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The short version: EPM (equine protozoal myeloencephalitis) is a neurologic disease caused mainly by the parasite Sarcocystis neurona, which horses pick up from feed or water contaminated with opossum feces. Exposure is common and disease is rare: depending on region, 15% to 89% of US horses carry antibodies, yet the national estimate is only about 14 new cases per 10,000 horses a year. The hallmark signs are uneven (asymmetric) incoordination and muscle wasting on one side. Diagnosis rests on a serum-to-spinal-fluid antibody ratio, not a blood test alone. Three FDA-approved drugs (ponazuril, diclazuril and sulfadiazine/pyrimethamine) succeed in about 60% to 67% of horses, roughly 10% to 20% relapse within one to three years, and only 10% to 20% return to full athletic work. Below: how horses get it, the signs and look-alikes, testing, every approved treatment with its numbers, recovery odds, prevention, and the plain truth that hemp has no role in treating it.
What EPM Actually Is
A parasite that wanders into the brain and spinal cord of a host it was never meant for.
EPM stands for equine protozoal myeloencephalitis: inflammation of the spinal cord (myelo-) and brain (encephalitis) caused by a single-celled parasite (protozoan). The 2016 consensus statement of the American College of Veterinary Internal Medicine (ACVIM), written by eight of the leading EPM researchers in North America, is the reference vets use. It states that EPM can be caused by Sarcocystis neurona or Neospora hughesi, but that the majority of cases are due to S. neurona (Reed et al., Journal of Veterinary Internal Medicine 2016).
The horse is a dead-end host. S. neurona normally cycles between the opossum and small mammals. The opossum (Didelphis virginiana) is the definitive host in North America: the parasite reproduces in its gut and it sheds infective sporocysts in its feces. Skunks, raccoons, armadillos and cats act as intermediate hosts, carrying cysts in their muscle that infect the next opossum that eats them. A horse that swallows sporocysts in contaminated feed or water becomes infected, but the parasite cannot complete its cycle in a horse. In the few horses whose immune system does not clear it, it reaches the central nervous system and causes damage wherever it settles.
That last point explains why EPM is so unpredictable. The consensus statement notes that signs vary because the parasite can damage both the grey and the white matter at multiple sites in the brain and cord. Wherever it lands decides what you see. Neospora hughesi is the less common cause, and its full life cycle is still unknown, although several studies show it can pass from mare to foal across the placenta.
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EPM by the Numbers
| Figure | What it measures | Source |
|---|---|---|
| 15% to 89% | Share of US horses with antibodies to S. neurona (exposure), depending on region. | Reed et al. 2016 (ACVIM) |
| 14 ± 6 per 10,000 | Estimated yearly incidence of clinical EPM in US horses 6 months and older (national NAHMS survey). | Reed et al. 2016 |
| Under 1% | Share of exposed horses that go on to develop clinical disease. | Helber et al. 2026 |
| 6× in fall, 3× in spring and summer | EPM risk compared with winter, the lowest-risk season. | Reed et al. 2016 |
| 2.5× | Higher risk on premises with opossums, and on premises with a previous EPM case. | Reed et al. 2016 |
| About one third lower | Reduction in EPM risk when wildlife was kept away from feed. | Reed et al. 2016 |
| 88–93% / 83–100% | Sensitivity / specificity of the serum:CSF titer ratio (SAG2, 4/3 ELISA), the most accurate test. | Bedenice & Johnson 2022 |
| 60%, 67%, 61.5% | Success rates in FDA field trials for ponazuril, diclazuril and sulfadiazine/pyrimethamine. | Bedenice & Johnson 2022 |
| 10 times | How much more likely EPM horses treated with an anticoccidial drug were to improve than untreated horses. | Reed et al. 2016 |
| 10% to 20% | Successfully treated horses estimated to relapse within 1 to 3 years, and the share expected to return fully to athletic work. | Bedenice & Johnson 2022 |
Sources: Reed et al. 2016, ACVIM consensus · Bedenice & Johnson 2022 · Helber et al. 2026
"Since EPM occurs only in a small percentage of horses infected with S. neurona, it is extremely important that an EPM diagnosis is not merely based on serum antibody testing."
