Page 1130 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1105

VetBooks.ir no recovery from chronic locoweed poisoning and usually been ingesting the plant for weeks to months
these animals continue to show signs of neurological
prior to the appearance of signs. The toxic agent of
these plants is known as repin, a sesquiterpene lac-
disease when excited.
tone with high affinity for neural tissue. Studies in
NIGROPALLIDAL ENCEPHALOMALACIA rats suggest that repin exerts its neurotoxic effect by
inhibiting dopamine release. Other neurotoxic com-
Definition/overview pounds, including glutamic and aspartic acids, have
Nigropallidal encephalomalacia is a plant toxicosis also been isolated from Centaurea plants.
that has only been reported in horses, although don-
keys and mules may also be susceptible. Clinical presentation
Young horses (<3 years old) appear to be more fre-
Aetiology/pathophysiology quently affected, but it is not apparent whether this
Yellow star thistle (Fig. 10.57) (Centaurea solstitia- relates to age susceptibility or more frequent inges-
lis) and Russian knapweed (C. repens) are capable of tion of the plants. The onset of signs is acute, and a
causing this disease. Malta star thistle (C. meliten- sudden lack of coordination of facial and oral move-
sis) may also be capable of causing this disease, but ments is most apparent. Effective eating or drinking
this has not yet been confirmed. Yellow star thistle is not possible, although many affected horses display
is an aggressive weed found mainly in the USA, in ineffective chewing movements (Fig. 10.58). Other
California, Oregon and Idaho. Russian knapweed signs are head oedema, yawning, ataxia, muscle
has a broader distribution throughout the inter- tremors, hypertonicity of the lips and tongue giving
mountain states of the USA. These plants are not a fixed facial expression and involuntary lip twitch-
considered palatable, but some animals appear to ing. Death, if it occurs naturally, is usually a result
develop a taste for the plant and will consume it pref- of starvation, dehydration or aspiration pneumonia.
erentially. Most poisonings occur in the spring when
the plant is young and green, but it remains toxic Differential diagnosis
when dry and thus poisoning can occur as a result EPM; dental abnormalities; fractured jaw; glossitis;
of ingestion of contaminated hay. Up to twice the pharyngeal abscess/neosplasia; oesophageal obstruc-
horse’s body weight has to be consumed before signs tion; guttural pouch pathology; lead toxicosis; botu-
of toxicity develop and, as a result, the animal has lism; arboviral encephalitis.

10.57 10.58

Fig. 10.57 Yellow star thistle
(Centaurea solstitialis). (Photo
courtesy WA State Noxious Weed
Control Board)

Fig. 10.58 Horse with
dysphagia secondary to ingestion
of yellow star thistle.