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What is Black Locust Tree Poisoning?

The bark, seeds, leaves and twigs of the black locust tree are toxic to humans, cattle, poultry, sheep, and horses.  The black locust grows 40-100 feet tall and is most common in the southeastern states of the United States. The tree usually flowers in May or June. The flowers are creamy white and are not toxic. Other parts of the tree are highly poisonous; symptoms of locust tree poisoning can occur within one hour of consumption.  

If your horse is showing symptoms of locust tree poisoning it is important to call a veterinarian immediately. The horse should be removed from the pasture, to prevent him from ingesting any more toxins.  Black locust tree poisoning in horses can be fatal. The toxins rapidly attack the central nervous system and the gastrointestinal system.

The black locust tree (Robinia pseudoacacia) is extremely poisonous if consumed.  A horse only needs to ingest 0.04% of his body weight to be poisoned by the black locust tree.

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Symptoms of Black Locust Tree Poisoning in Horses

Symptoms of black locust tree poisoning may include:

  • Colic
  • Weakness
  • Renal kidney failure
  • Tachycardia (fast heartbeat)
  • Depression
  • Loss of appetite
  • Labored breathing
  • Bloody diarrhea
  • Paralysis
  • Pupil dilation
  • Cold limbs
  • Esophageal irritation
  • Gastrointestinal irritation

Causes of Black Locust Tree Poisoning in Horses

  • Horse ingests black locust tree bark, seeds, leaves and/or twigs
  • Machinery that is used to make feed is contaminated by black locust tree
  • Black locust poison can pass through digestive tract, manure can contaminate the pasture

Diagnosis of Black Locust Tree Poisoning in Horses

The equine veterinarian will take a medical history of your horse and will note the clinical signs you have observed and when they started. The veterinarian will then perform a physical exam on your horse which may include taking the horse’s temperature, heart rate and blood pressure. He may listen to the patient’s heart, gastrointestinal tract, and lungs with a stethoscope. The veterinarian may also palpate the horse’s abdomen and limbs. An ocular exam may also be performed on the patient due to pupil dilation.  The veterinarian may want to see the pasture where the horse usually forages.

There are no specific tests to verify that a horse has been poisoned by the black locust tree. The veterinarian may choose to do blood tests in order to rule out other disorders, but if the ingestion of the tree was witnessed, or the branches of the trees are obviously chewed, the diagnosis may be based on the symptoms alone.

Treatment of Black Locust Tree Poisoning in Horses

It is important to prevent further ingestion of the black locust tree by your horse. He should be removed from the pasture and placed in a quiet stall for the veterinary consult and treatment. In some cases, horses will be transported to a large animal facility for care. The veterinarian will start an intravenous line for administration of fluids to keep the horse hydrated and to maintain the electrolytes balance. 

Your horse may be given activated charcoal orally;  activated charcoal helps prevent the toxins from entering the bloodstream and organs.  Sometimes activated charcoal is mixed with the food additive sorbitol.  The sorbitol will help the horse move his bowels and eliminate the toxins within his stomach. 

The veterinarian may prescribe non-steroidal anti-inflammatory drugs (NSAIDs). NSAIDs can also help with pain management. Supplements with omega-3 fatty acids may also help with inflammation of the digestive tract.  If the patient has esophageal irritation his diet may need to be temporarily changed.  The veterinarian will let you know what the patient is able to eat. Stall rest, once home, may also be recommended for the patient.

Recovery of Black Locust Tree Poisoning in Horses

Recovery of black locust tree poisoning may take days to several weeks.  Patients that are treated shortly after ingesting the black locust tree have a better prognosis.  Follow-up visits will be necessary to monitor the horse’s progress. Horses recovering from black locust tree poisoning should not be ridden until the veterinarian has verified a full recovery.

It is important to avoid the recurrence of locust tree poisoning.  It is essential that the tree be removed from the pasture.  Pastures should be checked for other toxic trees or weeds.  A local agricultural agent can help you determine what plants need to be removed for the safety of your horse.  He can also assist in suggesting what grasses and legumes are beneficial for horses. 

New horses should be quarantined.  This will help avoid contaminating the pasture with toxic seeds or plants passed in the horse’s manure. Additionally, quarantining a new horse can also help determine if he has any contagious health conditions that can infect the current horses in the pasture.