Nile Virus, Eastern Equine Encephalytis, etc. With the symptoms you have described, I would consider arthritic changes in the neck, a chronic lameness from pain or possibly Lymes disease.
There are several treatment options including Marquis, Navigator, diclazuril and a combination of pyrimethamine and sulfadiazine marketed as Rebalance. However, Navigator is no long in production and Rebalance tends to be out of stock so vets and owners go through compounding pharmacies to have pyrimethamine and sulfadiazine compounded for use. Diclazuril has never been mass produces in a formula for treating EPM because the cost of doing so and the fact that an already effective treatment (Marquis) is on the market would likely lead to the same fate as that of Navigator---a product that doesn't sell enough to warrant production.
Marquis is labelled for use for 28 days and actually kills the parasite. This product is more expensive than the Rebalance but it requires a much shorter treatment time. Marquis also has a low incidence of adverse effects.
Rebalance (pyrimethamine/sufladiazine) prevents reproduction of the parasite and treatment must be continued for a minimum of 3 months to allow for the parasites that are present to all die without leaving new generations in the central nervous system of the horse. It is often recommended that this product be given for 6 months or more. Rebalance must be given on an empty stomach.
About 70% of horses recover with any of these treatments. Recovery is considered a 1 grade improvement of neurological deficits as graded on the Mayhew scale 1 month after treatment ends. However, horses who are recovering from EPM can improve for more than a year after treatment. But rehabilitation must be slow and cautious for the safety of the horse and the handlers.
Recovered horses also can have days when they appear to relapse and neurological symptoms reappear or worsen. This may actually be from a reinfection or it may be from an inability to compensate for neurological deficits left over from their initial infection. Horses will over time learn to compensate for neurological deficits and appear normal or at least more normal, but overwork, illness, injury, excessive heat and humidity or anything else which affects their energy level or concentration may lead to a reappearance or worsening of neurological deficits.
Other supportive care includes Banamine or DMSO to help with inflammation. 8,000-10,000 iu of Vitamin E daily, a good balanced diet, safe environment for horses with balance issues, free choice exercise.
Licensed Veterinary Technician