Nearly two years after I bought her, River was diagnosed with EPM (Equine Protozoal Myeloencephalitis). The diagnosis was based on high titers of Sarcocystis neurona as registered on UC Davis‘s Sarcofluor test. (She was negative for Neospora hughesi on the NeoFluor test.) My vet ordered the test based on River’s wellness exam and my description of a series of symptoms. In hindsight, I really should have suspected long ago. In my defense, detecting subtle signs of EPM in horses can be difficult.
Jump to:
Clue #1: stringhalt
Clue #2: Fluctuating lameness
What should have been an early clue: difficulty backing
Obvious signs of EPM in horses with which I was already familiar
How River’s symptoms have tracked her stress
Clue #1: Stringhalt
River came to me with stringhalt
I really should have seen this one from the beginning. If I had known more about EPM, I would have!
River got off the trailer on September 9th, 2023 with stringhalt. Very obvious at the trot (not at all at the walk). Much worse on the right side. Considering she had had an extensive and expensive pre-purchase exam, I was surprised. No mention had been made of stringhalt. (And that’s something you learn in the first year of 4H horse!)
Since I wasn’t going to send her back, I never mentioned the stringhalt. It also got better. By the time I took River to the chiroprator, Jeff Shelley said it was minor and should give her no problems.
The stringhalt has come and gone
By the time River did her first endurance ride at Fire Mountain, there was no sign of stringhalt. Last May, after River was dead lame after slipping in the mud at Cache Creek Ridge, my now retired vet Doug Anez did a thorough workup, including flexion tests. No stringhalt.
After she tied up at Cuyama Oaks in March, River showed a tiny bit of stringhalt. You had to look very closely to see it. We vetted many times at Treasure of the Sierra Madre, and not once did anyone notice stringhalt.
Then on May 20th, River had part of her annual wellness exam. The salient issue was a sore right shoulder, which was no surprise. I had not done her daily stretches for over two weeks so the vet (now Dr. Jill Marquez) could see her at baseline. More interesting was the pronounced stringhalt after the right stifle flexion test.
Exactly one week later I took River to the clinic to do the computerized gait analysis. (The first part had been at home, because I wanted them to float Fantazia‘s teeth too.) Her right shoulder was 100% fine… a nice validation of my stretching exercises! The stringhalt was less pronounced. However, now she was slightly lame on all of her other three legs, most on the left fore. After her left rear hock flexion test, she was clearly lame on her left hind.
Above: River looking very healthy at the beginning of May. At this point she has slight stringhalt in her right hind with no flexion.
Clue #2: Fluctuating lameness
Detecting subtle signs of EPM in horses often boils down to trying to find explanations for fluctuating lameness. The biggest hint was stringhalt, which as a neuromuscular condition can be clearly linked to neurological symptoms of EPM. However, River had other fluctuating lamenesses.
The computerized gait analysis, continued
Once she started favoring the left hind, she remained noticeably (to the human eye as well as the computer) lame on it throughout the rest of the flexion tests. She was left-rear lame on hard and soft surfaces, in a straight line and to the right and left on the lunge line.
There were three possible explanations.
First, it could be a sore hock. River is eight, and we do a lot of steep downhill work that is hard on hocks. This theory was unsupported by beautiful clean hock x-rays.
Second, it could be the result of a small superficial wound that River had since April 17, the day before we went to Huasna for the Treasure of the Sierra Madre ride. A small cut had turned into a hard knot. She had never been lame on it, and it was not sensitive to prodding. But it was there, and we did x-rays. They were beautifully clean.
The final possible explanation we came up with was that she’d been overloading the left hind due to the stringhalt in the right. The problem was that the stringhalt had improved… and had been fluctuating anyway.
Above: My vet, Dr. Jill Marquez, holding River’s leg back and up for a left stifle flexion test. River is wearing the apparatus for the computerized gait analysis.
Previous fluctuating lameness
I am the first to admit that I see lamenesses even when a vet cannot. I have a healthy imagination for lameness. One might say it’s almost a psychosis. However… River has certainly caused more lameness hallucinations in me than other horses.
