When EPM Strikes

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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Evaluating Pasture Quality for Horses

By Kentucky Equine Research Staff

In Arthur Conan Doyle’s The Hound of the Baskervilles, the renowned fictional detective Sherlock Holmes assesses the quality of a portrait by stating simply, “I know what is good when I see it.” For some horsemen, high-quality pasture may be viewed in the same way—difficult to describe but easy to identify.

What attributes separate high-quality pasture from middling or poor pasture? Some characteristics include species and growth stage of plants, density of desirable plants, weed population, and presence and extent of sacrificial areas. These barren areas are often found around feeders, waterers, or fencerows because of high traffic.

Pasture quality is often intrinsically linked to pasture care. Are soil tests conducted to inform owners how to fertilize? Are pastures mowed and dragged regularly? Are weed-control measures performed?

Kentucky Equine Research prompts horse owners to complete a detailed form when they request a ration evaluation. Every component of a horse’s diet must be depicted accurately. One question asks owners to describe their pastures in one of five ways: drylot, sparse, moderate, lush, or high in oxalates. By knowing the quality of pasture, especially the availability of forage, nutritionists can estimate potential intake levels and the nutrient contribution to the diet.

Drylot. A drylot is any area where horses are turned out and able to move around freely, but the area contains no forage. Drylots are often sand or gravel runs connected to a stall or turnout paddock. A drylot is ideal for horses that must consume carefully regulated diets such as easy keepers or those diagnosed with certain metabolic diseases. Horses with drylot turnout only will be entirely dependent on supplemental forage sources such as hay, chaff, forage cubes or pellets, or alternative fiber sources. If horses spend more than an hour or two in a drylot, forage should be provided to them.

Sparse pasture. Sparse pasture provides some forage for horses, though quality may be poor. The forage may be overmature, which can be determined by the presence of thick stems and seedheads, or it may be yellowing or brown, as in the case of drought-stricken plants. Sparse pasture may be due to overgrazing, which often leads to a flush of weed growth. Sacrificial or loafing areas may start small but spread wider, giving weeds a chance to take a foothold. Sparse pasture may also be due to inappropriate management, such as infrequent mowing, inadequate weed control, or lack of fertilization. This pasture will not support optimal dry matter intake per day but may be appropriate to meet the needs of easy keepers.

Moderate pasture. Moderate pastures may have a mix of grasses interspersed with weeds. Pasture will be unevenly grazed but will have minimal bare spots. Plant height will typically be inconsistent. Moderate pasture generally does not have consistent care with some mowing and minimal or no fertilization. Like sparse pasture, moderate pasture will not support the maximum dry matter intake per day.

Lush pasture. Lush cool-season pasture contains thick stands of grasses with few or no bare areas and even distribution of grazing across the pasture with minimal weed proliferation. Plants are kept at a height of 6-10 inches by regular mowing. This pasture will support maximum rate of dry matter intake. Cool-season grasses grow best during cool, moist periods of the year, such as in spring or autumn. They thrive in the temperature range of 59-77° F. Common cool season grasses include tall fescue, orchardgrass, ryegrass, Kentucky bluegrass, and bromegrass. Warm-season grasses grow best during warm periods of the year, flourishing when temperatures are in the range of 86-95° F. Bermudagrass and bahiagrass are typical warm-season grasses. When grasses are faced with temperatures outside of their ideal range, growth may slow considerably, decreasing yield.

Legumes such as clover, lespedeza, and alfalfa are most often used in pastures as components of mixes with grasses. Both grasses and legumes provide essential nutrients to horses, though legumes will convey more energy and protein as well as certain minerals.

The fifth category, high-oxalate pastures, pertains primarily to horse owners in tropical regions. Oxalates bind calcium in the gastrointestinal tract of the horse, thereby prohibiting its uptake into the bloodstream and its subsequent use throughout the body. Ingestion of these plants is typically not an issue unless they represent a large fraction of available forage or if oxalate-laden plants are more palatable than other species in the pasture landscape. Kikuyu, buffel, pangola, and green panic grasses all are potentially dangerous because of high oxalate levels.

Read more: Evaluating Pasture Quality for Horses