Bedenice & Johnson, Animal Frontiers, 2022. With up to 89% of horses exposed in some regions, a positive blood test on its own says very little.Which Horses Get EPM
Any horse can. Some situations stack the odds.
EPM usually appears in a single horse on a farm, although small clusters do happen. The risk factors in the consensus statement are worth knowing because several are under your control:
- Age. Young horses (1 to 5 years) and older horses (over 13) had higher risk in one retrospective study. In a post-mortem survey from 10 diagnostic centers, 61.8% of EPM cases were in horses 4 years old or younger.
- Season. Risk was lowest in winter, three times higher in spring and summer, and six times higher in the fall.
- Opossums and wooded land. Opossums on the property raised risk 2.5-fold and wooded areas 2-fold. A previous EPM case on the same premises also raised it 2.5-fold.
- Stress. Heavy exercise, transport, injury, surgery and foaling have all been linked to higher risk, probably through temporary immune suppression. Racehorses and show horses had higher risk than breeding and pleasure horses. If your horse travels a lot, our guide to trailering and travel stress covers what the research shows about the trip itself.
- Breed. Thoroughbreds, Standardbreds and Quarter Horses show up most in case series. A 2022 review cautions that this probably reflects how many of those horses there are and how they are used, not a true breed weakness.
Two factors lowered risk in the same study: keeping wildlife away from feed (about one third lower) and having a creek or river as the water source (about half). The second one is surprising and is probably about horses not drinking from troughs and buckets that wildlife can reach.
The Signs of EPM
Uneven, often subtle at first, and easy to mistake for lameness.
The ACVIM statement is blunt: there are no signs that are unique to EPM. But it also names the two that should put EPM at the top of your vet's list, an asymmetric gait and focal muscle atrophy. Signs can come on suddenly or creep in over weeks, and occasionally they settle down only to come back days or weeks later.
| Sign | What it can look like | Why it happens |
|---|---|---|
| Asymmetric ataxia (incoordination) | Stumbling, tripping, one hind leg swinging wide, crossing over or stepping on itself, worse on one side. | Damage to the spinal cord tracts that tell the brain where the limbs are. |
| Focal muscle atrophy | Wasting of one hindquarter, along one side of the topline, the shoulder or the face. | The parasite has damaged the grey matter that supplies those muscles. |
| Weakness | Toe-dragging, sinking when you pull the tail to one side, difficulty backing up or on hills. | Motor pathways in the cord are affected. |
| "Lameness" that doesn't block out | An odd gait abnormality that no nerve block or joint injection fixes. | The problem is neurologic, not in the leg. |
| Problems under saddle | Uneven stride, trouble with leads or lead changes, a horse that suddenly feels unbalanced. | Often the earliest signs a rider notices. |
| Head tilt, facial paralysis | Drooping ear, eyelid or lip on one side, leaning or falling to one side. | Brainstem and cranial nerve involvement. |
| Difficulty swallowing | Dropping feed, food or water coming back down the nose. | Brainstem involvement; can be the first sign. |
| Bright, alert, no fever | The horse looks well apart from how it moves. | EPM horses are typically not painful and rarely feverish, which helps separate EPM from some look-alikes. |
Severely affected horses can have trouble standing, walking or swallowing, and the consensus statement warns the disease can progress very rapidly. A horse that is wobbly is also a safety risk to you. Do not ride a horse you suspect is neurologic until your vet has examined it.
