A introduced weed that spreads rapidly in cultivated and disturbed soils along roadsides and in fields.
Yellow star thistle is widely distributed in the western States and is spreading.(Maddox Dm et al. Weed Science 33, 315-327, 1985)
Horses appear to be the only animals affected.
Recent evidence indicates that a complex compound isolated from yellow star thistle, and also probably present in Russian knapweed (2,3-dihydro-3,5-dihydroxy-6-(H)
-pyran-4- (DDMP) is responsible for causing nigropallidal encephalomalacia. The compound inhibits the dopamine transporter system of the brain, and uniquely affects horses. The toxin specifically acts on the dopaminergic nigrostriatal pathway that has a coordinating and inhibitory effect on the nerves affecting prehension and chewing of food (cranial nerves, V, VII, IX affected). The facial muscles become hypertonic giving the horse a fixed or wooden appearing facial expression. Affected horses cannot bite-off and chew their food in a coordinated manner. Swallowing however, is unaffected. The plant is toxic in the green, and dry state to horses only.
The toxic dose of green plant is equivalent to the body weight of the horse eaten over a period of weeks.Thus horses can eat the plant without becoming poisoned as long as they do not consume the plant continuously. Horses however find the plant quite palatable.
An annual herbaceous weed, branching from the base up to 3 feet tall. The branches are winged. Leaves are densely haired. The basal leaves are deeply lobed, while stem leaves are small and entire. The yellow disc flowers are surrounded by stiff spines 1-2cm long.
The plant spreads via its seeds that are dispersed when the plant is incorporated in hay. The seeds may also pass through the intestinal tract of horses and remain viable in the manure.
Often referred to as 'chewing disease' because affected horses try endlessly to bite-off, hold and chew food without success. The mouth may be held open and the tongue protrudes or is crled from side to side. Horses act as if they are 'choked'. Mouth abrasions and ulcers may also develop as a result of trauma to the gums and tongue. Weight loss can be severe.
Hypertonicity of facial muscles affecting normal movement of the lips, cheeks and tongue. Weight loss is severe as the horse cannot eat. or drink.
There is no known effective treatment. The brain lesions are permanent. Euthanasia is recommende to avoid the horse starving to death.
Inhalation pneumonia may develop as a result of the animal's attempts at chewing and swallowing food.
Prehension and chewing of food (cranial nerves, V, VII, IX)are affected causing hypertonicity of the facial muscles.
Horses unable to prehend and chew their food are highly suggestive of Russian knapweed and yellow star thistle poisoning. Confirmation of the diagnosis is based upon the presence of typical brain lesions of nigropallidal encephalomalacia.
Malta thistle (Centaurea melitensis) is a similar noxious weed of the South western States but with shorter spines on the bracts. It is not known to be toxic.
1. Farrell RK, Sande RD, Lincoln SD: Nigropallidal encephalomalacia in a horse. J Am Vet Med Assoc 158:1201-1204, 1971.
2. Fowler ME: Nigropallidal encephalomalacia in the horse. J Am Vet Med Assoc 147:607-616, 1965.
3.Larson KA, Young S: Nigropallidal encephalomalacia in horses in Colorado. J Am Vet Med Assoc 156:626 628, 1970.
4. Young S, Brown WW, Klinger B: Nigropallidal encephalomalacia in horses caused by ingestion of weeds of the genus Centaurea. J Am Vet Med Assoc 157:1602-1605, 1970.