First some fixed, explicable lamenesses
The right fore
From the beginning, I have known her right shoulder was off. This was easily addressed with stretches. After her slipping and fall at Cache Creek Ridge and subsequent partial ride at Montana de Oro (read about my rider option there), River was clearly lame from what we supposed was a pectoral muscle pull, also on the right.
She also had a stone bruise in the right front hoof, easily diagnosed with hoof testers. This happened when she stepped on a rock in the Western Mojave xpride last year.
A corn in the left front hoof
My fault for letting her go too long between shoeings. This followed shortly after she recovered from the stone bruise and was also quickly diagnosed by my farrier. It was annoying (I had to keep that hoof totally clean), but in a few months was history.
Then the mysteries
Once in a while, we would be trotting along and River would start limping. I’d get off and look for stones in hooves. (Fantazia becomes dead lame with a stone caught in her hoof!) River has never had a stone in a hoof, but!
At the Eastern Mojave ride in February, she started limping. I cleaned out her feet, and she was better. Dave Nicholson said there was nothing wrong with her, after watching her trot out at the beginning and end of the hold. She finished sound, if tired. She was stiff but not lame the next day. I chalked it up to shoes not being perfect. We addressed that.
At Cuyama Oaks she tied up. Tying up is not caused by EPM, but it was still a mystery. More later.
On day 2 of this year’s Treasure of the Sierra Madre in Huasna, River suddenly started limping when I got back on after leading her a while. I got off, loosened the cinch, and started back. After a short time, she started jigging completely soundly. I chalked that one up to a too tight cinch, but my vet was clearly skectical of that explanation.
At home I would occasionally wonder….
There have been myriad other moments on training rides when I have thought River was favoring one leg or the other. It always disappeared within a short time. I always thought I was imagining it.
In the arena, I’d occasionally thing she was off in tight bends. But it was never consistent enough for me to think it would be detectable at the vet, with blocking.
Identifying the subtle signs of EPM in horses is very difficult, but I really should have suspected. I did know fluctuating lameness is a sign. To the knowledgeable, stringhalt is a red flag. There were other signs I missed.
Above: These videos were taken the day after my vet persuaded me to test for EPM. If you look vrty closely, you can see the stringhalt.
What should have been an early clue: difficulty backing
If I had known River better when she arrived from Texas, I might have clued on to this one.
When I got her home, River did not want to back out of the trailer. In fact, she refused. I let her turn around, thinking she’d simply never been asked. (The first time I had to back Beroni out of a straightload, it took 10 minutes. He was an excellent backer… he’d just never needed to back out of a horse trailer. He kept trying to turn around!)
River was also reluctant to back under saddle, but this didn’t surprise me at all. She had been a racehorse. They almost never have to back under saddle (just when training in the starting gates).
Weirdly, when I decided to teach her to back out three weeks later, she just did it. Since then I have learned that she prefers to back out of the trailer. She’s always reluctant to turn and walk out (I do this to keep her from backing through poop). About the same time, she became quite willing to back up under saddle.
I did know that difficulty backing could be a sign of EPM.
Above: WSG Amigo, ridden by my cranky-looking son Ian (underdressed on a cold nasty Oklahoma day!). Amigo had more symptoms of EPM than River does, but a much lower s. neurona titer.
Obvious signs of EPM in horses with which I was already familiar
About eight years ago, when I was still in Oklahoma, I trained a horse for some friends. “Amigo” was (and is) a Morgan gelding. He came to me with some issues, but those were quickly fixed. Amigo was an anxious, rather needy horse who loved and trusted people. He would follow his human anywhere. No fear of stepping down or into anything. (He once followed me through a people-door into the office at a stable we rode at!)
Amigo suddenly worrying about stepping into dark patches on the ground or down steps. He’d always loaded and unloaded easily; he suddenly started worrying about stepping up or down from the trailer. Always easy to shoe, he started having trouble holding a hind foot in the air for shoeing. Never spooky, he started looking everywhere. From backing smoothly under saddle, he began to resist even rocking back.