What Can Mimic EPM
This is the question to keep asking, because so much looks like it.
| Look-alike | What usually sets it apart |
|---|---|
| Wobbler syndrome (CVSM) | Signs are usually symmetric and worse in the hind limbs; focal muscle wasting is uncommon. Neck X-rays help. |
| Equine herpesvirus-1 (EHV-1) | Often a fever shortly before or at onset, a history of respiratory disease or abortions on the farm, symmetric hind-end weakness, bladder and tail problems. Unlike EPM, it is contagious. |
| Equine motor neuron disease | Weakness and muscle loss rather than incoordination; your vet may check vitamin E levels. |
| West Nile virus | A mosquito-borne virus; your vet can test for it. |
| Lyme neuroborreliosis | Tick exposure; can cause neurologic signs that overlap. See our guide to Lyme disease in horses. |
| Trauma, tumors, abscesses, toxins | History of a fall, other systemic signs, or exposure to plants such as creeping indigo. |
| Ordinary lameness | Improves with a nerve block; a neurologic exam is normal. |
Differential list summarised from the ACVIM consensus statement, which also names equine degenerative myeloencephalopathy, rabies, migrating parasites, lead poisoning and others.
How Vets Test for EPM
A blood test can rule EPM out. Only blood and spinal fluid together can rule it in.
The ACVIM statement sets out three steps, and a presumptive diagnosis is strongest when all three are met:
- A full neurologic exam that confirms signs consistent with EPM and locates where in the nervous system the problem is.
- Ruling out other causes, for example with neck X-rays, vitamin E levels or tests for other infections.
- Testing serum and cerebrospinal fluid (CSF) together to show the horse is making antibodies inside its nervous system (intrathecal antibody production), not just carrying them in its blood.
Why a blood test alone is not enough. Because so many healthy horses have been exposed, a positive serum test has what the consensus statement calls a low positive predictive value. A negative serum test is far more useful: it usually means the horse has not been infected and your vet should look elsewhere. The one catch is a very recent infection, so if signs have only just started, the statement recommends repeating the blood test in 10 to 14 days.
Why the serum:CSF ratio works. Normally the blood-brain barrier keeps antibody levels in spinal fluid far lower than in blood, at a fairly constant ratio of more than 100 to 1. When the parasite is in the nervous system, the horse makes antibodies there and the ratio drops. Using a cut-off of 100 or below, the SAG2, 4/3 ELISA serum:CSF ratio showed a sensitivity of 88% to 93% and a specificity of 83% to 100% in two independent studies, the best of any test (Bedenice & Johnson 2022). For N. hughesi the equivalent test is the NhSAG1 ELISA ratio. When a result is borderline or the blood-brain barrier may be damaged, vets use a refined calculation called the antibody index (AI) or C-value.
Where the fluid is taken matters. A 2026 study of 27 horses found that in EPM-affected horses, fluid taken from the lumbosacral site (near the hips) had higher antibody titers and a lower serum:CSF ratio than fluid taken near the head, which could change how a borderline result is read. The antibody index was less affected by the collection site (Furr, Journal of Veterinary Internal Medicine 2026). It is a fair question to ask your vet which test and which site they plan to use.
FDA-Approved Treatments, Side by Side
Three drugs, broadly similar results. The choice is usually about practicality and your horse.
The ACVIM statement recommends that every EPM horse be treated with one of the FDA-approved antiprotozoal drugs, plus supportive care matched to how sick the horse is. Treated horses in one study were 10 times more likely to improve than untreated horses.
| Drug (brand) | How it is given | Labeled course | FDA field-trial success | Main cautions |
|---|---|---|---|---|
| Ponazuril (Marquis) | Oral paste. Loading dose of 15 mg/kg on day 1, then 5 mg/kg daily. | 28 days | 60% (28 of 47) | No adverse effects noted in the trial. The loading dose reaches steady blood and CSF levels by day 2. Half a cup of vegetable oil may raise absorption by up to 15% (unpublished data). |
| Diclazuril (Protazil) | Alfalfa-based pellets top-dressed on feed at 1.0 mg/kg daily. | 28 days | 67% (28 of 42) | No important adverse reactions reported. No loading dose needed; oil does not improve absorption. |
| Sulfadiazine + pyrimethamine (ReBalance) | Given by mouth: sulfadiazine 20 mg/kg + pyrimethamine 1 mg/kg daily. No hay for 2 hours before or after a dose. | Minimum 90 days | 61.5% (16 of 26) | Can cause mild anemia, low white cells and platelets, poor appetite, hives and diarrhea. Pyrimethamine causes birth defects; a fatal syndrome was seen in foals of mares treated late in pregnancy. Adding folic acid is not recommended. |
Doses and trial figures: Bedenice & Johnson 2022. Steady-state, absorption, toxicity and the folic acid warning: Reed et al. 2016. "Success" in these trials meant improving at least one neurologic grade (two for ReBalance) or the spinal fluid turning antibody-negative, not a full cure.
