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What is Yellow Flag Poisoning ?

Yellow flag iris (Iris pseudacorus or water flag) is a perennial plant that grows across the wetlands, marshes, forests and ponds of regions of the United States and Canada. Able to flourish on both wet and dry ground, yellow flag iris is typically found growing in the woods, and along rivers and streams. While all parts of the yellow flag iris are poisonous, the roots and leaves closest to the soil – and those most available to wildlife -  are the most toxic. The distinguishable characteristics of the plant include its height, which may soar to over four feet tall, the substantial width of its large blue-grey leaves, and its large sunny yellow flowers that appear in April and flower through mid-summertime.

A common characteristic of most poisonous plants is the acrid taste that typically discourages livestock from ingesting it in large quantities. Poisonous plants not only give off a bitter taste, but also cause a burning sensation on the tongue. One sample is usually enough to satisfy curiosity and dissuade the horse from further consumption. Horses will not consume a toxic plant such as the yellow flag unless under a certain set of circumstances. In case of over-grazing or during times of food scarcity, horses will go in search of any kind of vegetation, including those they would otherwise avoid. Only the hungriest of horses will ingest enough of the yellow flag iris to become significantly ill. The plant is disagreeable enough that levels of toxicity typically fall between mild to moderate. If ingested, however, symptoms of toxicity will likely include gastro-intestinal distress. If the horse consumes a moderate amount of yellow flag iris, additional symptoms may include drooling and lethargy. Within 24-48 hours of the poisoning, most horses show vast improvement.

In the United States, the yellow flag iris grows predominately in New England, the Southeast and California. Many states have seen instances of widespread gastroenteritis in livestock due to shipments of hay containing the stems and flowers of yellow flag iris. Because there are so many species of poisonous plants, horse owners are encouraged to have their hay evaluated for any type of contamination. While the iris does not cause debilitating illness in most livestock, other plants, such as all members of the nightshade family, can cause neurological damage, failure of vital organs, and in some cases, death. It is recommended to stick with one supplier, or to carefully grow and manage your own hay.

After ingesting a small quantity of the poisonous yellow flag iris, a horse may experience signs of gastro-intestinal distress such as diarrhea.

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Symptoms of Yellow Flag Poisoning in Horses

The symptoms of yellow flag iris poisoning are typically mild to moderate, and generally recede within 24-48 hours. Symptoms include:

  • Drooling
  • Diarrhea
  • Lethargy

If a horse consumes this poisonous plant in large quantities, severe symptoms may arise and will require immediate treatment. These include:

  • Drooling
  • Tremors
  • Ataxia
  • Convulsions
  • Confusion and anxiety
  • Erratic behavior
  • Sudden death

Causes of Yellow Flag Poisoning in Horses

The  toxic aspects of yellow flag include Irisin, a plant juice that is found in all members of the family iridaceae. Pentacyclic terpenoids (zeorin, missourin and missouriensin) are the chemical compounds found in all parts of the plant, especially the rhizomes, which are the underground horizontal stems that produce the shoot and root system.

Diagnosis of Yellow Flag Poisoning in Horses

Diagnosis will require a history of the horse, including observations of symptoms, as well as any physical and behavioral changes since the poisoning. The veterinarian will conduct a full exam of the horse, and likely conduct a complete blood count (CBC), a serum chemistry profile, an abdominal ultrasound and other radiographs. Further testing will depend upon the degree of poisoning. In cases of suspected plant poisoning, it is always helpful to provide the veterinarian with samples of any unfamiliar plants in the grazing area.

Treatment of Yellow Flag Poisoning in Horses

If eaten in small quantities, the yellow flag will cause mild to moderate gastrointestinal distress and diarrhea. Some horses, unfortunately, develop a taste for some poisonous plants; in this case, the horse should be relocated until the offending plants and weeds are eliminated, either by manual removal, mowing or safe chemical control. 

On occasion, horses seek out particular plants due to a nutritional imbalance or deficiency. In this case, work with your vet to find the best diet for your horse.

Medications, such as a diuretic, are given to flush out the remaining toxicity. Another common treatment for poisoning is activated charcoal. Supportive care for nausea or diarrhea will help the horse restore feelings of health and well-being. Not all horses will return to same level of performance preceding the poisoning.

Recovery of Yellow Flag Poisoning in Horses

Continue medication as directed. Remove the horse from the pasture until poisonous plants are eliminated. Pasture maintenance will be essential going forward in order to prevent further poisoning. Manual hand-pulling and frequent mowing are helpful, and safe herbicides are available to reduce or eliminate yellow flag irises in the grazing area.