Juglans nigra - BLACK WALNUT
TOXICITY RATING: Moderately toxic, depending upon length of exposure.
ANIMALS AFFECTED: Horses, dogs, possibly other animals.
CLASS OF SIGNS: Laminitis, breathing problems, gastroenteritis.
PLANT DESCRIPTION: These familiar trees are recent additions to the list of poisonous plants. Little information is yet available about their toxicity. Black walnuts are large (60-80 foot) forest trees often planted as ornamentals. The bark has characteristic broad, round ridges. The leaves are alternate, pinnately compound, 1 to 2 feet long, with 13 to 23 sharply toothed, tapered-pointed leaflets (fig. 45). Often there is no terminal leaflet. The fruit is a very rough nut enclosed within a clammy glandular husk, 2 to 4 inches in diameter (fig. 45A).
SIGNS: Horses are most often affected. When horses are bedded on wood shavings containing more than about 20% of black walnut shavings (which tend to be dark in color), clinical signs of laminitis (inflammation of the laminae in the hoof) can occur within 12 to 18 hours of contact, but 24 hours may elapse before signs manifest. Consumption of the shavings may cause signs of laminitis as well as mild colic. Affected horses become unwilling to move or have their feet picked up, are depressed, may exhibit limb edema and signs of laminitis. Difficulty in breathing (increased rate and depth) may be noted. Horses on pasture may show mild respiratory signs from pollen or fallen leaves. Poisoning in dogs is reported occasionally when the seed hulls are consumed, causing stomach upset and diarrhea (gastroenteritis).
The toxin causing equine laminitis has not been clearly identified. It was once thought to be juglone, but this is not believed to be the toxin. Juglone is produced by the tree which limits the growth of other plants in the vicinity. Walnuts are also lethal to earthworms.
FIRST AID: Remove black walnut shavings immediately; cooling the hooves and legs with a hose can help. Call a veterinarian if signs are severe or if shavings were consumed. Prompt removal of shavings typically results in complete recovery. Respiratory signs in horses usually do not require treatment unless severe or long-lasting. Stomach upset in dogs will resolve when hulls are no longer eaten.
SAFETY IN PREPARED FEEDS: Reports are not clear concerning the safety of black walnut leaves in processed feeds. The bark and seed hulls are toxic, and these should never be incorporated into any feeds. In the interest of safety, any part of the black walnut plant should not be allowed to contaminate feeds.
PREVENTION: Do not use shavings containing black walnut, limit access of horses to pastures with walnut trees. Purchase bedding shavings only from reputable dealers. Do not let dogs eat walnut hulls.