2. Astragalus and Oxytropis spp. (locoweeds).
Of the 500-plus Astragalus and Oxytropis species, about 20 are considered locoweeds (A. lentiginosus, Photo 2a; and O. sericea, Photo 2b).
Photo 2a: Astragalus lentiginosus (spotted locoweed) is a native plant that is well adapted to grow in the dry rocky areas
of North America. It is a prolific seed-producer with long-lived seed banks. A perennial herb (~0.5 m) that has alternate
and pinnately compound leaves, it has pealike flowers of variable numbers on axillary racemes. These develop into pods that
contain small (2-4 mm) kidney-shaped seeds. Though growth patterns vary with location and precipitation, most germinate with
the fall precipitation; remain green through the winter and mature early in the spring when other green feeds may not be available.
Photo 2b: Oxytropis sericea (white-point locoweed).
Horses are uniquely sensitive and readily eat locoweeds, resulting in frequent poisoning. The locoweed toxin, swainsonine,
inhibits cellular enzymes, resulting in impaired cellular metabolism and storage disease.
Signs of poisoning begin after several weeks' ingestion and include depression and reluctance to move. Many animals abort,
have weak foals or foals with birth defects. With continued exposure, animals develop intention tremors, proprioceptive deficits,
loss of condition with irreversible neurologic changes.
With stimulation, poisoned animals become neurologically unstable, with fits ranging from anxiousness to maniacal aggressive
bouts, which are dangerous for the poisoned animal and by-standing animals or caretakers.
Many of the clinical signs resolve when exposure is discontinued, but previously poisoned animals may relapse with changes
ranging from mild proprioceptive deficits to convulsive-like fits.
Diagnosis is easily made by documenting exposure and identifying the characteristic histologic changes. Swainsonine can be
identified in blood and tissues of poisoned animals, but it is quickly cleared within days of discontinuing exposure. New
biomarkers are being developed to better identify poisoning and predict recovery.
There are no treatments for locoweed poisoning. As horses have to ingest locoweed for several weeks to be poisoned, avoiding
prolonged exposures is essential. Many previously poisoned animals can recover and function as reproductive animals. However,
any poisoned animal is likely to have permanent neurologic lesions that will likely impair function. Previously poisoned horses
should not be ridden or worked.
3. Leucoencephalomalacia (moldy corn).
This disease is caused when corn molds, Fusarium verticillioides or F. moniliforme, produce toxic fumonisin. Fumonisin alters sphingolipid synthesis and cell-membrane integrity, resulting in severe, irreversible
neurologic disease that causes characteristic softening or liquifactive necrosis of the cerebral white matter. Once horses
develop the disease, they rarely recover.
Clinical signs are neurologic and range from hypersensitivity, ataxia and posterior weakness to convulsions. Lesions are not
confined to the brain, as high fumonisin doses produce liver necrosis.
Most animals develop disease within a few days of eating contaminated grain. It has been reported that concentrations at or
above 8 ppm fumonisin B1 in feed are toxic. As horses are especially susceptible to fumonisin poisoning, moldy corn should
not be fed to horses. Prepared feeds may also be tested for contamination.
4. Conium maculatum (poison hemlock, Photo 3).
These noxious weeds generally are not palatable, and it is common to see horses walking between tree-sized poison-hemlock
Photo 3: Conium maculatum (poison hemlock) is a biennial, but in favorable locations it may be a perennial. It grows along
fence lines, in irrigation ditches and in other moist waste places. It may be 2 m to 3 m tall. The hollow stem usually is
marked with small purple spots. Leaves are delicate, like parsley, and it has a white taproot. Poison hemlock has white flowers
that grow in small, erect clusters. Each flower develops into a green, deeply ridged fruit that contains several seeds. After
maturity, the fruit turns grayish brown.