Black Walnut
Black walnut (Juglans nigra) is a widely distributed, large native tree of North America growing 30 to 65 ft (10 to 20 m) in height. It is extensively used for its wood, aromatic oils, and edible nuts. Its wood is a rich, dark-colored, closed-grain hardwood commonly used for furniture and gunbarrel stocks. Horses are poisoned by contact with black walnut wood shavings or sawdust that may be present in bedding. The shavings range in color from purplish-black to coffee-brown and especially when freshly cut have a distinctly sweet or acrid odor, which indicates it is most toxic. In contrast, pine, fir, ash, oak, and most softwoods commonly used for bedding are all light colored. Bedding containing as little as 5 to 20% black walnut shavings can cause poisoning in horses.
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The toxin responsible is not known. Juglone, a naphthoquinone present in the roots, bark, nuts, and pollen of a variety of the walnut tree family has been proposed as the toxin. The variable severity of signs associated with black walnut shavings is poorly understood, but is apparently not related simply to contact of the horse's skin with the walnut shavings. The poisoning is due in part to the ingestion or inhalation of a toxic substance present in black walnut.
Clinical signs begin 8 to 24 hours after contact with black walnut shavings or sawdust. They are characterized by decreased feed intake, lethargy, depression, and/or edema of the lower legs, which in severe cases may extend up to the chest. Colic, respiratory distress, and rarely death may occur. The most common symptom is varying degrees of lameness due to founder (laminitis). The laminitis results in foot pain, foot warmth, and increased digital pulse pressure. If affected horses are removed from the source of the black walnut shavings early enough and treated for laminitis, they recover without the severe laminitis consequences of hoof deformity and rotation of the toe bone (third phalanx).
Although not all dark-colored wood shavings are black walnut, it is safest not to use any wood shavings that contain any dark wood unless it is known for certain that it is not black walnut. Poisoning of horses in pastures containing black walnut trees is also a concern, particularly during pollen shedding in the spring, and in the fall when the leaflets are shed from the walnut leaves.
A recently identified syndrome of leg edema, fever, and laminitis of horses eating hoary alyssum (Berteroa incana) has been reported. The symptoms resemble those of black walnut poisoning. Affected horses generally recover once they are removed from the plant source. Hoary alyssum is a pervasive weed of the northeastern and north central United States and Canada.
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