Black walnut tree poisoning occurs when horses come into contact with toxic compounds in the black walnut (Juglans nigra) tree. [2]

The roots, bark, wood, nuts, pollen and leaves of the tree contain a chemical that is poisonous to horses upon ingestion or skin contact. [2]

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. [2]

Horses that come into contact with black walnut shavings, sawdust or tree materials can develop mild to severe symptoms and acute laminitis in only a few hours. [10][15]

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. [20]

Symptoms can present differently in horses and range from mild to very severe. Common signs include: [2][15][20]

  • Nervousness
  • 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. [20] 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. [20]

Researchers suspect that a compound in black walnut extract triggers a systemic inflammatory response in the horse, resulting in adverse effects such as lameness and laminitis. [23]

Laminitis

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. [6]

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. [10] 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. [14]

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. [11] 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.

Liver Disease

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

Black Walnut Tree and Laminitis in Horses

The black walnut tree (Juglans nigra L.) is a hardwood tree known for its dark brown lumber and edible nuts. [3]

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. [15] 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. [1]

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. [14] 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. [14]

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).

Juglone

Juglone (5-hydroxy-1,4-naphthalenedione) is an organic compound that can be found in the roots, hulls and fresh leaves of Juglans trees, such as black walnut. [5][12]

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). [12]

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. [4] It has low water solubility, so it does not spread over large distances. [5]

Juglone is known to have allelopathic effects on other plants and animals, negatively influencing their growth and survival. [5]

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. [17]

Oral administration of large amounts of juglone did cause mild laminitis in some ponies. [17] However, in field studies toxicosis is reported following skin contact and not ingestion. [15]

Other studies have found that juglone is not responsible for triggering laminitis. [18]

Aqueous Extract

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.

An aqueous (water-based) extract from the heartwood reliably causes laminitis in experimental models. [19][21][22]

The aqueous black walnut extract increased vasoconstriction when tested directly on isolated equine arteries and veins. [19] In another study, BWE was shown to cause laminar inflammation. [21]

Mycotoxins

Penitrem A is a toxin found in mouldy walnut hulls from the black walnut tree. Ingestion of mouldy hulls can cause serious acute illness that requires treatment.

Penitrem A is a tremorgenic mycotoxin produced by fungi and known to cause neurotoxic effects in several species.

Exposure to mouldy food containing Penitrem A has led to outbreaks of neurological disorders in horses, cows, sheep and dogs. This compound affects the central nervous system and causes trembling and potential brain damage. [9]

If you think your horse has ingested walnut hulls and is demonstrating clinical signs of toxicosis, call your veterinarian immediately.

Prevention

Bedding Alternatives

The best way to prevent black walnut poisoning is to choose an appropriate bedding material for your horse.

Good bedding is absorbent, cost-effective, comfortable and relatively easy to clean. Regular inspection of all bedding materials is recommended.

Straw, shavings and peat moss are popular bedding materials in North America. [7] Softwood shavings, such as pine, Douglas fir and spruce are also safe, cheap and comfortable bedding for horses.

Other dangerous shavings that should be avoided are cypress and red maple as they could cause severe skin rashes and irritation.

Only purchase shavings from reputable suppliers and avoid suppliers that handle Black Walnut hardwood products. If traces of black walnut shavings are included in softwood shavings, these products should not be used for horses. [2]

Black Walnut in Pasture

Keep horses away from pastures containing black walnut trees, especially during the spring when the pollen is shed. [5] The pollen can cause allergic reactions in horses and humans.

Research shows that a mature black walnut tree has a toxic zone extending 50 to 60 feet from the trunk. [4][5] Inside this radius, the tree’s root system secretes juglone into the soil, which can be detrimental to plants and livestock in the area.

Removing toxic trees and plants from your pasture can help to keep your horse healthy. Hire a tree service to remove black walnut trees and roots from your pasture and ensure the mulch is removed from your property. Keep horses away from the sawdust to avoid toxicosis.

If trees cannot be removed, build a fence around the perimeter of the toxic zone to prevent horses from ingesting fallen walnuts, hulls, leaves and bark.

Some horses chew on tree bark and branches if turned out in a pasture with trees. These behaviours may help horses meet their fibre requirements and reduce boredom. [16]

Chewing and stripping bark can be dangerous if pastures contain toxic trees. Chewing the bark of black walnut trees and consuming soil in the toxic zone can quickly lead to poisoning.

Branches should be trimmed or pruned regularly and kept out of reach of the horse. Any fallen branches or nuts should be removed promptly from the pasture.

Diagnosis

To diagnose a horse with black walnut tree poisoning, your veterinarian will conduct a physical exam and take note of presenting symptoms. An evaluation typically consists of the following: [13]

  1. History of pre-existing or co-existing conditions or diseases
  2. Identification of the cause of symptoms
  3. Identification of any existing complicating factors

Physical Evaluation

Your veterinarian will examine all four hooves for physical characteristics indicative of laminitis before palpating the digital pulse and evaluating the hoof wall temperature.

Several clinical signs can indicate acute laminitis, including:

  • Increase in skin temperature of the pastern and hoof wall
  • Swelling and edema of the coronary band and pastern
  • Sensitivity around the frog, sole and across the foot

The veterinarian may use large pincers designed to apply pressure to specific parts of the hoof (hoof testers) to test for sensitivity, pain and structure integrity during this examination.

