Dr Andrew Hoffman
One indication to suspect plant intoxication is neurologic signs – typical signs in cases of plant poisonings include blindness, inability to grasp food with the mouth and chew food, ataxia, convulsions, and depression. Horses that exhibit asymmetric (on one side of the body only) signs or spinal cord disease (as in EPM or West Nile Virus infections) are not likely to have plant poisonings.
The most bizarre behavioral changes are caused by long-term ingestion of locoweeds (Oxytropis spp, Astragalus spp) . These horses are hyperexcitable, with a high stepping spastic gait, head bobbing, coupled with severe weakness. There are many non-toxic forms of locoweeds so it is important to make a correct identification. Curious young animals are most susceptible, since developing neurons are most vulnerable to the effects of the toxin, called "swainsonine" (an indolizidine alkaloid).
A dropped jaw and inability to grasp food and swallow is a sign of Yellow Star Thistle and Russian Knapweed (both Centaurea spp) . The mouth is open and the tongue protruds. As previously mentioned, the horse is drooling as well. These horses rarely if ever recover.
Falling forward and weird behavior
Sagebrush (Artemisia) ingestion causes abnormal behavior, and falling toward the front end – you can smell the culprit toxin on your horse's breath. The toxin is a monoterpenoid, so has that volatile smell.
Horsetail is that straight, bamboo-like segmented grass, sometimes found with a rattle-snake tail end that releases spores. You see it along the roads just about everywhere. This plant (Equisetum spp) , which is almost always ignored by your horse despite his wanting to eat everything along the road, makes an enzyme that breaks down Thiamin – your horse becomes short on Thiamin.
Reluctance to move, blindness, ataxia, are signs. Bracken Fern (Pteridium spp) and Sensitive Fern , quite common in woodlands in the USA, also may contain the Thiaminase enzyme. Bracken Fern toxicity is different in horses than in other species – in horses there is progressive depression, hindlimb weakness and ataxia, blindness, recumbency (lying down and unable to rise) and even death. In cattle, a fatal aplastic anemia or bladder tumors are signs of toxicity.
Certain plants contain toxins called "tremetols." These, akin to their name, release tremetols that cause intense whole body tremors, inability to swallow and choking signs. Since the esophagus is made of the same muscle as your horse's legs, it's spastic too. Culprits include White snakeroot, Jimmyweed, Rayless Goldenrod, or Burrow weed.
Sitting or falling when backed, cystitis
Consumption of Sudan or Johnson grasses (both Sorghum spp) affects the back end or your horse, causing weakness and ataxia behind, as well as paralysis of the bladder, anus and tail. Urine dribbles freely from the perineal region. There is loss of sensation around the tail, anus, and vulva, a cardinal sign of this toxicity.