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NEUROLOGIC DISEASE INDUCING PLANTS
Behavioral alterations, blindness, inability to ingest and chew food, incoordination, depression, convulsions, and other physical abnormalities are all indicators of nervous system disorders. The brain, spinal cord, and peripheral nervous systems are susceptible to a variety of infectious, toxic and congenital diseases that are often indistinguishable clinically. A variety of plants that grow throughout North America are known to produce neurologic abnormalities in horses and, therefore, should be considered in the differential diagnosis of nervous system disorders.
Locoweeds and Milkvetches
Milkvetch Neurologic Poisoning
Nitroglycosides or nitrotoxins have been demonstrated in some 263 species of milkvetch (Astragalus spp.). These plants are found growing in vast areas of rangelands in the western United States, Canada, and northern Mexico, and have been associated with severe livestock losses. Horses, although not frequently affected, are susceptible to poisoning by these nitrotoxins.
Nitrotoxin-containing Astragalus contains at least two toxic compounds. These toxins, once absorbed from the digestive tract, act primarily on the respiratory and central nervous system, causing initially depression, incoordination, and hindleg weakness. Difficulty in breathing, weight loss, and paralysis of the hindquarters develop as the animal continues to eat milkvetch. Animals appear to recover if they are removed from the source of the plants before neurologic signs become too severe.
Locoweed Neurologic Poisoning
Locoism results from the cumulative effects of the toxin swainsonine, named after its isolation from various species of Swainsona (Darling pea) in Australia. It has subsequently been demonstrated in Astragalus lentiginosus, A. mollisimus, and A. bisulcatus, amongst others, and Oxytropis lambertii, and O. sericea in North America. Signs of poisoning do not become evident until animals have consumed significant quantities of locoweed over many weeks and the toxic threshold is reached. Some horses develop an addiction for locoweed, seeking it out even when normal forages are present. Young animals are most severely affected, as maturing nerve cells are more vulnerable to the effects of the toxin. The toxin causes a generalized lysosomal storage disease, eventually causing irreversible nerve cell damage similar to the disease mannosidosis in people.
The most noticeable feature of locoism is the change in the normal behavior of affected horses. Incoordination, high stepping gait, head bobbing, marked excitement, and overreaction to various stimuli are typical of locoism. Some horses become totally unpredictable in their response to handling and may fall down when being haltered or ridden. When left alone they become progressively depressed and lose weight due to their impaired ability to take food into the mouth. If removed from the source of the locoweed and fed a nutritious diet, horses will show improvement and appear relatively normal after several months. However, if the horse has been chronically affected by locoism, the animal will recover only partially and will remain a liability to human safety. The prognosis for locoed horses therefore is always poor.
Pregnant mares that consume quantities of wooly loco (A. mollisimus) in early gestation may produce foals with various limb deformities. The teratogenic effects of the Astragalus will be discussed further in the section on teratogenic plants.
There is no proven effective treatment for locoweed poisoning in horses. Further access to the plants should be prevented immediately and in every year thereafter, as horses may retain an addiction to the plants from year to year.
Yellow Star Thistle and Russian Knapweed
Horsetail, Marestail, Horserush, or Snake Grass
White Snakeroot and Crofton, Jimmyweed, and Burrow Weed
Bracken Fern
Johnson and Sudan Grasses
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