When the parasite crosses the blood/brain barrier, a horse has the potential for developing neurologic defects due to direct neuronal damage and inflammation caused by the protozoa.
In 2013, a University of California- Davis study found 3,100 blood samples taken from horses in 42 states tested positive for the antibodies associated with the parasites that cause EPM. In 24 of those states, horses tested positive for having the antibodies against both Sarcocystis neurona and Neospora hughesi. Just because a horse has been exposed to opossum scat does not mean it will develop EPM. It all depends on the status of the immune system and stress level of the individual horse.
EPM is difficult to diagnose, treat or prevent and no vaccine is currently available. EPM is a master at disguise and often the symptoms mimic other diseases.
Mild depression and subtle gait abnormalities are often confused with lameness or injury. Neuronal damage caused by EPM can also cause upper airway malfunction making it difficult for the horse to eat and swallow, causes the horse to roar or changes the horse’s whinny. Depending on the progression and severity of the neurologic damage, the symptoms become more severe and the horse can potentially collapse or become unable to stand.
“The reason for the wide variety of clinical manifestations is that the protozoa have the ability to randomly infect many sites within the central nervous system,” Abraham said.
In addition to presenting as another disease or injury, EPM is unpredictable. A field of horses may all be exposed to the parasite, but some are able to develop antibodies against it and others are not. Bryan’s entire herd of horses eat the same hay and graze the same pastures as Ida B, but thus far none of the others have shown symptoms of EPM.
Research and diagnostic testing has shown not all horses exposed will develop EPM, and the annual incidence of EPM is less than 1 percent according to research by EPM expert Steve Reed, DVM.
(Reed S. Neurology is not a Euphemism for Necropsy: A Review of Selected Neurological Diseases affecting horses; Proceedings 54th Annual Convention AAEP; San Diego, CA. 2008:78-109.)
Risk Factors and Treatment
Exposure is the most obvious risk factor. Age and stress are others.
“Stress-induced immune suppression may be associated with susceptibility to developing disease after exposure,” Abraham said.
Horses age 1 to 5 and those older than 13 are at a higher risk for neurologic damage than other horses. Weaning and undergoing social regrouping can be one reason younger horses may be more susceptible to EPM. The start of rigorous training regimens may be another reason young horses are at a higher risk than middle-aged horses.
Horses greater than 13 years of age may be undergoing life changes. They may be retired from showing or entering the breeding shed, which is a new stress in their life. Additionally, as horses age their immune system may not be as robust and when it is challenged it may not be able to protect the horse from developing the disease after being exposed to the sporocysts, which is the infective stage of the disease.
Currently, there is no vaccine and prevention is challenging. However, good horse keeping can reduce an opossum’s opportunity for contaminating a horse’s environment. Limit opossum and other wildlife’s access to feed. Avoid feeding on the ground, instead use raised feeders. Keep feed rooms closed and minimize grain spillage by sweeping areas where horses drop grain out of feed tubs.
Regularly clean water tubs and replenish with fresh water. Wildlife may be drinking out of the same areas as your horse. When an animal jumps up for a drink they are contaminating those areas.
Schedule regular wellness visits with your veterinarian. Your veterinarian may be able to pick up on small clues early on. A slightly droopy ear could be an early sign of paralysis. Uneven muscle definition under the horse’s forelock can indicate muscle atrophy. Bryan has always scoped all her horses at age 2. That way she knows if the horse will have any respiratory issues before training begins. Because of this routine, she had a baseline for Ida B and knew the flapper paralysis had developed after exposure to EPM.
“I know she was clear at that point,” she said. “It’s helpful to have past information to compare to.”
Abraham says early detection and treatment are critical to long-term prognosis. Research shows only 60-70 percent of diagnosed horses will improve and only 15-25 percent will recover completely. Once diagnosed, horses have a risk of relapse. Ten to 20 percent of horses that have already been diagnosed will relapse within two years.
Although those signs can be seen with other causes such as trauma, wobblers, other causes of encephalomyelitis in horses, EPM can present with any combination of neurologic signs.
“Occasionally we will see just one problem, such as the inability to swallow (dysphagia), but we often see signs suggesting multiple sites of infection,” Abraham said.
Once treatment begins, a horse’s response to the medication can vary. Some horses show gradual improvement after the first 24 hours of treatment, whereas it may take other horses a few days to demonstrate improvement.
“Sometimes we’ll see signs worsen as soon as treatment is initiated because the medications kill the protozoa and that can set off a significant inflammatory response,” Abraham explained.
Often, horses will also receive anti-inflammatory drugs to counteract that response. Treatment can be difficult because veterinarians are often trying to target multiple sites in the central nervous system, and as the protozoa migrate they cause physical damage. There can be destruction of portions of the brain or multiple segments of the spinal cord.
“This damage and therefore the clinical signs can be permanent, even once we have eliminated the protozoa,” Abraham said.
Time is the only true predictor of how a horse will respond to treatment.
Nearly 18 months after the initial diagnosis, Ida B appears to be 100 percent recovered. She has regained and retained her weight. She’s back in training and Bryan is preparing her for derbies this summer. Not all horses are that fortunate. Horses with more severe cases and lingering neurologic symptoms can be easily fatigued and have difficulty supporting their hind ends.
While it may seem the incidence of EPM is on the rise, veterinarians can’t confirm that is necessarily the case.
“What I can say for certain is that, in the last few years, we have developed better tests that can more effectively diagnose EPM. So perhaps we are just doing a better job of being able to diagnose the disease,” Abraham concluded.
This article was originally published in the June 2019 issue of Barrel Horse News.