How long treatment really lasts. The two newer drugs are labeled for 28 days, but the consensus statement says most horses are treated 6 to 8 weeks or longer, as long as they are still improving. The decision to stop should be based on the neurologic exam, and the statement specifically does not recommend re-testing antibodies to decide when to stop. In practice vets also often use ponazuril at higher doses or for longer than the label, depending on the response.
Supportive treatment. For moderately or severely affected horses, vets frequently add a non-steroidal anti-inflammatory such as flunixin meglumine for the first 3 to 7 days of antiprotozoal treatment, to stop the horse getting worse while the drug takes hold (our guide to Banamine for horses covers that drug). Horses at risk of going down may get a short course of corticosteroids and DMSO. Natural vitamin E, typically 10 to 20 IU/kg a day, is widely used as an antioxidant for the damaged nervous system; the consensus statement is honest that its benefit has not yet been shown experimentally. Immune-stimulating products are used anecdotally by some vets. One warning from the 2022 review: levamisole can be metabolised to aminorex, a stimulant that is banned in performance horses, so tell your vet if your horse competes.
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Recovery Odds and Relapse
Most treated horses get better. Fewer get completely back to where they were.
Here is the realistic picture from a 2022 review of neurologic disease in sport horses (Bedenice & Johnson, Animal Frontiers 2022):
- About 60% of EPM horses improve by at least one neurologic grade with treatment, whichever drug is used.
- Only 10% to 20% return completely to normal athletic performance.
- 10% to 20% of successfully treated horses are estimated to relapse within 1 to 3 years after treatment stops.
- Horses with mild (grade 1) signs may do considerably better, and early recognition and treatment likely gives the best outcome.
Why relapses happen. The drugs stop the parasite multiplying but may not wipe it out. In a mouse model, mice treated with diclazuril stayed normal while on the drug, but all developed neurologic signs within 60 days of stopping, and live parasites were recovered from their brains (Hay et al., Journal of Parasitology 2019). In horses, a 2026 multi-university study cites earlier work in which S. neurona was confirmed in the nervous system of 30 of 53 (56%) EPM horses that had already been treated, and in its own small group 4 of 9 horses had relapsed at least once (Helber et al., Frontiers in Veterinary Science 2026). The same study found more degenerative damage in horses that had had signs for more than six months, one more reason not to wait.
If signs come back, the consensus statement's advice is simple: have the horse reassessed. A relapse, a new infection and a completely different problem can all look the same.
Can a horse with EPM be ridden again? Many can, especially for pleasure riding, once your vet is satisfied the horse is safe. A horse with remaining incoordination is more likely to stumble or fall, which puts the rider at risk too. Let the neurologic exam, not the calendar, decide when riding starts again, and build back slowly.
Preventing EPM
There is no vaccine. Prevention is about opossums, feed and water.
A killed S. neurona vaccine was tested in 70 horses in a challenge study and failed to prevent neurologic signs; in the long-term arm, the vaccinated horses' signs were, if anything, slightly worse, although that difference was not statistically significant (Saville et al., Veterinary Parasitology 2017). The ACVIM statement's prevention advice centres on cutting exposure to opossum feces:
- Store feed in sealed containers. Metal bins with tight lids for grain, pellets and supplements. Close feed-room doors at night.
- Clean up spills daily. Dropped grain draws opossums into barns.
