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What is Yarrow Poisoning?

Achillea millefolium, more commonly known as yarrow, is a stiff-stemmed plant that grows wild throughout the United States, and parts of Europe and Asia. It has clusters of small white flowers with yellow centers and is frequently found growing wild along roadways. Historically this plant has been used as a treatment to stop bleeding and reduce bruising, however, in large enough doses the various glycoalkaloids, saponins, and other chemical compounds contained in the plant can cause damage to the mucous membranes, gastrointestinal upset, and occasional photosensitivity.

Yarrow, a stiff-stemmed plant with clusters of small white flowers, can be utilized to help heal bleeding or bruising in small quantities. In large quantities, however, it can be toxic.

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Symptoms of Yarrow Poisoning in Horses

Your horse may present with irritation to the mucous membranes, particularly in the mouth, and gastrointestinal upset which could include the development of diarrhea or colic. Signs of photosensitization can occur on any part of the body but are most often found in places where the hair is sparse or in in areas of low pigment. These signs can include: 

  • Bumps on skin
  • Mouth ulcers
  • Oozing sores 
  • Peeling or reddened skin

Types

There are several ways in which yarrow has been used for the healing of horses in the past.

  • Bruises - This plant is an excellent external treatment for bruising as well, reducing both the pain and the swelling
  • Calming agent - The scent is said to soothe horses and may be planted near pastures to ease anxieties
  • Hemostatic - Yarrow is also used externally to slow or stop bleeding as well as supporting circulation to the peripheral blood vessels; in small amounts, it has been known to reduce or eliminate internal bleeding as well

It is essential that you consult a veterinary professional before starting any treatment plans to avoid risking a reaction with another medication or overdosing on the plant.

Causes of Yarrow Poisoning in Horses

Achillea millefolium contains several toxic compounds which can cause harm to horses that eat large quantities of this plant. These compounds can include glycoalkaloids (notably the glycoalkaloid achilline), monoterpenes, and lactones. These naturally occurring chemicals are not only responsible for the toxicity of the plant, but also the healing abilities.

Diagnosis of Yarrow Poisoning in Horses

The proper identification of the plant in the pasture may be sufficient to make an initial diagnosis. If you can obtain a sample of the plant, this will assist in the correct identification; there are a few plants that look comparable to yarrow, but are sometimes even more poisonous, such as White Snakeroot and Poison Hemlock. Your veterinarian will ask you questions regarding information about the amount of plant material consumed, how long it has been since it was eaten, and which part of the plant was ingested. They will also need to get a comprehensive history of the horse which includes getting as much knowledge as possible about the horse’s diet, medications, and environmental elements. 

Standard blood tests typically include a biochemistry profile and a complete blood count which will be evaluated to see if any infections are present as well as establishing the levels of liver and kidney enzymes found in the blood. Phototoxicity is generally more conspicuous on areas that have low pigment or that are the most exposed to the sun. A sample of any skin that has been affected by phototoxicity will be evaluated by cutaneous cytology. This microscopic examination will help reveal if fungal infections, mites, or allergies are causing the rash. If that does not locate the underlying cause, a biopsy of the tissue will be required for further evaluation.

Treatment of Yarrow Poisoning in Horses

The horse should immediately be moved out of the sun if any photosensitivity is seen. If the consumption of large quantities of the plant material was recent, then your veterinarian may choose to perform a gastric lavage in order to physically purge as much of the toxin from the digestive system as possible. Activated charcoal is also commonly administered prevent further absorption of the toxic compound into the bloodstream. There is no antidote for these compounds, making treatment other than decontamination mostly supportive. Treatments may include IV fluids to prevent dehydration as well as combinations of electrolytes and sugars in order to adjust for any imbalances that may have developed. 

Avoiding sunlight for 48 hours after the exposure will significantly reduce the likelihood that skin damage will develop, however, if skin damage occurs, medications such as corticosteroids, anti-inflammatories, and antihistamines may be prescribed for the animal as needed to ease the discomfort. Medications like this may be administered either orally, topically, or by injection.

Recovery of Yarrow Poisoning in Horses

The prognosis for horses who have eaten toxic quantities of yarrow is usually relatively good, although liver damage is possible with most poisons, and yarrow root is no different. Infections and infestations can develop if photosensitive skin is not kept cool and clean. Flies are often attracted to the skin while it is damaged, and will need to be kept away to avoid maggot infestation. 

Although the initial symptoms of photosensitivity are reduced after about 48 hours, some sensitivity to the sun may remain. Removing as much of the yarrow from the grazing pasture as possible will help prevent ingesting overabundant quantities of this plant.