Liver disease is not common in horses, but when it occurs it is usually due to toxins the horse has eaten. Signs of liver disease include decreased appetite, depression, colic, yawning, behavioral abnormalities, and yellowish coloration of the mucous membranes.
Many plants contain a chemical called pyrrolizidine alkaloids, a class of plant toxins associated with disease in humans and animals. Horses ingesting these plants over a long period can develop liver disease.
Some of the plants known to contain this chemical are Senecio and groundsel. Horses do not generally eat these plants unless there is nothing else to eat or it is baled in the hay.
Cocklebur is another plant common in some western states, and at the 2-leaf stage, seedlings contain a large amount of a toxic chemical. The seedlings appear early in the spring before many other plants are growing, and are readily consumed by horses. It is important in the early spring to make sure horses have good quality hay and do not have access to cocklebur seedlings.
Aflatoxin, another common toxin, is produced by molds on multiple different grains, especially when the plants have been stressed by drought or insects. The mold can grow on stored grain if stored in warm and damp conditions, and the toxin can be present without seeing mold on the grain.
Always buy your horse’s feed from reputable sources, making sure that it is stored in a cool, dry location. If the grain has not been evaluated for aflatoxin, you can ask about having it sent off for testing.
Blue-green algae is another toxin concern. Under certain conditions, these microscopic organisms undergo rapid growth called algal blooms on stock ponds and produce a toxin that affects the liver in horses.
If your horse has access to a farm pond with the colored sheen of an algal bloom on the top, remove your horse from this pasture and use another source of water.