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Equine Protozoal Myeloencephalitis (EPM)

November 2017 - AAEP is on Stall Rest

AAEP is taking the month of November off from "Ask the Vet", but will be back in December to answer your equine health questions concerning winter weather care for your horse with expert, Dr. Christine Tuma.



Click here to read this month's questions and answers.
  1. It has been two years since I have ridden my Thoroughbred consistently. After six weeks of Marquis medication, I treated him herbally and he seemed to be fine. Then he had a relapse and I am treating him with Silver Linings herbs this time. I could not afford the cost of Marquis again and am hoping he will continue to improve. Do horses ever recover completely without future relapses? (View Answer)
    I'm sorry to hear your gelding has not responded to treatment for EPM. Unfortunately, some horses do not fully recover from the damage the protozoa cause within the nervous system. EPM, as you probably know, is caused by protozoal organisms Sarcocystis and Neospora, which are transmitted to horses through the feces of opposum, raccoon, and domestic cats. The organisms cannot complete their life-cycle in the horse but do create significant damage when they reach the brain and spinal cord. Sarcocytis and Neospora are classified as coccidia. Medications against coccidia, including Marquis and Protazuril, are able to slow the progression of the protozoa but do not kill all of the organism in a host. Horses under stress (heavy training, shipment, young or old age, or concurrent disease) are most at risk for contracting EPM. A stressed immune system has a difficult time fighting a coccidia infestation. The organisms populate inside the horses nerve cells of the brain and spinal cord causing permanent nerve damage and symptoms of lameness, unbalance, and odd behavior. Because the medications can only slow the organisms but cannot fully eradicate the organisms from the horses nervous system, a recurrence can occur if the horse experiences a new cause of stress. Multiple rounds or extended lengths of treatment with the coccidiostats may improve relapsing symptoms. Unfortunately, there are occasional patients in whom the nerve damage is too great and neurologic deficits remain the same or worsen with age and time. Safety of the horse and his handlers, as well as quality of life, becomes a consideration in severe or chronic cases. Be sure to contact your local veterinarian for the best diagnostic and treatment plan for your own horse. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  2. How can I help prevent a recurrence of EPM? Should I treat prophylactically every six months? (View Answer)
    Sarcocystis and Neospora, the organisms found to be responsible in EPM, tend to cause new damage when a horse undergoes a stressful period. Nutritional deficiency, illness, severe weather changes, or transport are all potential sources of stress and a compromised immune system in horses. Managing your horse's daily activities to create a consistent and solid stable routine will go a long way toward decreasing stress. Preventing carrier animals such as opposum, raccoons, and cats from defecating in the feed areas and hay mangers reduce the spread of the EPM organisms. Prophylactic treatment of a previously diagnosed EPM horse is controversial. Some studies have identified that occasional dosing of currently available medications may be useful in decreasing the organism numbers in a horse, while other studies find that a longer duration of initial treatment is more effective. Some clinicians recommend dosing medications during periods of stress. An Ohio State University handout reminds us that "intermittent treatment may increase the risk that parasites infecting a horse develop drug resistance. Therefore, we do not recommend intermittent or periodic treatments." I suggest discussing options with your local veterinarian to formulate the best treatment and preventative plan for your situation. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  3. When my horse was 3-years-old, she developed signs of EPM - sudden ataxi and a weird gait that lasted a few months. Although she recovered quickly and was not confirmed as EPM, she definitely had a neurological problem. For a few years after, she stumbled at times. Now at 11-years-old, she has almost no problems. Does EPM go completely away with time or is it possible she had something else going on? (View Answer)
    Neurological symptoms, including ataxia (difficulty coordinating limb movement), loss of balance, or change in behavior can be associated with a number of conditions affecting the central or peripheral nervous system. EPM protozoa cause damage to the spinal cord and nerve cells, which leads to neurological symptoms in the horse. EPM can be tricky to diagnose (false negatives are possible) but other reasons for damage to the nervous system (trauma, intervertebral disk disease, arthritis, viral infections) may lead to long lasting or recurrent neurological deficits. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  4. Are their any new treatments since Ponazuril, or the like, that have been approved? (View Answer)
    FDA-approved treatments for EPM include ponazuril (as the brand name Marquis) and most recently diclazuril, under the brand name Protazil. Re-Balance is a branded treatment that incorporates the original combination of pyrimethamine and sulfa medications. All of these medications have their pros and cons; Re-Balance must be dosed for many months at a time, and Marquis and Protazil are not inexpensive. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  5. If scat is removed from pastures, does it greatly reduce chances of horses contracting EPM? (View Answer)
