We now have a likely explanation for how Red came to reinjure his right hind ankle. He originally injured the ankle in July, and after 30 days off from work with icing daily, he was looking pretty good and we started slowly back to work. After about two weeks, we were up to 8 minutes of (very nice) trot work, when one day, he suddenly tripped badly behind - so badly he almost fell down behind - and strained the ankle once again. More time off, more icing. He was having trouble walking down hills - dragging his hind toes. At his last farrier visit about 6 weeks ago, he was very fidgety and hard to trim. And picking his hind feet has become a bit difficult.
Hmmm . . . something looks familiar . . .
We ran a neuro exam and drew blood to check for EPM - and sure enough, that's what's going on (again). Red had EPM (phenotype 5) several years ago and made a full recovery. He's had a couple of minor symptom flare ups (no active EPM - we tested and the results were 2-2-2 - more on that below) due to stress or vaccinations - EPM horses, even if clear of infection, can have temporary reoccurrence of minor symptoms due to these things, and a little Banamine or passage of time clears things up.
When we did the neuro exam, his hind legs weren't quite normal on the turning test - he tended to slightly drag the left hind toe when that leg was on the outside, and the right hind didn't step across as far on the turning test. His skin sensation and reflex responses were poor in his hindquarters, particularly on the right side. And his right hind foot placement test was very abnormal - he'd leave it crossed over behind his left hind indefinitely. The reason he tripped and reinjured his ankle is that he couldn't accurately feel where his hind legs, and particularly his right hind, were.
Here is a link to the Pathogenes web site that describes the antibody test and how its results are interpreted. In my layperson's summary, a 2-2-2 (there are three phenotypes tested for) is a horse that is free of infection. A number below 4 is a horse that is likely free of infection. A horse with a number of 16 or above has an active infection and is likely to be symptomatic. Red's results were 8-2-2. This, together with the neuro abnormalities, indicates at the very least that his immune system has been exposed - this time to phenotype 1 (not the type he had before) and is ramping up antibody production. Phenotype 1 apparently typically produces more acute symptoms.
I might add that this is a new test - it isn't a spinal tap or a Western blot - it's an ELISA antigen test done from a blood draw, and is very specific - a horse that has been previously exposed but didn't get sick or a horse (like Red) who'd had a previous infection and fully recovered to a 2-2-2 antibody level (essentially zero) also wouldn't test with higher numbers without a new exposure.
Now, there are some unanswered questions. Does this mean he has an active infection? Considering that he has real symptoms, probably yes, although the numbers are borderline - you don't get an 8 without something happening in the immune system. As the Pathogenes site indicates, changes in titer levels are more indicative than absolute titer levels, and retesting in two to four weeks to see if the numbers are going up or down would tell us more. It could be that he was going to fight it off on his own, but we're treating him anyway with at least 30 days of low-dose decoquinate, which will likely resolve things. We may retest him in a couple of weeks to see if his numbers are coming down - if not, we may move to a higher dose with combined immune stimulants.
Can a horse that has had a particular phenotype get a new active infection from the same phenotype? Not in my experience so far - Dawn has had phenotype 1 only, Pie had two separate infections with phenotypes 1 and 5, and Red previously had an infection with phenotype 5 and now appears to have an infection with phenotype 1 (the third number in the test results is for a phenotype that doesn't appear to usually produce symptoms in horses). But within each phenotype, there are several different strains. I don't know if the science is far enough along to answer that question, but I'm hoping that horses get some immunity from reinfection with phenotypes they've already had.
But at least it's good to have an explanation of what happened with Red, and he should improve with treatment. His ankle is almost back to normal, and we'll be starting up our walk work again - but no trotting until he's neurologically normal. It's also a good reminder of how common EPM really is - not that I could ever forget!
If you're new to this blog, there is lots (and lots and lots) more information about EPM on the EPM and Lyme page - see the tab under the header.
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And, on a lighter note, some Norman cuteness, with his usual girlfriend - you can see his unusual eye coloring (he's a champagne, and has amber eyes):