Black walnut tree poisoning occurs when horses come into contact with toxic compounds in the black walnut (Juglans nigra) tree. 
The roots, bark, wood, nuts, pollen and leaves of the tree contain a chemical that is poisonous to horses upon ingestion or skin contact. 
Horses are particularly at risk of poisoning from exposure to black walnut shavings. The hardwood shavings of the tree are sold as animal bedding in North America. 
You can help to keep your horse safe by removing black walnut trees from your pastures and carefully selecting the bedding you use. Call your veterinarian immediately if you suspect your horse has been in contact with or ingested black walnut shavings or products.
Signs of Black Walnut Tree Poisoning
Horses exposed to black walnut products show clinical signs of toxicosis within a few hours or days. In one case report, clinical signs developed 8 – 10 hours after exposure to wood shavings. 
- Breathing difficulties
- Reluctance to move or have hooves touched
- Increased digital pulse and warm hooves
- Edema of the limbs (stocking up or leg swelling)
- Gastrointestinal issues (colic, diarrhea)
- Respiratory distress
- Lethargy and depression
- Loss of appetite (anorexia)
- Fever or increased body temperature
- Increased heart rate
- Convulsions, seizures, muscle tremors
- Unusual abdominal sounds
If your horse displays any of these symptoms or has recently been exposed to black walnut products, contact your veterinarian immediately.
Clinical signs typically resolve a few hours after being removed from wood shavings, and many horses fully recover without long-lasting complications.  However, horses that develop acute laminitis may have a longer recovery period.
Effects of Black Walnut on Horses
Case reports show that bedding a horse on black walnut shavings can lead to toxicosis in only a few hours. 
Laminitis is a serious and painful condition of the equine hoof characterized by tissue inflammation and damage. Several underlying factors are known to contribute to the condition. 
Finger-like protrusions of tissue called laminae are found between the hoof wall and coffin (pedal) bone. In the healthy hoof, laminae act as an anchor, preventing the coffin bone from sinking down or rotating in the hoof under the weight of the horse.
However, exposure to black walnut toxins can trigger acute lamellar inflammation and separation from the hoof wall.  Clinical signs can worsen quickly, and immediate intervention is necessary to prevent long-lasting damage.
Researchers believe that a toxin from walnut shavings is absorbed through the skin around the coronary band, increasing blood pressure in the hoof and inducing acute laminitis. 
While acute laminitis can be treated, a horse that develops the condition once will be at increased risk of getting it again. Lifelong management changes are required to support the horse.
Coffin Bone Displacement
If the laminae become severely weakened, the coffin bone can detach from the hoof wall and rotate or sink down in the hoof capsule towards the sole.
Coffin bone detachment causes severe pain and can permanently alter the hoof structure. This advanced stage of laminitis is referred to as founder.
Horses with advanced laminitis may have uneven hoof wall growth and dropped soles wherein the natural arch of the hoof decreases.  In severe cases, the coffin bone can protrude through the sole of the hoof.
If your horse is showing symptoms of laminitis or founder, call your veterinarian immediately.
Horses that ingest toxins found in black walnut could be at risk of developing liver disease. Liver problems can also occur if the horse consumes mycotoxins from mould in walnut hulls.
The liver plays an important role in filtering and breaking down waste products, toxins, hormones, drugs and other chemicals in the horse’s bloodstream. Consuming large concentrations of toxins can damage the liver, affecting the horse’s ability to remove toxins from the blood.
If you believe your horse has ingested a toxic substance, call your veterinarian immediately for treatment.
Black Walnut Tree Identification
The black walnut tree (Juglans nigra L.) is a hardwood tree known for its dark brown lumber and edible nuts. 
It can be identified by its straight trunk, dark and ridged bark and coarse branches. The tree has compound leaves and produces green and rounded clusters of nuts in the fall.  The husks of these nuts turn brown after falling from the tree.
Black walnut trees can be found throughout the central and eastern parts of the United States, southern Canada and in other regions at low elevations. These trees grow in moist and rich soils and are rarely found in dense woods. 
Shavings & Bedding
Black walnut can be found in some livestock bedding materials, but should never be used as bedding for horses. Walnut shavings are dark in colour and have small pores.
Research shows that shavings containing more than 5-20% black walnut are toxic to horses.  Aged walnut shavings that have been exposed to air for several months are less dangerous than fresh shavings, but should still be avoided by horses.
If exposed to walnut shavings in a stable or group environment, multiple horses will likely develop symptoms of poisoning simultaneously. 
Toxicity to Horses
Researchers have identified several chemicals in black walnut trees which may be poisonous to horses.
Some toxic effects are attributed to the chemical juglone, while others are attributed to a component in aqueous Black Walnut Extract (BWE).
Traces of juglone can also be found in other trees such as the butternut tree (Juglans cinerea), hickory tree (Carya spp.), pecan tree (Carya illinoinensis) and English walnut (Juglans regia). 
The soil within a 50 – 60 foot radius of the tree can also become contaminated with juglone as it is excreted from the root system.  It has low water solubility, so it does not spread over large distances. 
Juglone is known to have allelopathic effects on other plants and animals, negatively influencing their growth and survival. 
Effects on Horses
Juglone was presumed to be the toxic agent responsible for triggering laminitis, but this chemical does not consistently induce laminitis in experimental models.
In one study, topical application of juglone to feet resulted in skin irritation but did not produce laminitis. 
Other studies have found that juglone is not responsible for triggering laminitis. 
Researchers now believe laminitis is induced by a component in black walnut heartwood. Heartwood refers to the dead, inner wood found in the tree’s center.