Page 932 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

884 SECTION | XIV Poisonous Plants

VetBooks.ir Habitat and Distribution palatable species. Ingestion often occurs for several
months or more before an abrupt onset of neurological
Yellow star thistle is most abundant in the western United
dysfunction is observed. Impaired eating and drinking
States from central California north through Oregon,
are often the first observable signs. Depression and
Washington, and Idaho. Smaller invasions are reported in
hypertonicity of the lips and tongue follow, and a
many states east of the intermountain region, and
constant chewing may be observed, hence the name
although these have not received the attention of the
“chewing disease.” Abnormal tongue and lip postures
larger invasions in the west, they have the potential to
may be observed, and other neurological signs include
rapidly spread under the right environmental conditions
locomotor difficulties such as aimless walking, drowsy
(Panter, 1991; Burrows and Tyrl, 2001; Knight and
appearance, and inactivity with the head held low. The
Walter, 2001). Russian knapweed has invaded very large
neurological disease is considered permanent, and
areas of the intermountain region of the western United
although some improvement may be seen, difficulty
States and the Great Plains. Again, smaller populations
eating and drinking may preclude long-term recovery.
have invaded areas of the Midwest and northeastern
Often, the disease progresses to dehydration, starvation,
United States and have the potential to expand rapidly.
and bizarre behavior including submergence of the head
Although the aggressive nature of these species threatens
in water to allow water to flow into the esophagus or
rangelands and prohibits optimum utilization, a greater
lapping water like a dog. Centaurea repens appears to
threat is the risk to sensitive or threatened native plant
be more toxic than C. solstitialis, but prolonged inges-
species and the balance of plant biodiversity.
tion is required by both before disease appears. The
amount of plant ingested to induce the clinical effects
Toxicology is reported to be 60% or more of body weight for
C. repens and 100% or more of body weight for C. sol-
The compounds isolated from knapweeds include a large stitialis. Intermittent grazing can prevent disease, indi-
class called sesquiterpene lactones. Although the puta- cating that there is not a cumulative effect but, rather, a
tive toxin causing the neurological disease in horses has threshold must be exceeded before neurological signs
not been specifically identified, six of these compounds are observed (Cordy, 1978). Once neurological signs
have been screened for cytotoxicity in an in vitro neuro- are observed in horses, prognosis for recovery is poor
nal cell bioassay. The rank order of activity is and euthanasia should be considered.
repen . subluteolide . janerin . cynaropicrin . acroptil-
in . solstitialin (Riopelle and Stevens, 1993). Toxicity
of solstitialin A-13 acetate and cynaropicrin to primary
cultures of fetal rat substantia nigra cells has been dem- Pathology
onstrated. These sesquiterpene lactones are quite unsta- The lesions are very specific and limited to the globus
ble, and it has been hypothesized that they are pallidus and the substantia nigra (nigropallidal encephalo-
precursors to the ultimate neurotoxin. Also, there are malacia), where distinct pale yellowish to buff-colored
aspartic and glutamic acids present in these plants, and foci or softening and cavitation are seen (Cordy, 1978).
they possess neuroexcitatory properties. The lesions are typically bilateral and symmetrical. This
specificity of the lesions for the basal ganglia has
Clinical Signs prompted more investigations into unraveling the myster-
ies of human diseases associated with dopaminergic path-
Thus far, only yellow star thistle and Russian knapweed ways, such as Parkinson’s or Huntington’s disease, and
have been implicated in toxicoses in the United States tardive dyskinesia. This disease in horses is often called
and only in horses. Apparently, ruminants are not equine parkinsonism. This unusual disease is manifest by
affected, and the Centaurea spp. may be useful forage for an almost immediate onset after prolonged ingestion, sug-
sheep and goats. However, in other countries, toxicoses in gesting an all-or-none type of acute neurological crisis.
ruminants have been reported. For example, in South The lesions develop quickly and completely, and progres-
Africa, C. repens fed to sheep at 600 g dosages for 2 days sive stages of degeneration rarely occur except for some
caused an acute digestive upset and pulmonary edema changes in the adjoining neurons adjacent to the necrotic
and ascites. In Azerbaijan, C. repens is reported to cause foci in the globus pallidus and the pars reticularis of the
a neurological disease in buffalo similar to that which has substantia nigra (Cordy, 1978). Microscopically, there is
been described in horses. However, no neuropathology extensive necrosis of neurons, glia, and capillaries within
similar to that seen in horses was observed in the buffalo. sharply defined margins of the involved brain centers.
Toxicity generally occurs in summer and fall when Occasionally, lesions may be observed in the gray and
forage is depleted and horses are forced to graze less white matter of the brain.