This disease is very common in North Carolina and is often referred to as the “Possum Disease” or EPM.
What causes EPM?
It is caused the protozoal parasite Sarcocystis neurona. The protozoa is carried by infected opossums and transmitted in the feces and ingested by the horses. From there the parasite migrates from the intestines and into the brain or spinal chord.
What are the clinical signs?
Signs may vary based on the number of parasites and where in the central nervous system they are attacking. Direct damage and killing of nerves cells versus swelling and inflammation around them will determine if the signs are permanent or simply temporary once the disease is treated. Signs may occur slow and progressively or seemingly overnight. They may also be mild or severe.
Common clinical signs include:
Abnormal gait or lameness
Muscle atrophy esp. of the hind quarters (usually unilateral)
Facial paralysis (usually unilateral)
Dysphagia (unable to chew or swallow)
How is EPM diagnosed?
The most definitive way to diagnose EPM is with testing of the cerebrospinal fluid. This method requires a spinal tap, usually under general anesthesia. It can be a risky procedure and fairly costly. Transportation to a surgical facility is also needed. I would not recommend this method of diagnostics to be performed in the field. Another method is through SAG antibody testing using a blood sample. This test is more accurate than the previously used blood test that resulted in a lot of false positives. The test is economical and generally takes anywhere from 2-7 days to get a result. I must point out that in many cases, waiting for test results to come back can simply take too long and a response to therapy is as definitive as you get.
How is it treated?
There is only drug currently available and approved for use in horses to treat this disease. The product name is Marquis (manufactured by Bayer company) with the drug name ponazuril. This method of treatment is shorter and carries less risk of drug induced side effects that the older form of treatment. The current recommendation is 1 or 2 rounds of therapy, each round consisting of 28 days of a weight appropriate amount of drug once a day.
This therapy is relatively costly and run anywhere from $850-$1000 per round of therapy. Additional supportive care including IV fluids, antibiotics, anti-inflammatories and hospitalization may add to the cost of therapy.
What is the prognosis?
If caught early and the clinical progression is not too severe or too rapid, then the treatment is very effective. Some horses can return to work with minimal permanent damage. However, these cases are often not caught in time to avert the really bad clinical signs and they often progress way too fast for therapy to be successful. The horses become so uncoordinated that they are a danger to themselves and everyone around them and euthanasia is elected in those horses along with the horses that become unable to stand or eat for long periods of time.
An example of EPM.
This horse started showing some mild signs on day 1 and 2, but progressed rapidly. The mare was completely unable to swallow and started showing signs of aspiration pneumonia on day 3 and was euthanized on day 4. The horse was taken to necropsy and diagnosis was confirmed at that time.