Depends how much is eaten. Vitamin B1 is the treatment. Most animals won't eat it unless there is nothing else but young animals may and some sheep I had once seemed to have a taste for it
I understand they have to consume large quantities for it to have an affect, producing what is called 'bracken staggers'.
We have a small amount of bracken in our top field and the horses occasionally nibble at it to no apparent harm. However some horses can get a 'taste' for it, which is when the plant needs to be fenced off or removed.
Thanks everyone, have enlarged their field and included a bit which has a little bit of bracken growing in it. Seems like I don't have to be too worried, but should get rid of it when I see it.