I've been thinking about my horses' four cases of EPM (out of three horses, that's a lot). And I started thinking about the circumstances - moving barns. And in Pie's and Red's cases, moving from one geographical area to another, and then in Pie's case, moving barns again. And then I started thinking about possums, and looked up some more information about them. I knew that they originated in South America, and are the only marsupial in North America.
And then I looked up their geographical range - here's a map from National Geographic - the light-colored area is the possum's range - I don't know the date of this map or how exactly correct it is but it'll do for this purpose:
My apologies to readers who aren't from the States, since the state boundaries aren't indicated. You will note that they generally aren't present in the far northern tier of states, or the mountain West. Over (a very long) time they have extended their range to the north, but they apparently haven't reached Montana, the upper part of Wisconsin, or most of Minnesota.
Now here's the interesting part. Pie was bred and raised in Montana and lived there until he was a weanling. Then he moved to Minnesota, in an area somewhat to the north of Minneapolis. And Red was bred, raised and always lived a few miles from Eau Claire, Wisconsin. All of these areas seem to be outside the usual range of the possum - hence it is likely that neither Pie nor Red were ever exposed to the organism that causes EPM before they moved to live with me. When they moved to the old barn, both developed infections with strains 5 and 6. But Dawn did not - she had been living there for almost 10 years and also was bred and raised in Illinois, which is within the possum's range. It is likely that she was previously exposed to strains 5 and 6 and either didn't get infected or had an infection and developed immuity - she did have an odd neurological episode several years ago involving a very stiff neck and some odd neuro signs like twitching - we attributed it to an atypical rhino (she'd been vacinnated for the usual rhino strains) but who knows? It's also possible that exposure to the EPM organism as a foal, either through the environment or as a result of getting antibodies in mare's milk, could confer some protective immunity.
And then Dawn and Pie moved to the new barn and both, within a matter of weeks, developed active infections with strain 1 - neither had obviously been exposed to this before. Strain 1 tends to be more symptomatic and severe than 5/6, so it's likely that I noticed the symptoms more quickly than I would have, and I was also familiar with what sort of symptoms were likely to be there so I noticed them. Different barn, different hay - one of the most likely vehicles to transmit EPM is hay, and there's not a thing the hay farmer can do about it.
I corresponded with Dr. Ellison (the researcher who developed the new blood test and treatment that is in clinical trials) by e-mail, and she agreed that moving from a familiar geography to a new one can result in infection when the horse is exposed to the EPM organism for the first time or to a new strain. She recommended preventative treatment with the low-dose decoquinate powder when I move Red back to the new barn in June, starting when he moves.
I could have done without the geography lesson, but I must admit it is interesting . . .