ORIGINAL: feddoc
The physiology ofa horse is not much different from us humans. In other words, they should acclimate themselves to altitude like we do...plenty of water, not a lot of strenuous riding the first day or so. If you can, stop and pull them out of the trailer and give em a break too.
not quite accurate...
horses are dramatically different atleast as tohow they process oxygen, they are not nearly as prone to altitude issues...horses can put about twice the amount of oxygen in their blood than humans, causing them to be less prone to these issues, partially caused by a much larger spleen to store and oxygenate red blood...
the bigger issue to be most concerned about is dehydration...especially with the long drive...
having carted my critters all over the country hunting the elusive wapiti (2.5k to 14k feet elevation), the most critical things (besides the legal ones above) are rest and water...some horses are NOT good travellers...would highly suggest hay bags or if you have a trough, keep it full for them, but then you have to make sure you get them lots of water at your stops...get them out, walk them around...let them rest out of trailer for awhile...with good access to 5 gallon buckets of water...
water is your biggest issue IMHO, they can live wihtout food for awhile, but you cannot let them be without water...