    Pastures can be contaminated with Sarcocystis oocysts from the feces of infected possum or other wildlife vectors. Certainly, removing feces from the grass would be ideal to prevent contamination. Unfortunately, it would be difficult, if not impossible, to clear all fecal material from a pasture or paddock. Managing the possum population by humanely removing the animals from the area or preventing access to a paddock or pasture is recommended. Pastures with areas of woods or bordering wooded areas will be more likely to have an opposum population. Keep garbage or other possum feed sources secured and away from the paddocks and pastures. Stalls and barns may be cleaned with power washers to remove feces and oocysts. Secure your grain and hay sources to prevent possum access and monitor horses closely for potential EPM symptoms. Managing the opposum population is your best bet for decreasing your horses' exposure to EPM organisms. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  6. My racehorse had a fairly bad case of EPM three years ago and was successfully treated with Marquis. Now at age 10, she has immune system stress during weather changes, often in spring, with low white blood counts. Would it be unwise to breed this horse later if blood counts return to normal? (View Answer)
    The EPM research group at UC Davis have found that the organisms of EPM, Sarcocystis and Neospora, do indeed transmit from the mare to the foal in utero. A previously treated mare that is no longer showing neurologic signs can transmit the organisms under times of stress, such as late gestation, directly to the foal. The mare herself can have a relapse of EPM at this high stress time. While infected foals have been treated successfully from birth for the EPM organisms, the risk of neurological deficits and loss of performance warrants serious consideration if you decide to breed a previously infected mare. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  7. My horse went through treatment for EPM two years ago and has recovered. He is now 25-years-old. Is there any supplement he should be receiving that would help resist a relapse? (View Answer)
    Vitamin E is an excellent antioxidant that helps support muscle and nerve health and function. It, as well as other antioxidants and anti-inflammatories such as MSM, are not specific to reducing the chances of contracting EPM, but do improve the health and immune strength of a horse. Some studies suggest that thiamine or folic acid may improve the nerve health, especially following medical treatment of EPM, and lower the chance of a relapse. A healthy horse that receives well-balanced nutrition, consistent and appropriate exercise, and has quality shelter and care has a lowered risk of contracting EPM. Managing pasture and feed contact with opposums or other vectors decreases the risk of exposure. Use critical thinking about any supplement that claims to be a definitive prevention for EPM or other disease. Keeping your horse healthy is the best prevention we have! Sandi Farris, DVM, Harmony Veterinary Services, LLC
  8. My horse recovered from EPM two years ago. Ever so often, he will do something that makes me worry about a relapse. Is it likely that he could relapse? What percentage of horses become sick again? (View Answer)
    Research answers vary as to the percentage of relapse from EPM, with numbers from 10-28% being found in a quick search of published papers. The likelihood of a relapse seems to increase in older, stressed, or chronically ill horses. Horses that received only a short, interrupted course of treatment or no treatment at all seem to relapse more often. The species of protozoa can have an impact on relapse, with Neospora being somewhat harder to treat and clear. Fortunately, Neospora has been found to be the primary organism in a minority of EPM horses. Ask your veterinarian which species of EPM your horse was considered to be infected. This information may help define the chances of a relapse. Keeping your horse healthy, feed a well-balanced diet, preventing access of opposums to your grain and pasture, and reducing stress is your best prevention against a relapse. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  9. Is EPM contagious by horse to horse transmission? A horse where I board had joint surgery, never recovered properly, and went downhill. Eventually, EPM was diagnosed. Despite treatment, he was euthanized several months later. Are the other horses at a greater risk because of this? (View Answer)
    I am sorry to hear of the loss of the surgical horse. EPM protozoal organisms are a dead-end parasite in the horse, which means that they cannot complete their life cycle and be shed in feces or urine by the horse. So, although EPM is not contagious directly between horses, a shared environment may be a source of infection. Vectors of the protozoa Neospora and Sarcosystis include the opposum, raccoon, and domestic cats. Feces of these infected animals contain the protozoa and will contaminate shared feed sources of horses. Be sure to manage potential contamination by preventing wildlife access to stored hay and grain. Utilize appropriate measures to limit the opposum population in the stables and keep stress in the horses as low as possible in endemic areas. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  10. I have a three-year-old filly that had EPM. She has a great appetite but is losing weight. I want to build her up and use her for breeding as she would replace her dam, a very old and hard to find bloodline. Will she have trouble foaling due to the damaged muscles? Is the uterus an affected muscle? She has never received any medications. At times, she looks great but then loses weight again. She is alert and has never lost her appetite. (View Answer)
    The EPM research group at UC Davis have found that the organisms of EPM, Sarcocystis and Neospora, do indeed transmit from the mare to the foal in utero. A previously treated mare that is no longer showing neurologic signs can transmit the organisms under times of stress, such as late gestation, directly to the foal. The mare herself can have a relapse of EPM at this high stress time. While infected foals have been treated successfully from birth for the EPM organisms, the risk of neurologic deficits and loss of performance warrants serious consideration if you decide to breed a previously infected mare. Contact your veterinarian for a full work-up and examination of your mare before deciding to breed. This examination will also help identify any concurrent issues that may be contributing to her weight loss and muscle strain. It may be necessary to treat your mare with one of the FDA approved EPM coccidiostats on the market today. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  11. I have a mare that had EPM 5 years ago. She could hardly stand or walk, but was treated aggressively for the condition and recovered. In fact, she was able to return to racing and had no reoccurance. Currently, she is in foal and due any day. Could her previous bout with EPM affect the foal? (View Answer)
