The Braken Fern, or Eagle Fern is common across the United States, although not likely to be found in desert ecosystems. A horse that has ingested Braken Fern may exhibit signs of neurological distress, such as uncoordination, blindness, and depression.
It generally takes a large quantity of Braken Fern consumption for a horse to be effected, as the toxin (Thiaminase- which inhibits the absorption of Thiamin) is fairly low. However, some horses seem to enjoy Eagle Fern, and will search for it above other foraging options.
If you suspect your horse has ingested Eagle Fern, contact your veterinarian immediately. If you catch it before the signs are too severe, your veterinarian may be able to administer medications, such as Thiamin, to reverse the effects.