Since my own horse had Equine Protozoal Myleoencephalitis, or EPM, a few years ago and I have participated in discussions on forums on the subject, I get a lot of email from people wanting to know about the diagnostic procedures. I have done quite a bit of reading on the subject, both on the Net and in periodicals such as The Whole Horse Journal.
It has been known for quite some time now that blood tests for EPM are non-conclusive, since so many horses (approximately 50% in the midwestern United States) have been exposed to the disease and will produce a positive result, even though they do not have the disease.
The definitive test for EPM is considered to be the Western Blot test or the spinal fluid test (CSF), which is where spinal fluid is tested for the antibodies that will be produced if the horse is actively infected by Sarcocystis neurona, the protozoal organism responsible for EPM. The antibodies will only be present in the case of an active infection.
The spinal tap involves inserting a long needle into the horse's spinal cavity and withdrawing a sample of the spinal fluid. This test can be performed at two different locations on the horse's spine. When Annapolis had his tap done, it was performed at the lumbosacral space, between the last lumbar and first sacral vertebrae of the spine, right under the horse's croup.
For this procedure he was given a local anesthetic and the site was shaved and scrubbed. While he was still standing a long needle (about 8 inches long and I shudder to even think about it) was inserted into the lumbar cistern and the fluid was withdrawn. The only evidence afterward was a small shaved rectangle on his croup with a stitch in it, which dissolved and came out after a few days.
The danger with this procedure is that the horse, since it is standing and aware of what is going on, may become startled and jump, possibly injuring himself or his handlers. Luckily, Annapolis stood quietly.
Once the fluid was collected, it was sent for analysis to a laboratory in Lexington, KY, which was gathering extensive information on EPM and was receiving fluid samples from all over the country. According to an article from the University of Missouri-Columbia, it was suggested that the samples be sent to this lab at the beginning of the week, so that they could be processed fresh as the sample deteriorate quickly, making diagnosis more difficult.
The other location from which spinal fluid may be drawn is the cisterna magna, a fluid-filled space behind the horse's brain. This location requires the horse to be given a general anesthetic and laid out, since any movement from the horse may mean a slip of the needle into the brain tissue and result in death. Given this danger, and the fact that the animal may thrash around an injure itself as the anesthetic takes effect, I am glad that the team at Texas A&M opted to go for the lumbosacral space in Annapolis' case.
Given the inherant dangers and the expense of the spinal tap, some owners decide to go ahead and treat the horse as if he has EPM anyway. But according to an article in the Whole Horse Journal, this can have serious consequences. For example, if the horse appears to get better with the treatment, and has not had a spinal fluid test, the owner may decide the horse doesn't really have EPM at all and stop the treatment before it has had time to be completely effective. Alternatively, if the horse does not have EPM, the owner is spending a lot of money on a treatment which will not cure the problem the horse actually has. EPM treatment is extremely expensive and to lay out that kind of money before confirming the horse has the disease is very costly.
As The Whole Horse Journal point out in their article, there are a number of diseases and injuries which may produce similar symptoms to EPM.
As public awareness of EPM has grown, veterinarians are seeing more and more suspected cases each year. However, it only makes sense to consider all the possibilities and use all the means available to make a positive diagnosis before proceeding with a treament plan.