Radiographs

A standard radiograph is recommended to assess horses with severe symptoms of laminitis. A radiograph can produce a precise lateral view of the hoof and determine whether the coffin bone has sunk or rotated due to black walnut poisoning. [13]

While the radiograph is not an emergency procedure, it can be helpful for developing a treatment plan that is specific to your horse.

Treatment

If your horse shows signs of black walnut poisoning, contact your veterinarian immediately. Identify and remove potential sources of toxins.

Immediately remove shavings containing black walnut from your horse’s stall to alleviate symptoms and lower the risk of developing laminitis. Thoroughly wash your horse’s legs with soap and water to eliminate traces of black walnut and prevent further absorption of toxic compounds.

Supportive treatment should begin as soon as possible under the supervision of a veterinarian. if your horse shows warning signs of laminitis, follow our Emergency Protocol for Laminitis in Horses.

Pain Relief

If your horse is demonstrating signs of acute laminitis or toxicosis, pain-relieving medications may be prescribed or administered by a veterinarian. [15]

Mild sedatives or non-steroidal anti-inflammatory drugs (NSAIDs), such as phenylbutazone (Bute) or flunixin meglumine (Banamine), may be administered to horses for pain relief.

In rare cases, beta-adrenergic blockers (prazosin) or calcium channel blockers (nifedipine) may be used as a treatment to reduce blood pressure. While the use of these drugs is fairly new, they have demonstrated promising results in treating black walnut toxicosis. [14]

Bathing and Icing

Horses exhibiting clinical signs of black walnut poisoning often have swelling of the legs.

Applying ice packs to the legs under an adhesive bandage, cold-hosing affected limbs or soaking the limbs in ice baths can reduce inflammation. [15]

Hoof Support

Horses with acute laminitis due to black walnut poisoning can develop chronic laminitis with permanent changes to the structure of the hoof.

To support recovery, these horses may require corrective trimming and shoeing to keep the hoof wall at equal lengths and reduce pressure in the hoof.

Several support materials are used to distribute the horse’s weight across the foot instead of relying on the frog alone to withstand pressure. [8] These materials include:

  • Thick and soft bandages
  • Styrofoam
  • Silicone impressions
  • Commercial pads or boots

Contact your veterinarian and farrier to determine a treatment plan for your horse when dealing with acute laminitis due to black walnut poisoning.

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References

  1. Brooks, M. Effect of black walnut trees and their products on other vegetation. West Virginia Agricultural and Forestry Experiment Station Bulletins. 1951.
  2. Cassens, D. L. & Hooser, S. B. Laminitis Caused by Black Walnut Wood Residues. Purdue Extension. 2005.
  3. Dana, M. N. & Lerner, B. R. Black walnut toxicity. West Lafayette, IN, USA: Department of Horticulture, Purdue University, Cooperative Extension Service. 2001.
  4. Feeley, C. Black Walnut: The Killer Tree. Iowa State University Extension and Outreach. n.d.
  5. Funt, R. C. & Martin, J. Black Walnut Toxicity to Plants, Humans and Horses. Ohio State University. n.d.
  6. Hunt, R. J. Equine Laminitis: Practical Clinical Considerations. American Association of Equine Practitioners (AAEP). 2008.
  7. The National Farm Animal Care Council (NFACC) Code. Code of Practice for the Care and Handling of Equines. NFAAC. 2013.
  8. O’Grady, S. E. & Parks, A. H. Farriery Options for Acute and Chronic Laminitis. American Association of Equine Practitioners (AAEP). 2008.
  9. Pitt, J. I. Mycotoxins: Mycotoxins – General. Encyclopedia of Food Safety. 2014.
  10. Pollitt, C. C. Equine laminitis. Clinical Techniques in Equine Practice. 2004.
  11. Sendel, T. et al. Horse Foot Health. Ontario Ministry of Agriculture, Food and Rural Affairs. 2012.
  12. Strugstad, M. & Despotovski, S. A summary of extraction, properties and potential uses of juglone: a literature review. 2012.
  13. Swanson, T. D. Clinical Presentation, Diagnosis, and Prognosis of Acute Laminitis. Vet Clin North Am. 1999.
  14. Wilson, D. A. Clinical Veterinary Advisor. The Horse. 2011.
  15. Wright, B. et al. Black Walnut and Butternut Poisoning of Horses. Ontario Ministry of Agriculture, Food and Rural Affairs. n.d.
  16. van den Berg, M. et al. Browse-related behaviors of pastured horses in Australia: A survey. J Vet Behav. 2015.
  17. True RG, Lowe JE. Induced juglone toxicosis in ponies and horses. American Journal of Veterinary Research. 1980.
  18. Mouithys-Mickalad, A. Effects of Juglone on Neutrophil Degranulation and Myeloperoxidase Activity Related to Equine Laminitis. Front Vet Sci. 2021.
  19. Galey FD, Beasley VR, Schaeffer D, Davis LE. Effect of an aqueous extract of black walnut (Juglans nigra) on isolated equine digital vessels. American Journal of Veterinary Research. 1990.
  20. Uhlinger, C. Black walnut toxicosis in ten horses. J Am Vet Med Assoc. 1989.
  21. Loftus, J. Early laminar events involving endothelial activation in horses with black walnut- induced laminitis. Am J Vet Res. 2007.
  22. Belknap, J. Black walnut extract: an inflammatory model. Vet Clin North Am Equine Pract. 2010.
  23. Colorado State University. Black walnut. Guide to Poisonous Plants.