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What is Fireweed Poisoning?

The fireweed plant can be easily identified by a characteristic yellow 13 petal-like ray. Found worldwide, they are more common in Australia, Hawaii, and Florida. They often are found in paddocks and along fence lines, or in ground that is sparsely populated with grass, has been cultivated or disturbed, and areas wrought with drought. Once established, fireweed can reduce grazing pasture capacity, decrease growth rates of animals ingesting it, and incur a high cost to eliminate it.

Fireweed, or Senecio madagascariensis, is a pretty yellow flowered weed that is highly poisonous to horses. While many horses will not eat this bitter plant, most ingestions occur from contaminated feed. Fireweed poisoning is often gradual, and ingestion occurs over a long period of time. Unfortunately, the alkaloids in fireweed cause irreversible liver damage, which can result in liver failure or death.

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Symptoms of Fireweed Poisoning in Horses

Symptoms of a fireweed poisoning can vary, and relate to brain damage, liver failure, and skin issues. While most horses are poisoned through ingestion, there are cases where a horse can roll around in fireweed, causing skin hives and irritations. Due to the fact that most poisonings are from contaminated feed, doses are low over long periods of time, causing symptoms to appear months after an initial exposure. Symptoms include:

  • Poor growth rates in young horses
  • Loss of appetite
  • Loss of condition
  • Depression
  • Colic 
  • Progressive weakness
  • Reluctance to move
  • Difficulty swallowing
  • Difficulty breathing
  • Weight loss
  • Jaundice
  • Abdominal swelling
  • Diarrhea
  • Constipation 
  • Bloodstained feces
  • Rectal prolapse
  • Irritability
  • Sleepiness
  • Dullness
  • Poor performance 
  • Aimless wandering
  • Circling
  • Yawning 
  • Head pressing
  • Chewing motions
  • Pressing against or walking into objects, such as trees, gates or fences
  • Incoordination
  • Muscle twitching
  • Altered behavior
  • Pica
  • Reduced awareness
  • Photosensitivity that affects light colored skin areas 
  • Fluid in the lungs
  • Blindness
  • Liver failure
  • Sudden death 

Signs your horse has rolled around fireweed include:

  • Severe hives
  • Skin irritation

Causes of Fireweed Poisoning in Horses

Fireweed poisoning in horses is caused by the ingestion of the fireweed plant, which contains pyrrolizidine alkaloids. If a horse accidentally rolls around in fireweed, mild skin irritation can occur. However, if these toxic chemicals are ingested, they can cause liver damage and photosensitization, as well as a variety of other symptoms in your horse. All parts of the fireweed plant are toxic whether fresh or dried, including their seeds. Horses are often poisoned from repeated ingestions of fireweed, and toxicity can occur over a very long period of time. 2% to 5% of a horse’s body weight must be consumed before liver disease occurs. While horses will generally avoid bitter tasting fireweed, there are occasions when they could ingest it. These include:

  • Hay or feed that has been contaminated with fireweed
  • In overgrazed pastures, or pastures will little grass and lots of fireweed
  • Boredom
  • Lack of a better food source
  • Mineral deficiencies
  • Drought conditions

Diagnosis of Fireweed Poisoning in Horses

Diagnosis can be difficult if you have not witnessed your horse eating fireweed, or noticed it on your property. As with any plant poisoning, bring in a sample of any suspected plant your horse may have eaten to be identified during your veterinary visit. A diagnosis can be made upon a positive identification of fireweed, and any symptoms seen in the past or currently present. A physical exam is followed by a number of possible tests to confirm the presence of fireweed or pyrrolizidine alkaloids in your horse.

Tests can include biopsies of tissues, blood tests, and an ELISA test. If liver function is suspected to be compromised, liver function, liver enzyme tests, or a liver biopsy may be performed. 

If you or your veterinarian do not suspect fireweed to be the cause, your vet may order a series of other tests to eliminate other possible conditions, such as a mineral deficiency or internal parasites, before looking at the possibility of a plant poisoning.

Treatment of Fireweed Poisoning in Horses

By the time symptoms appear, liver function may have already been compromised by 80%. Liver damage is irreversible, and there is no antidote for pyrrolizidine alkaloids. Therefore, supportive care is often the only treatment recommended.

The first step of treatment is to remove fireweed from your horse’s reach. This may include bales of contaminated hay, or the removal of the plant from pastures. If your horse merely rolled in the weed, then a cool hose bath and anti-inflammatories should help your horse to recover.

Antioxidant and anti-inflammatory medications are prescribed for those horses who have ingested fireweed. Milk thistle extract can also help. A change of diet to a high energy and correct protein level feed may be needed. Fluid therapy may also be administered. Further treatments may be given for photosensitization symptoms as needed.

Recovery of Fireweed Poisoning in Horses

For horses that have only had skin contact with fireweed, recovery can be good. However, since symptoms of a poisoning from ingested fireweed occur after a long period of exposure, the condition is often quite progressed with significant liver damage once diagnosed. Recovery in these cases is poor. Prevent your horse from being poisoned by fireweed through management strategies aimed at controlling your horse’s exposure to the plant. These can include:

  • Learn to identify fireweed
  • Monitor pastures and areas your horse may frequent, including trails, for signs of fireweed
  • Eliminate fireweed from pastures through hand digging, mowing or herbicides 
  • Do not overgraze pastures
  • Monitor hay bales, grain and feed for possible contamination
  • Cultivate healthy legumes or grasses to compete with fireweed