Protozoal Infection in Horses
The life cycle of Sarcocystis neurona (the protozoa causing EPM) is dependent upon the opossum. Natural occurrence of the disease, therefore, is limited to areas harboring this animal (i.e North and South America). Opossums are especially prevalent in the southeastern United States: Alabama, Florida, Georgia, North Carolina, South Carolina and Tennessee. A horse becomes infected by ingesting the protozoa in feed, hay, pasture and/or water contaminated with opossum feces.
Subsequent to ingestion by the horse, the protozoa travel through the digestive tract and are absorbed into the bloodstream. Once in the bloodstream, one of two things happens to the protozoa:
- The horse elicits an immune response which clears the protozoa and prevents development of the disease. These horses have been “exposed” to the protozoa but do not develop neurologic signs.
- The protozoa cross the blood-brain barrier and enter the central nervous system (CNS). Once in the CNS, the protozoa live and reproduce intracellularly (within cells), thereby protecting themselves from the horse’s immune system. At some point (possibly during times of stress, illness or immunocompromise), the protozoa break out of the host cells (the latter of which are killed in the process). Once free, the protozoa proliferate within the CNS until they enter new cells which are destined to undergo similar demise.
The Role of Inflammation
Cellular death within the CNS results in inflammation of nervous tissue (known as neuritis). Neuritis is considered to be a major component of EPM. In fact, the horse’s inflammation associated with the infection (as opposed to the infection itself) may be responsible for the preponderance of clinical signs that we observe in affected horses.
It is our intention through SAG titer analysis (blood work) to measure this inflammatory reaction, thereby assessing the horse’s response to potential protozoal infection.
Moreover, a crucial part of successful EPM treatment entails addressing the neuritis that occurs as a consequence of infection (in addition to the infection itself). Between protozoal infection and its associated inflammation, successful treatment of neural inflammation may actually have the larger impact on alleviating clinical signs and resolving the disease.
Rates of Exposure
Historically, equine protozoal myeloencephalitis (EPM) was thought to affect only a small percentage (2%) of horses residing in the United States. Currently, approximately 11% of US horses test positive for SnSAG1 exposure. Obviously, the percentage of exposed horses varies greatly among geographic locations.
At The ATLANTA EQUINE Clinic, horses testing positive for one or more of the SAG1, SAG5 and SAG6 phenotypes are in excess of 50%. This suggests that horses in the southeast are at far greater risk for exposure as compared to horses in some other US regions.
We owe the increased rate of southeastern exposure to the local abundance of intermediate and definitive hosts for the protozoa.
As many of our clients have surmised, exposure rates can also vary between barns within the same geographic locality. Some barns approach an exposure rate of 100%, whereas other nearby barns house very few affected animals. The difference in exposure rates between local barns presumably correlates with the disparity between opossum populations at those locations. Of course, not all opossums are infected with the protozoa.
Should You Treat Your Horse for EPM?
In an attempt to answer this question, we should consider the following:
- The vast majority of horses living in the southeast have been exposed to Sarcocystis Neurona.
- It is not exposure to Sarcocystis Neurona that generally dictates the horse’s symptoms, but rather the horse’s response to exposure.
- The majority of horses tested by The ATLANTA EQUINE Clinic exhibited elevated SAG titers.
- SAG titer analysis does not measure the degree of protozoal exposure or infection, but rather the degree of inflammation associated with exposure/infection.
- The majority of horses exhibiting elevated SAG titers were treated with Orogin, which is a combination of decoquinate (an antiprotozoal drug) and levamisole (a dewormer and antiinflammatory drug).
- The vast majority of horses treated with Orogin responded positively (i.e. resolution of clinical signs and improved performance).
If your horse has EPM it may be affecting his/her health and performance. Assessing SAG titers (which costs less than $90) will tell you if your horse is currently eliciting a response to protozoal exposure. If the results of this test are suggestive of EPM infection, treatment (costing less than $400) is likely to improve the health and performance of your horse and is therefore probably worth considering.