Plants poisonous to horses: the neurotoxic variety - DVM
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Plants poisonous to horses: the neurotoxic variety


Treatment involves removing brackenfern from the horse's diet and treating with thiamine. Most animals need repeated thiamine treatments over several weeks for recovery.

  • 7. Sorghum spp. (Sudan, Johnson grass, sorghum and hybrids).

All these forages are used for both pastures and hay. When stressed, they accumulate cyanogenic glycosides and nitrates. Stored feeds and hay generally are safe, as the glycosides are hydrolyzed and the resulting cyanide dissipates.

After a week to six months (usually about eight weeks) of ingestion, poisoned horses lose control of the hind legs and bladder (equine cystitis-ataxia syndrome). Affected animals stumble and fall with rear-limb paresis and proprioceptive deficits. The nerves that innervate the urinary bladder also are damaged and the resulting paralysis causes incontinence, with cystitis, urine dribbling and urine scalding of hind legs. Abortions and fetal deformities (multiple arthrogyrposis) also have been linked to these forages. Poisoned animals may not recover.

The exact cause of poisoning has not been determined, though chronic exposure to hydrocyanic acid and lathyrogenic nitriles are likely causes. It probably is best to avoid exposure to these forages when they are drought-or frost-stressed. If there is a question of toxicity, most animal-diagnostic laboratories have inexpensive assays to measure cyanide-producing potential.
  • 8. Stringhalt.

Photo 6: Hypochoeris radicata (Flatweed, false dandelion) is a perennial weed originally from Europe. It commonly grows in disturbed soils or overgrazed pastures. It resembles the common dandelion, with basal clusters of irregularly lobed 7 cm to 30 cm leaves. he branching, flowering stalks grow up to 0.5 m with a single yellow flower terminally.
Stringhalt is an abnormal gait that generally is seen as goose-stepping or high-stepping of the rear legs. A couple of toxic plants have been associated with this disease. Chronic ingestion of Lathyrus hirsutus (singletary pea) seeds is thought to cause stringhalt. Hypochoeris radicata (Flatweed, or false dandelion, Photo 6) also has been implicated as the cause of stringhalt on several different continents.

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The lesions of stringhalt are non-specific and related to axonal degeneration in nerves that supply the pelvic limbs. Several surgeries have been used to minimize the hypermetric gait, but there is no proven treatment for stringhalt.

Stegelmeier is a researcher at the U.S. Department of Agriculture's Poisonous Plant Research Laboratory in Ogden, Utah.


Source: DVM360 MAGAZINE,
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