Yellow star thistle (Centaurea solstitialis
) is an annual weed found predominately in the western United States. Yellow star thistle is a grayish-green colored plant which has multiple rigid stems extending in all directions from its base. It produces bright, star-like yellow flowers that are shielded by long, spiny bracts. Yellow star thistle has a deep taproot, which allows it to thrive during dry, hot summers and drought periods. Yellow star thistle begins to experience significant early growth in late May to early June and flowers early to mid-July. It can often can go unnoticed until the plant begins to flower, but once the bright yellow, dandelion like flowers bloom, the plant is easily detected.
Yellow star thistle contains eight sesquiterpene lactones that consist of: solstitialin A, repin, subluteolide, acroptilin, janerin, cynaropicrin, lactones solstitiolide and episolstitiolide. These toxins have a cumulative effect on horses, meaning they accumulate in the body over time. Upon ingestion of 85-100% of their body weight results in the appearance of clinical signs of equine parkinsonism
. Ingestion of these plants causes irreversible neurological damage to the horse, and if left untreated, they will usually die from starvation, inhalation pneumonia, or dehydration. Yellow star thistle causes the same clinical signs as Russian knapweed (Centaurea repens)
, but is considered more toxic.