These were all classic signs of EPM. We did take some alternative measures to address the issues but none really worked. I suggested is owners test. His titers were around 200.
EPM is really tricky
By the time Amigo was tested, he was on his way to his owner in Texas. I had been riding him while waiting for my mare Lady to come off a lay up. I can’t remember how or if Amigo was treated, but he lives a life of ease so any subtle signs of EPM would go unnoticed.
The thing is, s. neurona antibody titers are a response to exposure to the protozoa. Every horse differs in its immune response. Other threats to the immune system can cause a flare up of EPM symptoms in a previously asymptomatic horse. I don’t know why Amigo was symptomatic with low titer, but I can speculate. I can also speculate on River’s fluctuating symptoms.
How River’s symptoms have tracked her stress
I assume that River’s PPE made no mention of stringhalt because she exhibitted no stringhalt that day. She came off the trailer with stringhalt because she had spent several days travelling from Texas.1
Travel increases the probability of EPM symptoms.2
Once River had settled in, the symptoms disappeared.
Then River tied up at the end of March. Suddenly, the stringhalt was back. She had weird fluctuating lameness. Tying up is not directly related to EPM, which is neurological. However, it is a stressor. We don’t know why River tied up. She seems to have recovered with attention to exercise, feed, and electrolytes. (Read One way to deal with a horse tying up.) But since tying up, she has exhibited many of the subtle signs of EPM in horses.
- **Having lived in Texas should have been a warning sign, according to my vet. After River’s titers came back, Dr. Marquez asked if she had ever been in Texas. When I said River had lived there for years, my vet said, “You should have told me! I would have recommended testing right away.” ↩︎
- So does being a racehorse. River had it coming from several sides. Texas, racehorse, travel…. ↩︎
Dealing with EPM involves avoiding unnecessary stressors
Stress weakens the immune system. Horses with weaker immune responses to the protozoa are going to have more symptoms. They will also have lower titers initially, while their organisms mount an immune response.
That’s why titers don’t mirror symptoms. Higher titers are a measure of immune response: they reflect the level of antibodies created by an organism to fight invasive bacteria, viruses, or protozoa. That’s how vaccines work: they stimulate an immune response with a small exposure that won’t overload the system (read about different types of vaccines and how they stimulate immune response). The antibody titer an organism develops in response to vaccination enables it to swiftly combat later infection. (Caveat: I am not a biologist or immunologist or any other -ist that really understands this. You’ll need to do your own research if you want any details!)
Low titers could mean little exposure. If the horse has had no need to mount a large immune response, it will have lower titers. Low titers could also mean that the horse’s immune system has not responded adequately to exposure. Said horse would theoretically be symptomatic with low titers.
Perhaps Amigo had low titers and noticeable symptoms because his immune response was insufficient to fight off the protozoal parasite. Similarly, River’s relatively high titer means that her body was responding to the presence of the protozoa by developing antibodies.
Stress limits the ability of the immune system to respond to pathogens (or corresponding antigens)
Even with high antibody titers, enough exposure to a pathogen can overwhelm the immune system. This is more likely to happen when the organism is under stress, even if that stress is entirely unrelated.
Stress requires the body to dedicate resources to something other than immune response. River’s anti-EPM resources were depleted (under my working hypothesis) after travel and after tying up. Hence, she because symptomatic. Her symptoms are subtle because her immunity has been good. Only because my vet and I looked very closely (and Dr. Marquez used her knowledge!) was River tested for EPM. Honesty I was skeptical.
Hopefully, treating River for EPM will enable her body to fight off the pathogen and then recover. The first sign that it’s working will be lessening symptoms. River’s symptoms were never obvious to me, although in the future I will be much more sensitive to the subtle signs of EPM in horses. I plan to do a stifle flexion test in a few days to gauge the success of the treatment.
My vet wants to do another titer test in a few months. I am not sure what it will tell us… but more on that later.
Coming up: A blog post on ways to treat EPM in horses.
Above: A week before we tested River for EPM. At this point she was showing some of the subtle signs of EPM in horses, but I did not recognize them as potential symptoms of EPM. Hence this blog post!