EPM testing: what does a test measure?
There are no pre-mortem EPM tests! The lack of a test to diagnose EPM by detecting live S. neurona parasites in the central nervous system (CNS), the definition of EPM, is due to the disease process, the organisms are gone. The next best diagnostic evaluator of an EPM horse, because one can't detect the live organisms, is a comparison between known diseased horses and clinically normal horses, and then finding the measurable differences between them. That is what I tried to do during my PhD work, and twenty-two years ago everyone had similar ideas. The goal was to find the definitive marker for horses harboring live Sarcocystis neurona parasites in the CNS tissue. If you understand what a test measures, and what changes in an animal after infection and how infection differs from disease, you will understand the limits and differences between tests that all of us developed. None reached the Holy Grail of EPM testing! However, there are useful tools to help you, the greatest one is knowledge. Knowledge lets you select and interpret the most useful tests.
Organisms, infections, and disease
Organisms are composed of proteins and the genetic machinery that makes those proteins. Nature is lazy, evolutionary speaking, when Nature finds a good thing, it keeps it. Many examples of protein pathways that are present in ancient bacteria continue to be used by humans and horses. It is no surprise some proteins of S. neurona, the pathogenic protozoa that can cause EPM, are similar to all living organisms and some fungi. Closer to our topic of understanding tests, S. neurona shares proteins and pathways with Apicomplexan parasites. Apicomplexans are a taxonomic group, a phylum of diverse parasitic protozoans, that have a complex life cycle, usually involving both asexual and sexual generations, often in different hosts.
Protozoa infect hosts (horses) with several possible outcomes. An outcome can be that the host is unhospitable, and the organism dies, most often the case with S. neurona infecting horses. You won't find S. neurona in the muscle of a horse. An evolutionary truce may allow the parasite to infect the host, remaining quietly and unobtrusively conducting its parasite duties with little or no consequence to the host. This unobtrusive relationship is often the case with S. fayeri infections in horses, called equine muscular sarcocystosis or EMS. The often benign sarcocysts are found in the horses muscles. A more serious outcome is that the infection takes a hold and a battle with the host ensues. Horses are inhospitable to S. neurona, horses are aberrant hosts for this parasite and some of these infections result in EPM. And progressive consequences of infections with unknown organisms cause chronic inflammation and ultimately PNE, S. neurona can do that too.
| horse is natural host | % horses in US with antibodies | produce sarcocyst in horse | can invade CNS | produces EPM-like signs |
S. neurona | no | 80% | no | yes | yes |
S. fayeri | yes | >80% | yes | no | ye |