Once regarded as the greatest health threat to horses, equine protozoal myeloencephalitis (EPM) is not in the news much these days. But that does not mean veterinary medicine has conquered EPM.
“I do not believe EPM is going away. I think it is just as prevalent now as it ever was,” said Dr. Steve Reed, an internal medicine specialist at Rood & Riddle Equine Hospital in Lexington who has been researching the disease for 25 years. Reed was part of a team at The Ohio State University that developed the disease model used to study EPM in living horses.
EPM is a neurological disease caused most commonly by the protozoa Sarcocystis neurona. The infection occurs when a horse consumes the sporocyst stage of the protozoa, which can migrate to the horse's brain and spinal cord to cause neurological damage. Another parasite, Neospora hughesi, has more recently been named as a source of EPM, although not much is known about the organism, including its definitive host.
“N. hughesi almost certainly is transmitted by ingestion of feces,” said Dr. Dan Howe, a parasitologist at the University of Kentucky's Gluck Equine Research Center and longtime researcher into the organisms that cause EPM. “There's no information about what the feces might come from, though. A good bet is that it's dogs, but that's never been proven.”
Coyotes also may be hosts. EPM caused by N. hughesi may be more prevalent on the West Coast, Howe said.
S. neurona sporocysts are shed in the feces of opossums and can contaminate feed and hay. Water also can be a prime source of transmission. In the pasture, areas under trees where fruit has fallen can be a hot spot of infection when the horse nibbles on the fruit and the grass around it that has been contaminated with feces from opossums dining on the fallen fruit.
Symptoms of the disease are a result of brain and spinal cord damage by the protozoans. In severe cases, horses will drag one hind leg and lose coordination. Common signs in earlier stages of EPM include loss of muscle tone and decreased sensation in the hindquarters, loss of tail-muscle tone, and uneven wear on hind shoes.
Three medications used to treat horses with EPM are: ReBalance, a combination of pyrimethamine and sulfadiazine produced by PRN Pharmacal; Protazil, a diclazuril drug manufactured by Merck Animal Health; and Marquis by Merial, which contains ponazuril.
“All of them claim to be effective at between 65 percent to 70 percent, so they all work about equally,” Reed said. “Candidly, what I've been doing is usually to start with either diclazuril or ponazuril for 28 days, and then I will go an additional month with ReBalance because (1) it attacks the protozoans from two different mechanisms, and (2) it's more cost-effective for the client.”
Some veterinarians recommend supplementing the horse with folic acid while it is being treated, but Reed doesn't feel that is necessary unless the horse develops anemia. In that situation, he discontinues the medication and supplements folinic acid, an active form of folic acid, until the horse rebounds and treatment can be resumed.
Dr. Martin Furr, now at Oklahoma State University after decades researching EPM at Virginia Tech, observed that a half cup of vegetable oil added to the horse's feed seemed to enhance the bioavailability of ponazuril, but Reed's personal preference is not to supplement vegetable oil. But, for ReBalance, he does advise that hay be withdrawn from the horse for at least one hour before and after administration.
Reed added, “I do think that some additional immunostimulant might be helpful for horses being treated for EPM, and the product that I've been using lately is Levamisole at between one and five milligrams per kilogram of body weight. I give that anywhere from the first 10 days to the full 30 days of the treatment.”
Prevention is key
Pristine feed room conditions and biosecurity around the farm help prevent horses from acquiring EPM. Some veterinarians recommend administering Protazil, which is available in alfalfa-pellet form, part of the month to prevent horses in high-risk areas from becoming infected.
An altered immune response may be a factor in acquiring EPM. If a horse is being treated for a health problem with a medication that depresses the immune response, such as steroids, immunostimulants are recommended to fortify the horse's ability to fight infection with EPM.
Fort Dodge Animal Health marketed a conditional EPM vaccine in the 1990s, but its production was halted when it proved ineffective. Currently, Zoetis, which took over Ft. Dodge, is contemplating work on a vaccine.
A significant breakthrough in EPM research is the identification of the complete genome sequence for S. neurona, work in which Howe and postdoctoral scholar Sriveny Dangoudoubiyam were involved.
“From a basic science standpoint, this is exciting,” Howe said. “It is going to lead to a lot of good, basic information. The genome is not going to immediately give us a vaccine or new drugs to use, but it will eventually lead to that.”
New to the Paulick Report? Click here to sign up for our daily email newsletter to keep up on this and other stories happening in the Thoroughbred industry.
Copyright © 2020 Paulick Report.