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What is Penstemon Poisoning?

Penstemon is also known as beard tongue and can be found widespread in the United States. It has tubular flowers that can be white, blue, pink or red. Penstemon poisoning can cause damage to the lungs, liver, immune system and the kidneys. If your horse is showing signs of selenium poisoning (such as cracked hooves, dull coat and weakness), he should be removed from the pasture and a call made to the veterinarian. 

Determining selenium amounts in the soil, water, hay, grain, supplements and pasture forage may be desirable, particularly if your horse is susceptible to tying up. The age of your horse and his current health status can influence selenium needs.

The herbaceous perennial penstemon (penstemon calycosus) is capable of storing high levels of selenium. Selenium is beneficial for the immune system and also acts as an antioxidant, but a high level of selenium is toxic to your horse. Therefore, if a horse consumes the leaves, flowers or stems of the penstemon he will experience a variety of problems.

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Symptoms of Penstemon Poisoning in Horses

Acute symptoms may include:

  • Respiratory distress 
  • Weakness
  • Depression
  • Diarrhea
  • Lack of appetite
  • Fever
  • Tachycardia
  • Teeth grinding
  • Incoordination
  • Fluids in the lungs
  • Damage to liver, kidneys and/or lungs

Chronic symptoms may include:

  • Dull hair coat
  • Loss of hair on the mane and tail
  • Anemia
  • Joint stiffness
  • Lameness
  • Weight loss
  • Hooves may have deformities, circular bumps

Causes of Penstemon Poisoning in Horses

Selenium poisoning in horses is caused by the ingestion of the penstemon plant in large or consistently small amounts.  The level of the selenium can vary from mild to severe. Horses typically do not eat the penstemon because it is not very palatable. However, your horse may eat penstemon while foraging in an overgrazed pasture. Penstemon may accidentally be mixed into hay as well.

Diagnosis of Penstemon Poisoning in Horses

Diagnostic tests that the veterinarian may recommend are a complete blood count (CBC), serum chemistry panel and a urinalysis. These blood tests can evaluate how the high level of selenium is; one to four ppm is considered toxic in a chronic situation.  An acute case of toxicity will show a level of 25 ppm. If the liver and kidneys have been affected, the selenium level will read above 4 ppm.

A hair analysis can indicate poisoning by penstemon; not only will the hair of the mane and tail be thin, the hair analysis can determine selenium concentrations that occurred months or years before the test was taken.  The condition of your horse’s hooves may point to toxicity with ridges, cracks, and sloughing evident.

Treatment of Penstemon Poisoning in Horses

Additional sulphur and copper in the diet can help counteract the effects of elevated selenium. Your horse should be fed a diet low in selenium and high in amino acids. Alfalfa hay is often recommended as is grazing in areas abundant with green grass. Horses that were diagnosed with anemia may need to have B12 shots. Supplementation with vitamin E is often suggested.  If the patient had cracked hoofs, the veterinarian may prescribe antibiotics to help prevent a bacterial infection.

The veterinarian may also refer you to a reputable farrier that can correct any hoof abnormalities and maintain the condition of the feet.

Recovery of Penstemon Poisoning in Horses

Once penstemon poisoning is treated, the recovery prognosis is good for the patient.  Follow-up visits will be required to monitor the horse’s progress. The veterinarian may want to have the selenium level rechecked, to ensure that the levels are going down.  

The horse should not be allowed in the pasture until the penstemon plants are removed. The surrounding soil should be checked for selenium levels as should any water source on the property. The pasture needs to properly be maintained, fertilized and routinely checked for toxic plants, with the application of horse safe herbicides performed regularly.