- Do not feed off the ground, and keep hay stored where wildlife cannot nest or walk on it.
- Provide clean, separate water and scrub buckets and troughs regularly.
- Remove what attracts opossums: pet food left out, open garbage, fallen fruit, and easy shelter under buildings. Talk to your local wildlife authority about humane removal if they are living in the barn.
- Manage stress around transport, hard training, surgery and foaling, which the research links to higher risk.
Some vets use preventive antiprotozoal dosing for high-risk horses. In experimental studies, daily or weekly ponazuril reduced but did not eliminate infection, and daily low-dose diclazuril reduced the rate at which foals became seropositive. The consensus statement says longer studies are needed before this becomes a standard protocol, so it is a conversation for your vet, not a do-it-yourself plan.
When to Call the Vet
Call the same day if your horse shows any new incoordination, stumbling, one-sided muscle wasting, a head tilt, a drooping face, or difficulty swallowing. Call immediately if your horse is struggling to stand, falling, or cannot swallow. Do not ride a horse you suspect is neurologic, and do not start a new drug, supplement or oil while you wait, so your vet can see the true picture.
What to Do If You Suspect EPM
- Stop riding and keep the horse safe. A small, level paddock or stall with good footing is safer than a large field or hills.
- Film it. Short videos of the horse walking straight, turning tightly and backing up help your vet, especially if signs come and go.
- Book a full neurologic exam, not just a blood test. Ask what look-alikes your vet wants to rule out.
- Ask about paired serum and CSF testing. A positive blood test alone only proves exposure.
- Start the prescribed antiprotozoal promptly and finish the course your vet sets, which is often longer than the 28-day label.
- Track progress with rechecks, and call your vet right away if signs return after treatment ends.
- Fix the source. Seal feed, clean spills, protect water and deal with opossums, for every horse on the property.
Where Hemp Fits — and Where It Doesn't
We sell hemp oil for horses. Here is the honest answer.
Hemp and CBD have no role in treating EPM. EPM is a parasite in the nervous system, and the only things shown to control it are the FDA-approved antiprotozoal drugs above. We searched the research literature for any study of CBD or hemp in horses with EPM and found none. If anyone tells you an oil, herb or supplement treats EPM, they are wrong, and waiting on one while the parasite does damage can cost your horse function it may not get back.
Here is what the equine hemp research actually covers, so you can see its limits. Our own equine formula was studied at Tarleton State University: a single serving lowered stereotypic stall behaviors such as pawing compared with a control oil (P = 0.03), and a dose study found no adverse effects at 0.3 or 0.6 mg/kg (Turner et al., Journal of Animal Science 2021). A separate randomised study of 20 horses with mild joint osteoarthritis found lower pain-scale scores when CBD was added to phenylbutazone (Interlandi et al., Frontiers in Veterinary Science 2024). Those studies were about behavior, tolerability and joints. None of them involved the nervous system or EPM.
So where does our oil fit? As part of a general daily routine for pleasure, rehab, retired and rescue horses, to support a calm, settled state and normal comfort. For a horse in active EPM treatment, do not add it without your vet's OK, and expect many vets to prefer you wait until treatment is finished so changes in the neurologic exam can be read clearly.
If you do use hemp later, with your vet's agreement:
- Know the safety signals. In one equine study, liver enzymes rose in 8 of 12 horses on CBD and returned to normal within 10 days (Yocom et al. 2022). A study in ponies flagged an increased insulin response, so take extra care with horses prone to metabolic syndrome (our write-up of that study). More in our horse safety guide.
- Competition horses: cannabinoids are prohibited under FEI, USEF and HISA rules, and no withdrawal time is published. Use hemp for pleasure, rehab, retired and rescue horses only.
- Check quality. Choose an oil with a published, batch-specific certificate of analysis. Ours are on our lab results page.
