For heaven's sake there are numerous medications you can take &, given the fact that Catherine regularly attends the Polo with William, I'm pretty sure she already takes something to alleviate the symptoms. I would hazard a guess that they are not really that severe. She has also been seen regularly at the races so it can't be as bad as people on these boards are making out.
For heaven's sake, Wymanda: we are talking a closed carriage here, which is housed normally not far away from the stables and has been used in connection with horses for generations. A carriage where already somebody had problems with allergies before.
You're not Catherine's doctor - I'm neither, but I suffer from heavy allergies and the idea to come that close to animals I'm allergic to in a surrounding which is potentially poisoned by their hairs is simply terrible. Of course they could clean the carriage carefully and try their best but why should they if Catherine can come safely by car? Why should they take this risk?
The fact that she attends polo, raxces and can ride in open carriages is not a contradiction per se. Was she ever seen at the races in a stable or at the "ring" - so in really close contact to the horses? Or was she just a spectator further away? I've only ever seen pics of her further away from the horses and can't remember any hugs given to William in his riding clothes.