This is the second in a series of articles introducing the toxic plants most likely to poison horses in North America.
Horses are relatively selective grazers and poisoning is comparatively rare, but when feed selection is restricted or when
toxic plants are included in prepared feeds, many horses are poisoned.
Unlike the hepatotoxic plants described in the first article, most neurotoxic plants are palatable and produce specific, clinical
disease and histologic lesions.
Here are some of them, along with associated clinical signs, lesions and sequelae of poisoning and recommendations for treatment
1. Centauria solstitialis (yellow-star thistle, Photo 1a), C. repens (Russian knapweed, Photo 1b), and C. melitensis (Malta star thistle).
Knapweed or yellow-star thistle intoxication is a disease of neglect, as horses must be forced to eat them. Poisoning occurs
when horses are kept on a small pasture with limited feed.
Photo 1a: Centauria solstitialis (yellow-star thistle) is a Mediterranean weed that has become established in many western
and southern states. It is invasive and dominates many roadsides and disturbed areas. Yellow-star thistle is a branching annual
with finely haired leaves that are lobed basally and linear on the stem. The disc flowers are yellow and the bracts are tipped
with stiff yellow spines. Photo 1b: Centaurea. repens (Russian knapweed) invades fields, pastures and roadsides. It is a persistent,
noxious weed that grows in all soil types, spreads by both seeds and rhizomes and is allelopathic. A perennial, Russian knapweed
is generally erect (~1 m tall) and the stems are covered with fine hairs. The leaves are alternate with serrated margins.
The flowers form thistle-like heads (1 cm) and vary from white to purple. The paper-like bracts have no spines. Most seeds
remain on the seed head, which is easily spread by animals.
As other forages are exhausted, some horses develop a taste for knapweed and may continue eating it even when supplemented
with other forage.
At doses of 50 percent to 200 percent of their body weight over 30 to 90 days, horses develop dysfunction of facial, mouth
and throat nerves and muscles.
This poisoning has been called "the chewing disease" (horses chew but can't swallow).
Next comes facial-muscle hypertonicity that causes "smiling," tongue lolling, protruding tongue and head tossing.Trying to
drink, some horses submerge their muzzle in the water.
These early neurologic signs become worse and lead to lethargy, loss of interest in food, dehydration, malnutrition, difficulty
breathing, incoordination, muscle tremors and severe depression.
Lesions include those of dehydration and starvation, in addition to damage to specific parts of the brain (the substancia nigra and globus pallidus — thus the name negropallidal encephalomalacia).
Horses are uniquely susceptible to this disease. The long exposures and lack of a smaller animal model have made it difficult
to positively identify the toxin.
As there is no treatment and the disease is irreversible, it is best to avoid exposure to the plants for prolonged periods.