- Dose by weight and start low. One pump of our 5,000 mg oil is about 20 mg. The Tarleton studies used 0.3 to 0.6 mg/kg per serving. Full chart in our horse dosing guide.
| Your horse weighs | Pumps per serving (about 20 mg each) |
|---|---|
| About 500 lb (pony) | 4–7 pumps |
| About 1,000 lb | 7–13 pumps |
| About 1,500 lb (draft) | 10–20 pumps |
General daily-routine ranges following the 0.3–0.6 mg/kg range used in the Tarleton studies of our formula. Not a dose for EPM or any disease. A bottle holds about 250 pumps.
Frequently Asked Questions
What are the first signs of EPM in a horse?
Early EPM often looks like a vague lameness or a training problem: an uneven stride, stumbling or tripping, the hind feet interfering, trouble picking up or changing leads, or a horse that suddenly feels unbalanced under saddle. Other early signs are muscle wasting on one side only (often the hindquarters, topline or face), a head tilt, a drooping ear, lip or eyelid, or difficulty swallowing. The classic pattern is that the signs are uneven from one side to the other, and the horse is usually bright, alert, not painful and without a fever. Any of these is a reason to call your vet for a full neurologic exam.
Is EPM in horses curable?
Many horses recover well, but not every horse is cured. In FDA approval field trials, 60% to 67% of horses treated with ponazuril, diclazuril or sulfadiazine/pyrimethamine were judged treatment successes, and treated horses were about 10 times more likely to improve than untreated ones. A 2022 review estimates that only 10% to 20% of affected horses return completely to full athletic work, and that 10% to 20% of successfully treated horses relapse within one to three years. Mildly affected horses that are treated early do best.
What can mimic EPM in horses?
Almost any neurologic disease can look like EPM. The main look-alikes are cervical vertebral stenotic myelopathy (wobbler syndrome), which is usually symmetric and worse behind; equine herpesvirus-1 neurologic disease, which often follows a fever or respiratory illness and can cause bladder problems; equine motor neuron disease; equine degenerative myeloencephalopathy; West Nile virus; Lyme neuroborreliosis; trauma; and plain musculoskeletal lameness. That is why a positive blood test alone is never enough to diagnose EPM.
What is the best supplement for a horse with EPM?
No supplement treats EPM. The parasite has to be controlled with an FDA-approved antiprotozoal drug prescribed by your vet. Natural vitamin E, often 10 to 20 IU per kg of body weight per day, is the supplement vets most commonly add as an antioxidant during treatment, although the ACVIM consensus statement notes that its benefit has not been proven experimentally. Hemp and CBD have no role in treating EPM and have never been studied in horses with the disease.
How do horses get EPM?
Most EPM is caused by the parasite Sarcocystis neurona. Opossums are its definitive host and shed infective sporocysts in their feces. Horses become infected by eating feed or drinking water contaminated with opossum feces. Skunks, raccoons, armadillos and cats are intermediate hosts that keep the cycle going, but horses cannot catch it from them directly. A smaller share of cases is caused by Neospora hughesi, whose full life cycle is still unknown.
Is EPM contagious to other horses?
No. Sarcocystis neurona is not passed from horse to horse, so an EPM horse does not need to be isolated. EPM usually strikes one horse on a farm at a time. If several horses share the same contaminated feed or water source, though, more than one can be exposed, which is why a confirmed case is a good reason to review feed storage and wildlife access for the whole barn.
How is EPM diagnosed in horses?
Vets use three criteria from the ACVIM consensus statement: neurologic signs that fit EPM, ruling out other likely causes, and testing blood and spinal fluid together to show the antibodies are being made inside the nervous system. The serum-to-CSF antibody titer ratio is the most accurate test available; a 2022 review reports a sensitivity of 88% to 93% and a specificity of 83% to 100%. A blood test alone mainly helps rule EPM out: a negative result makes EPM unlikely, but a positive one only proves exposure.
What is the prognosis for a horse with EPM?
