From: Heather
Hi Jessica; I am sending this to you to see if you can make heads or tails of the symptoms. At the barn where I worked last summer, a mare, ASA Gypsy Lee, died of what they believed to be some sort of poisoning. Her symptoms started as a blindness (she would just run into the walls and couln't seem to figure out what was happening so would keep running into them). No one was there. By the time I got there, not having any clue what was happening, she was bleeding from one nostril and one eye from hitting her head on the walls, she was sweating and shivering and very shaky on her legs. She was also urinating often. I pulled her away from the wall and walked her (very slowly) to the middle of the arena and held her while the trainer (who arrived at the same time I did) went to find the owners so they could get a vet out. Any time I let go of her she started staggering, so I just kept holding her. As I held her head, she was alert and listening to what I was saying to her and the other people as they came in as well as to her foal, but was obviously blind and in a lot of pain. I'm sure if she hadn't been such a calm mare (I was training her for their youngest daughter to ride, she was just a sweetheart and very patient), she would have been a lot harder to handle. When the vet got there, he gave her an IV because she was so dehydrated and also gave her a tranquilizer. She went down and I held her head (crying the entire time) as they tried to figure out what was happening, but decided she needed to go to the U of M vet school. Since she was tranquilized, they had to pull her into the trailer. She was still alive when they got there, but they put her down within an hour as there was nothing they could do for her. The tests they ran on her showed liver failure, but they never could pinpoint exactly what it was from. There was a theory that the pasture was being over grazed and the horses were forced to eat a weed they normally wouldn't eat. Would alsike clover cause this kind of reaction? She was a 15 year old Arabian mare with only a small star (black skin underneath, so don't think it would have shown the blistering reaction even if the clover was the culprit). They had one other mare too, a chestnut, that had the blistering and scabbing on her white socks and strip. She got really dehydrated too and ended up at the vet hopsital before it was really bad. She is fine now, I think because there was more warning with her. Was it alsike clover or something else? Does you have any ideas? Sorry this got so long, it was horrible to have her die and not have any real idea why it happened. She did leave behind one very tough two month old black Arabian filly (she came over and nuzzled her mom right before we loaded her and turned around and walked out into the pasture without so much as a neigh, it was like she knew and was saying bye to her). She never even acted like a foal that was being weaned, not a whinny or running the fence looking for mom, she just joined the rest of the herd (which was unusually quiet for the next few days). Any ideas on what would cause this?
Heather (remembering with sadness all those fun bareback rides on a very patient and sweet broodmare who would have made an excellent kid's horse and best friend)
The problem only seems to arise in hot/humid weather -- horses can graze alsike under dry conditions without becoming ill. Why this happens when the conditions become hot and humid isn't quite clear - - no particular toxin has been isolated. But what seems to happen is, unfortunately, clear: affected horses develop a characteristic photosensitization, especially in areas where there are white markings (light-skinned areas) or thin skin. Feet, lips, and noses are generally affected; because of this, and because the problem is associated with damp conditions, some people refer to this as "dew poisoning." The current belief is that the humid conditions produce certain mycotoxins or plant metabolites, and that these are what cause the disease.
In any case, the photosentization is often accompanied by signs of liver disease. Horses in this condition may show icterus (yellow or discolored mucous membranes, noticeable if you look at their gums and at their eyes). They may show other characteristic signs of liver dysfunction and pain -- yawning and "parking out" like show Morgans or Saddlebreds, but in this case the backward stretching of the legs is a reaction to liver pain: an attempt to "back away" from the sensation.
Horses that are found early and taken off the pasture usually manage to recover, and once the pasture is dry again, they can usually graze it without a recurrence. Sometimes years will go by without the conditions being "right" for this problem; many horses graze on alsike clover without ever developing liver damage.
The blindness, though, isn't typical, and neither is the physical incoordination. Those sound like symptoms of nervous system disorders, and they can indeed be caused by the ingestion of certain plants. The difficulty lies in knowing which plant and when it was eaten -- and there are other possibilities. Neurologic disease can be caused by a plant or plants, but it can also be a congenital disease -- or an infectious one. Was this mare the only horse affected in this way, or did the chestnut mare show changes in her liver enzymes too? Perhaps the mare that died was the only one in the pasture to eat a toxic plant, but perhaps it wasn't the pasture that caused the problem in the first place.
Liver disease is very tricky, because the liver doesn't work less and less well as it becomes more damaged; it can go on doing its job until MOST of it is damaged. This makes it almost impossible to notice the progression of liver disease -- the liver tends to work at full capacity until the damage is so wide-spread that it more or less stops working at all. In other words, your horse could be suffering from a diseased liver and still be completely normal in appearance and performance -- up to the point at which there was only, say, 31% of the liver still working, and then, with one more percent loss, and only (say) 30% of the liver working, at THAT point the balance would have been tipped from "enough functional liver to do the job and keep the horse healthy" to "no longer enough functional liver to do the job and keep the horse healthy." The shift can take place literally overnight, and by the time you KNOW that there's a serious liver problem, you may not be able to do anything about it.
If you suspect liver trouble, you can watch for overt physical symtoms, as above, and you can ask your vet to do bloodwork on the horse and have the liver enzyme levels checked. You can also ask your county extension agent about having your pastures checked for known toxic plants, and you can have the water on the property checked for toxins. It isn't always easy to identify the cause, though. Toxic plants can come up one year and not the next -- and the initial insult to the liver could have occurred at any time during the horse's life: last year, or the year before, or ten or fifteen years ago, in another pasture, at another stable, even in another state.
So, Heather, this is a long discussion which really comes down to "I don't really know the answer to your question." We just don't know enough about the liver, and liver disease, and the various neurologic diseases -- yet. I hope this information helps a little -- at least it tells you what conditions might make that alsike clover pasture dangerous -- and I certainly hope it never happens again!
- Jessica
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