OK - I'm going to have to answer with more questions, sorry about that! In general, I believe pred is known for causing an increase in appetite, not a decrease. Why is the horse on pred? Allergies?
When was the last time he was seen by a vet? Sounds like he needs to be seen now. Unless I miss my guess, he probably has an elevated temp, and at the very least, sounds like he feels really bad.
Usually when a horse starts losing weight, we look first at the teeth. In this case, I think it would still be a good idea but not for the usual reasons! Though there may be some contributing problems due to overgrown points on his teeth, his behavior suggests something that is also making him feel sick/feverish. A possible abcessed tooth could be a potential cause.
Beyond that, I'd look towards bacterial/viral infections - is there any nasal discharge? Is he dehydrated? What do his gums look like? Are they moist? Sticky? Dry? Pink? Grey? Bright red? All this can tell you a bit about what might be happening on the inside. Bloodwork done by a vet would be your best diagnostic tool.
Besides the fact that your horse is obviously sick, another reason to get the vet out pronto is that if he does have an infection, the pred could be making it worse. And I'd get the vet out today, 100-200 pounds in that short of time could mean something REALLY serious is going on!
One other thought, why is he on food supplements? A balanced feed should be adequate for most horses. Unless your vet says this horse has this problem and therefor needs this supplement, you could be messing him up even more by skewing his nutritional intake with supplements!
Good luck with him, please let us know what the vet finds out.
PH Cody
HorseHobbyist Community Director
