Wednesday, May 2, 2012

Geography and EPM

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 . . .

14 comments:

  1. Poor horses. . .but I am glad that you are well aware of the signs and noticed them soon enough!
    We are in the range here in Michigan, but I have not heard of any cases in my area. I have seen possums all over the place though, usually as road kill.
    Thank you for all the information, I really appreciate it having the knowledge!!

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  2. Makes me afraid to move Bodhi back down to the states from Canada. We already had one EPM scare where he tested negative. Eeek! What are the fist symptoms you noticed in your horses?

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    1. Golden the Pony Girl - first symptoms were minor gait abnormalities behind, much more subtle than being off - occasional tripping/toe dragging - one hind more affected in each of my horses - and lack of push/forward, and difficulty picking feet, fronts and/or backs - either feet planted to the ground or else trying to snatch feet away. Foot placement tests, turning tests and backing in hand on a hard level surface will tell you a lot. A big tip off is difficuly/reluctance to go a downhill slope. This is all by comparison to your horse's normal, which you should learn.

      If Bodhi's lived here before he may already have been exposed/be immune.

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    2. Thanks Kate, I also found your EPM page-- so helpful. Bodhi already has a lot of those symptoms as a baseline (toe dragging... lazy draft pony). But it's true you must know their normal I guess. I will be on a close watch when we move again. Thanks!

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  3. Part of horse ownership is detective work--trying to stay one step of ahead of any potential illness or injury...and this is a good example. If you can do something preventative to keep Red from getting it, that would be great.

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  4. I am in Washington and we have tons of opossums but I have only known one horse with a case of EPM.

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  5. Yes, it is interesting. Lots of opossums here in New
    Jersey, and I know of at least two cases of EPM. However, I am not sure of the horse's histories. I'm sure there are more around, but I do wonder how much of an immunity the horses do build up over time. Fascinating.

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  6. Hmm it sure is interesting how they both got it after moving from non infected areas. Being from Canada I was unfamiliar with this but now I know a lot more thanks to you and know what to look for if I ever come across a case.

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  7. Interesting. Thanks for the geography lesson. I think all of our horses have been exposed because those little rats (as I call them) are all over the place here.

    Hope all of yours are healthy soon.

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  8. Definitely interesting... it's almost as if those horses raised in an infected area have an immunity of sorts, and those who aren't, are more susceptible. Hopefully the preventative treatment will work for Red so he doesn't become infected again.

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  9. I agree that not being exposed before they moved to where you are could have a lot to do with why Pie has EPM again. When we moved from California to the East Coast, the land of Lyme ticks, both of my horses got really sick from it almost right away. They had no resistance to the disease. I appreciate all your research because EPM is always on my mind around here since my neighbor tells me that she has a mama possum and some babies in her woods. Hope you have healthy horses soon!

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  10. Hi there! i used to live in northern wisconsin on the st. croix river (twin cities was an hour away on the wisconsin side) for over 20 years, and we had LOTS of Opossums. Dead ones, live ones, and quite a few friends with horses who had EPM - Amery, WI, Montevideo MN, and River Falls, WI to name a few locales.

    I work in forestry as a trade, and probably have had more opportunity then the average person to notate when we have seen them. Possums are nocturnal, and hide very well during the day - its only when they are hit on the road does one really see them. Unless you work outdoors ;). Because winters have been very mild in the last 20 years in both Wisconsin and Minnesota, Opossums have been thriving, and by most standards, are there to stay. And not only is EPM alive and well in Wisconsin, but theres the issues of deer carrying leptospirosis which has the ability to cause blindness in horses. Living in the Northern woods is not all cracked up what its suppose to be... :S

    so maybe thats why I moved my horses to Montana ;)...


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    1. Well, there goes that theory for Red and Pie - guess I shouldn't have relied on that map - perhaps it was out of date. We are certainly seeing species migration here with lots of more southern species - birds are most noticable - moving into our area - ranges seem to be expanding north. Pie lived only briefly in MN - he came from Montana, so perhaps that explains something. But then there's Dawn, who's lived in IL all her life and got one strain and seems to be immune to the other. Go figure.

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