    The EPM research group at UC Davis have found that the organisms of EPM, Sarcocystis and Neospora, do indeed transmit from the mare to the foal in utero. A previously treated mare that is no longer showing neurologic signs, can transmit the organisms under times of stress such as late gestation, directly to the foal. The mare herself can have a relapse of EPM at this high stress time. While infected foals have been treated successfully from birth for the EPM organisms, the risk of neurolgic deficits and loss of performance warrants serious consideration if you decide to breed a previously infected mare. If your mare is due to foal, be prepared by speaking with your veterinarian about how to manage and treat a likely EPM positive foal for best results. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  12. My 16-year-old pony mare was treated with Marquis a year ago. She was diagnosed during a pre-purchase examination. She still continues to want to go a little crooked in the ring and definitely favors one direction over the other. This is not apparent on the trail. She is only ridden over very small fences and on the trail for short periods of time. Is there anything I should be doing for her? She is on a daily dose of Equioxx. (View Answer)
    Make sure to double check with your veterinarian about your next step or treatment plan. A current examination should include a specific neurologic exam to ensure your mare isn't suffering balance issues. Subtle neurologic deficits can be hard to see without the exam and could be dangerous if she tripped or fell. Anti-inflammatories such as fibrocoxib have been used to minimize secondary nerve cell damage effects. Ask your vet to help rule out muscle weakness or mild lameness that could also be contributing to her directional preference. Above all, use caution while jumping to prevent injury to your mare or her riders. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  13. I will be temporarily boarding with a neighbor until our fencing is completed. He informed me he uses pond water as his primary water source for his horses. My horses are use to tap or rural water and am concerned not only that they won't drink well, but they may be exposed to contaminants such as EPM and nitrates for starters. I was going to ask him to give them tap water during their brief stay in part so they won't have to readjust once at our place. Is pond water a potential source for EPM? I would appreciate your thoughts. (View Answer)
    Pond water or pasture surrounding a pond could be a potential contaminant source if opposum or other infected vectors (cats, possibly raccoons) have defecated in the area. The protozoa Sarcocystis and Neospora are passed into the feces of the opposum and enter the horse while grazing or consuming contaminated feed. Consider asking about the likelihood of opposum or other wildlife that may access the pond and pasture and make a decision based on the risk of exposure. Ponds, lakes, and running streams have been a legitimate water source for many horses but gathering information on EPM potential as well as other diseases in the local area, such as leptospirosis or giardia, may be in your horses best interest. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  14. Are there any supplements, such as vitamin E, or things to avoid in order to help prevent a young horse that is being trail ridden from contracting/developing EPM? (View Answer)
    Vitamin E is an excellent antioxidant that helps support muscle and nerve health and function. It, as well as other antioxidants and anti-inflammatories such as MSM, are not specific to reducing the chances of contracting EPM but do improve the health and immune strength of a horse. A healthy horse that receives well-balanced nutrition, consistent and appropriate exercise, and has quality shelter and care has a lowered risk of contracting EPM. Managing pasture and feed contact with opposum or other vectors decreases the risk of exposure. Use critical thinking about any supplement that claims to be a definitive prevention for EPM or other disease. Keeping your horse healthy is the best prevention we have! Sandi Farris, DVM, Harmony Veterinary Services, LLC
  15. My 21-year-old Tennessee Walking horse gelding was diagnosed in 2016 for EPM and treated with Marquis for 30 days. He was a little lame and had stumbled a few times, which was what sent me to the vet. At the time of diagnosis, the vet told me he should never be ridden again since he may stumble, fall and hurt his rider. He seems fine now-- runs in the pasture, gallops in for meals, slight stumbles perhaps once in 4-6 months. Is there a neurological test which can determine if he is still not safe to ride? He really wants to go with us on trail rides. (View Answer)
    You can always ask your veterinarian for a recheck at which time they will conduct a specific neurologic examination. I am certain they are using caution to prevent your horse or any handlers or riders from injury. An examination at this time may differentiate EPM neurologic signs from similar appearing symptoms of other disease. Subtle neurologic deficits may not be apparent without a specific neuro exam and your veterinarian can give you the best information on safety and risks to the rider and to your horse. Sandi Farris, DVM, Harmony Veterinary Services, LLC
  16. If a horse ingests contaminated feces of an opposum, does the horse automatically develop EPM? Or do some horses show resistance to the disease? (View Answer)
    No, not all horses seem to come down with EPM symptoms. Some horses are able to mount an immune response against the protozoa before the organisms reach the nervous system. Other horses don't fight off the protozoa effectively. If the Sarcocystis or Neospora reach the central nervous system, they can create damage to the nerve cells. The horse may not show neurologic symptoms for weeks to years once infected. Stressful times, including nutrition, weather, environment, or transport may lower their immune strength just enough to allow the organisms a chance to take hold and cause symptoms. Sandi Farris, DVM, Harmony Veterinary Services, LLC