About 60% of treated horses improve by at least one neurologic grade, according to a 2022 review in Animal Frontiers. A smaller group, around 10% to 20%, returns fully to athletic work, and 10% to 20% of horses that respond relapse within one to three years after treatment stops. Horses with mild signs that are diagnosed and treated early have the best outlook, and many go back to pleasure riding even when they are not quite normal on a neurologic exam.
How much does EPM treatment cost?
There is no reliable published average, and costs vary widely by region and by case. The main cost drivers are the neurologic exam, a spinal tap with paired blood and CSF testing, any tests to rule out look-alike diseases, the antiprotozoal drug itself (the label course is 28 days, but most horses are treated six to eight weeks or longer), anti-inflammatory and vitamin E support, and recheck exams. Ask your vet for an estimate for the work-up and for a full course of treatment before you start.
Is there a vaccine for EPM in horses?
No vaccine has been shown to prevent EPM. A killed Sarcocystis neurona vaccine was tested in a 70-horse challenge study and failed to prevent neurologic signs; in the long-term arm the vaccinated horses were, if anything, slightly worse, although the difference was not statistically significant. The ACVIM consensus statement does not include vaccination in its prevention recommendations. Prevention today rests on keeping opossums away from feed and water.
How can I prevent EPM in my horse?
Keep opossum feces out of what your horse eats and drinks. Store grain and supplements in sealed metal bins, clean up spilled feed every day, do not feed off the ground, block wildlife access to stalls, feed rooms and hay storage, and provide clean, separate water. In one risk-factor study, keeping wildlife away from feed cut EPM risk by about a third. Reducing stress around transport, hard training and surgery also matters, because stressful events are linked to higher risk. Some vets use intermittent preventive antiprotozoal dosing for high-risk horses, but the consensus statement says more research is needed before it becomes a standard protocol.
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The Short Version
EPM is a parasite infection of the brain and spinal cord that horses get from opossum feces in feed or water. Most horses are exposed, very few get sick, and the ones that do usually show uneven incoordination and one-sided muscle wasting. Diagnosis needs a neurologic exam and paired blood and spinal-fluid testing, not a blood test alone. Treatment is one of three FDA-approved antiprotozoal drugs, usually for longer than the 28-day label; about 60% to 67% of horses improve, and 10% to 20% relapse. Prevention means sealed feed, clean water and no opossums in the barn. Hemp has no role in treating EPM.
Lyme Disease in Horses · Banamine for Horses · Colic in Horses · Laminitis in Horses · CBD for Senior Horses · Is CBD Safe for Horses? · CBD Dosage for Horses
Marc co-founded HempMy Pet in 2016 with his business partner Natalie Mondine, the company's COO. HempMy Pet breeds its own hemp genetics, grows them on its own Colorado farm, and formulates every batch in-house under human-grade standards — seed to sale, nothing sourced overseas. Every batch is third-party tested and the certificates of analysis are published. More about us.
Our own 5,000mg equine formula was the oil studied in a research program at Tarleton State University by Dr. Kimberly Guay, Ph.D., with Trinette Jones, Ph.D. and William B. Smith, Ph.D. In a single-dose study published in the Journal of Animal Science (2021), stereotypic stall behaviors were lower on CBD than on the control oil (P = 0.03), and a dose study recorded no adverse effects at 0.3 or 0.6 mg/kg. The transport-stress arm found no significant change in cortisol, and we report that too. Read what was measured.
Our canine research: HempMy Pet also ran a 90-day study of 37 dogs with chronic joint discomfort, overseen by Dr. Robin Downing, DVM, MS, with Lori Kogan, Ph.D. and Peter W. Hellyer, DVM of Colorado State University. Of the 32 dogs that completed it, 30 (93%) showed a benefit and 94% of owners reported improved quality of life. In 2005 Dr. Downing became only the third veterinarian in the world credentialed as a Diplomate of the American Academy of Pain Management; she founded the International Veterinary Academy of Pain Management and was the AVMA's 2020 Bustad Companion Animal Veterinarian of the Year. See our clinical trials.
