Time is of the essence to improve chances of recovery
While horse owners have likely heard of Equine Protozoal Myeloencephalitis (EPM), fortunately, less than one percent of the horse population will develop the disease. The impact of this neurologic disease can be devastating, though, so it is important that horse owners know how to identify a potential case of EPM, how the diagnosis is made, the prognosis for recovery and how to manage the environment to help prevent EPM.
How Horses Contract EPM
EPM is most commonly caused by the protozoal parasite Sarcocystis neurona, which is transmitted to horses when they ingest food or water that has been contaminated with opossum feces. A less prevalent way horses can contract the disease is through the protozoal parasite Neospora hughesi. Very little is known about how this parasite is transmitted to horses.
What EPM Looks Like
Dubbed “the great pretender,” EPM presents in a variety of ways in horses. According to Amy Polkes, DVM, DACVIM, an equine internal medicine specialist and owner of the consultation service Equine IMED, clinical signs can range from mild to severe. It can be anywhere from a change in performance under saddle with weakness, tripping, and/or muscle loss to a horse that is acutely recumbent (down).
Other clinical signs a horse owner might observe include, but are not limited to:
- Muscle atrophy, most often asymmetrical
- Ataxia (lack of coordination or abnormal gait, often more pronounced in the rear hind limbs)
- Tripping, toe dragging, locking stifles, difficulty picking up correct lead, “bunny hopping” with hind legs
Diagnosing EPM
According to Dr. Polkes, if a horse owner notices any of these signs, it is important to have a veterinarian evaluate the horse as EPM can be progressive and early treatment is important for success. If there is a suspicion of neurologic disease, Dr. Polkes says, “It is very important to have a thorough neurologic evaluation by someone with advanced training, such as an internal medicine specialist or a veterinarian proficient in this area.”
During a neurologic evaluation, the veterinarian will assess proper nerve function and movement. The exam starts with observation of general behavior and body condition, with attention to any observed muscle atrophy. Next, a cranial nerve exam is performed to assess the function of the nerves around the head and neck, including vision, eye movement, tongue tone and specific responses to stimuli. During the active portion of the exam, the veterinarian will observe the horse walking in straight lines, circles and over different surfaces, checking for gait abnormalities such as toe dragging, stepping on themselves or losing their balance. Strength and coordination can be assessed with the “tail pull” exam by pulling the horse’s tail while they are walking forward. Proprioceptive deficits are common in horses with neurologic disease; they often are unaware of proper foot placement and may adopt a base-wide or base-narrow stance or leave a leg in a very abnormal and unnatural position. Dr. Polkes states, “A neurologic evaluation is the most important step in the diagnosis, as there must be neurologic deficits present for the diagnosis to be EPM.”
Diagnostic testing includes serum and cerebrospinal fluid (CSF) for antibodies to Sarcocystis neurona (or Neospora hughesi). A CSF tap can sometimes be done at the barn, or the veterinarian may want the horse transported to a clinic. The procedure is well tolerated with few complications and can provide important information for a more definitive diagnosis.
The presence of antibodies in serum only confirms exposure, but not necessarily active disease, as there are some areas of the country with high exposure, and those horses are likely to be positive on serum. The level of the serum antibody also does not determine the likelihood of disease, a positive test at any level only confirms exposure but a negative test makes it unlikely to be EPM. The most accurate test for active infection is to submit both cerebrospinal fluid (CSF) and serum together. The serum and CSF are both tested for antibodies, and a ratio is performed to determine if there is evidence of antibody production in the spinal fluid.
Diagnostic testing and treatment should only be done in horses with neurologic deficits and clinical signs consistent with EPM. Testing for EPM should not be part of an annual health check-up or pre-purchase examination.
Prognosis for Recovery
EPM can be treated with MARQUIS® (15% w/w ponazuril), which is administered as a paste. In general, about 66 percent of horses will either recover completely or well enough to be a pasture pet and about 33 percent will not respond well to treatment or have permanent issues from protozoal damage.
“I have had some great successes with treatment,” says Dr. Polkes. “A recent case responded so well to treatment with MARQUIS that he was back in the show ring within six months!”
Dr. Polkes says there are things horse owners can do to help manage the opossum population around their farms, such as keeping food sealed and stored properly, not leaving cat or dog food in places accessible to opossums, securing and disposing of waste in a timely manner and eliminating clutter.
For more information about EPM, click here.
About Boehringer Ingelheim
Boehringer Ingelheim provides innovation for preventing and treating diseases in animals. The company offers a wide range of vaccines, parasite-control products, and medicines for pets, horses, and livestock to veterinarians, animal owners, farmers, and governments. As a leader in animal health, Boehringer Ingelheim values that the health of humans and animals is deeply connected and strives to make a difference for people, animals, and society. Learn more at www.bi-animalhealth.com
IMPORTANT SAFETY INFORMATION: The safe use of MARQUIS in horses used for breeding purposes, during pregnancy, or in lactating mares has not been evaluated. In animal safety studies, loose feces, sporadic inappetence, lost weight, and moderate edema in the uterine epithelium were observed. For use in animals only. Not for human use. Keep out of reach of children.
MARQUIS® is a registered trademark of Boehringer Ingelheim Animal Health USA Inc. ©2024 Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All rights reserved. US-EQU-0263-2024
This content originally appeared on Boehringer Ingelheim Animal Health and is published here with permission.
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