St. John's Wort Poisoning in Horses

St. John's Wort Poisoning in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

Most common symptoms

Anemia / Bleeding / Diarrhea / Lethargy / Premature Labor / Wound


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Most common symptoms

Anemia / Bleeding / Diarrhea / Lethargy / Premature Labor / Wound

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St. John's Wort Poisoning in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

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What is St. John's Wort Poisoning?

St John’s wort, or Klamath weed, is a plant that generally competes with other more useful plants in many pastures. As a matter of fact, large amounts of it, when noted in an area, can decrease property values. For animals, ingesting varying amounts of it can cause some problems which are largely inconvenient in terms of production for the owner but, more importantly, also very unhealthy for the animal, even to the point of death of the animal in question.

St John’s wort (hypericum perforatum) poisoning in horses is a toxicity which occurs as a result of the equine consuming the weed which contains hypericin, a chemical known to cause photosensitization in horses, sheep, cattle and goats.

Symptoms of St. John's Wort Poisoning in Horses

While the mature stage of this particular plant is not generally the first preference of your horse when grazing in the pasture, the horse will graze on it when other sources of food are not readily available. Sometimes, the plant will be more attractive to for consumption by your horse in its earlier stages. Regardless of the plant’s stage of maturation, the symptoms which are listed below can be seen within 5 hours or less of consumption of an amount toxic to the horse. These are some of the early signs:

  • Agitation
  • Head Rubbing - eventually rubbing the skin raw to the point of bleeding

  • Intermittent weakness of hind limb, with knuckling over
  • Panting
  • Confusion
  • Depression

As the toxicity progresses, you may also note:

  • Diarrhea - usually mild

  • Skin around the forehead and eyes will swell and become inflamed
  • Increased body temperature (hyperthermia) 

As the horse continues to eat the plant, the swelling and inflammation around the head and ears will increase, which results in the horse rubbing its head against solid objects to attempt to assuage the irritation. This will exacerbate the rubbing of the skin to a raw and bleeding state, with scabbing even being apparent.



The photosensitizing component of St John’s wort is the main contributor to the above mentioned symptoms. There are basically three types of photosensitivity involved in the chemical reaction involved in the ingestion of St John’s wort by your horse:

  • Primary hypersensitivity occurs after the plant is eaten, absorbed into the intestinal system and then travels to the skin, where the hypericin gets altered chemically by ultraviolet light through the skin - skin cell damage occurs, killing areas of skin which are white or lightly pigmented

  • Secondary hypersensitivity occurs with interaction of the chemicals in St John’s wort and oxygen after it has been absorbed into the bloodstream - the resulting changes to the chemical hypericin damage red blood cells
  • Another type (called hepatogenous photosensitivity) is also used in connection with any liver poisoning which may occur as a result of the liver’s inability to metabolize any photosensitive chemical

Causes of St. John's Wort Poisoning in Horses

There is really only one cause for St John’s wort poisoning and that cause is simply the ingestion of the plant by the horse. The ensuing chemical reaction which takes place inside the horse’s body (intestinal system and bloodstream) after it is eaten causes the photosensitivity, resulting in the damage to skin cells and to the red blood cells when the chemical hypericin is subjected to light and oxygen. Basically, the chemical reaction of the hypericin contained in St John’s wort to light and oxygen is at the root of the poisoning of your horse.

Diagnosis of St. John's Wort Poisoning in Horses

Diagnosis of St John’s wort poisoning will likely be done by thorough physical examination, history given by the owner of exposure to photosensitizing chemicals, lesions or hepatotoxins, characteristic lesions and observance of the horse’s behavior, with special attention being given to photophobia combined with erythema (appearance of windburn or sunburn) and edema (swelling) of hairless and non or lightly pigmented skin. Your vet may do blood work to obtain some specific biochemical values to help identify the cause of the signs and symptoms, especially as they apply to some levels of specific blood proteins.

You may be the first one to diagnose the problem since you will be closest to the horse when the symptoms are discovered. If you have never dealt with this type of poisoning, your vet will be your go-to source for information and treatments. Based upon the extent of the poisoning in your horse, and this assessment will be made contingent on the degree to which the symptoms are presenting and the results of any lab testing that has been deemed appropriate, your vet will recommend treatment options.


Treatment of St. John's Wort Poisoning in Horses

If St John’s wort poisoning is suspected, the most immediate treatment would be initiated by the owner in the removal of the horse from the pasture from which he has ingested the plant and keeping him in the shade. By avoiding sunlight, you will limit the chemical reaction that takes place within the horse’s body that causes the photosensitivity to wreak its havoc. The rest in shaded areas will likely need to be maintained for a period of 4 to 7 days, giving the horse the opportunity to shed through its excretions (urine and feces) the built up amounts of hypericin in its system. Once it has shed all of the accumulated hypericin, you can return the horse to the sunlight, making sure it has fresh, clean water and feed. The horse should be monitored to assure there is no further chemical or clinical response.

Of course, extra care and attention should be given to the broodmare as St John’s wort poisoning can result in weak or dead foals at birth as well as poor performance in young sucklings due to the mare’s ability to pass the poisoning on to her offspring through the milk. Other treatments may include the administration of corticosteroids and treatment of the wounds which have resulted from the attempts by the horse to relieve the irritation of the skin.

Recovery of St. John's Wort Poisoning in Horses

The prognosis for the horse with liver involvement is usually poorer than for those horses having primary photosensitivity. The latter group has a good prognosis with even the more severe lesions healing reasonably well with appropriate wound treatment. It would logically appear that, to prevent future episodes with your equine population, some sort of weed control measures would need to be implemented to reduce the availability of St John’s wort to your animals. These measures might include hand-weeding, broad scope weed control or even burning the field or pasture. Some of these options will, of course, have a greater effect on the pasture overall health than others but broad scope options (meaning broadcasting weed killers) can have poor effects on any animals in the area or grazing in the pasture. Your vet can guide you in balancing these concerns.

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St. John's Wort Poisoning Average Cost

From 301 quotes ranging from $1,000 - $5,000

Average Cost


St. John's Wort Poisoning Questions and Advice from Veterinary Professionals

St. John's Wort Poisoning Average Cost

From 301 quotes ranging from $1,000 - $5,000

Average Cost