This article is general information for horse owners, not veterinary advice. It is reviewed and updated as new peer-reviewed evidence is published, and where the evidence is thin or negative we say so.
How We Sourced This Article
Every figure above comes from one of these. Where we could read the full paper we did; where only the abstract is public we used the abstract.
- Reed SM, Furr M, Howe DK, Johnson AL, MacKay RJ, Morrow JK, Pusterla N, Witonsky S. Equine Protozoal Myeloencephalitis: An Updated Consensus Statement with a Focus on Parasite Biology, Diagnosis, Treatment, and Prevention. Journal of Veterinary Internal Medicine, 2016. PMC4913613 · doi 10.1111/jvim.13834. Supports: causes and life cycle, seroprevalence 15–89%, incidence 14 per 10,000, risk factors and seasonality, signs, look-alikes, the three diagnostic criteria, serum and CSF testing, the three FDA-approved drugs, 28-day label vs 6–8 week practice, supportive care, the 10-times improvement figure, pyrimethamine warnings and prevention.
- Bedenice D, Johnson AL. Neurologic conditions in the sport horse. Animal Frontiers, 2022. PMC9197298 · doi 10.1093/af/vfac036. Supports: early signs under saddle, not contagious, serum:CSF sensitivity and specificity, drug doses and FDA field-trial success rates, levamisole and aminorex, prognosis and the 10–20% relapse estimate.
- Helber L, Wagner B, Leeth CM et al. Persistence of Sarcocystis neurona and histopathology in horses with equine protozoal myeloencephalitis. Frontiers in Veterinary Science, 2026. PMC13124509. Supports: under 1% of exposed horses develop disease, parasite confirmed in 30 of 53 previously treated horses (citing Boy et al.), 4 of 9 relapsed, more damage in chronic cases.
- Hay AN, Witonsky SG, Lindsay DS et al. Sarcocystis neurona-Induced Myeloencephalitis Relapse Following Anticoccidial Treatment. Journal of Parasitology, 2019. doi 10.1645/18-193. Supports: relapse within 60 days of stopping diclazuril in a mouse model.
- Saville WJA, Dubey JP, Marsh AE et al. Testing the Sarcocystis neurona vaccine using an equine protozoal myeloencephalitis challenge model. Veterinary Parasitology, 2017. doi 10.1016/j.vetpar.2017.09.012. Supports: the vaccine failed to prevent neurologic signs in 70 horses.
- Furr MO. Influence of collection site on cerebrospinal fluid test results in horses with equine protozoal myeloencephalitis. Journal of Veterinary Internal Medicine, 2026. doi 10.1093/jvimsj/aalag135. Supports: CSF collection site changes titers and ratios; antibody index less affected.
- Turner PL, Guay KA, Jones T et al. The Effects of Feeding Cannabidiol Oil on Behavior of Stalled Horses. Journal of Animal Science, 2021. doi 10.1093/jas/skab235.008. Supports: the Tarleton behavior finding on our formula.
- Interlandi C, Tabbì M, Di Pietro S et al. Improved quality of life and pain relief in mature horses with osteoarthritis after oral transmucosal cannabidiol oil administration. Frontiers in Veterinary Science, 2024. PMC10876772. Supports: the joint osteoarthritis finding.
- Yocom AF, O'Fallon ES, Gustafson DL, Contino EK. Pharmacokinetics, safety and synovial fluid concentrations of oral CBD in horses. Journal of Equine Veterinary Science, 2022. PubMed 35307550. Supports: liver enzymes in 8 of 12, normal within 10 days.
Disclosure: HempMy Pet funded the Tarleton State research on our equine formula and sells the product linked on this page. We had no involvement in any other study cited above. Brand names of EPM drugs are given only so owners can recognise them. Spotted an error? Email support@hempmypet.com and we will correct it.
These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. This article is general information, not veterinary advice. EPM must be diagnosed and treated by a veterinarian. Consult your veterinarian before use, especially if your horse is pregnant, nursing, competing, under treatment, or taking medication.
