EPM in Horses - Eliminate Possum Muck
The Frank J. Milne State-of-the Art Lecture at the recent 2008 AAEP Conference was delivered by Dr. Stephen Reed of Rood and Riddle Equine Hospital in Kentucky. The lecture was entitled “Neurology is Not a Euphamism for Necropsy: A Review of Selected Neurological Diseases Affecting Horses.” The three major diseases that were reviewed were cervical stenotic myelopathy (Wobbler syndrome), equine herpesvirus (EHV-1) myeloencephalopathy, and equine protozoal myeloencephalitis (EPM).
EPM was first identified in horses in the 1970s, but it took more than twenty years before the causative agent, the protozoal species Sarcocystis neurona, was identified. Shortly thereafter, the definitive host of S. neurona was identified as the opossum (specifically the Virgina opossum (Didelphis virginiana) in North America). In the opossum, the parasite lives and reproduces in the animal’s intestine without causing much damage, and oocysts are passed in the feces. Horses, on the other hand, are aberrant or “dead end” hosts – if they ingest the parasite from the opossum feces, they cannot spread it to other animals of any species. However, in a horse, the parasite can leave the intestine and migrate through the animal’s tissues, ultimately reaches the brain and/or spinal cord. Damage to the nervous system caused by migration of the parasite. Although EPM has been identified in horses outside of the Americas, the vast majority of these cases are horses that were imported from or at one point traveled to the New World.
EPM commonly gets blamed for a lot of minor abnormalities in horses (mostly gait abnormalities), often without being properly diagnosed. Part of the problem is that exposure to the parasite in the New World is very common (>50% of horses in many areas), but definitive diagnosis of disease due to EPM can be very difficult. The classic sign of EPM is pronounced asymmetrical muscle wasting (note the profound muscle wasting of the left gluteal muscle in the picture) in different parts of the body. There are very few other diseases that result in this kind of condition, so a fairly confident diagnosis can be made in these cases. Horses with more subtle or less classic signs are more problematic.
- If a blood test is positive for antibodies to S. neurona, it only means the horse has been exposed to the parasite, not that it has EPM.
- If the blood test is negative it means the horse probably hasn’t been exposed to S. neurona, but there have been a few reports of EPM caused by protozoa from the Neospora genus as well, which throws another wrench in the works.
- We used to think that the way to diagnose EPM was to test the horse’s cerebrospinal fluid (CSF) for antibodies. The theory was that antibodies from the bloodstream could not get into the CSF because of something called the blood-brain barrier, so any antibodies found in the CSF would have to have been produced in response to parasites in the central nervous system itself. Unfortunately, it’s since been discovered that some antibodies from the bloodstream can leak into the CSF, even in a normal horse. However, analysis of CSF can still provide other information in some neurological horses, so CSF taps are still performed in some of these animals. It’s important to realize, though, that this procedure is also not without its risks.
The consensus statement on diagnosis of EPM published by the ACVIM in 2002 is available on the equIDblog Resources page. It is a few years old now, but still contains a lot of good information.
Watch for more information on EPM in an upcoming post!
Opossum photo credit: M. Anderson, 2007.
