Pigweed Poisoning in Horses

Pigweed poisoning causes high levels of nitrites to be absorbed into the bloodstream.  The nitrates are then able to convert the hemoglobin into methemoglobin. The hemoglobin is the protein in the red blood cells that carry oxygen to the different organs.  Methemoglobin inhibits the normal release of oxygen to the body.

Pigweed (Amaranthus retroflexus) is a common annual weed found throughout the United States.  The weed can grow three to four feet; the flowers are green and prickly and the plant has oval shaped leaves. The pigweed’s leaves, roots and stems are toxic.

If your horse is showing signs of pigweed poisoning he need to be removed from the pasture he has been foraging in and placed in a quiet stall.  Contact a veterinarian as soon as possible. Pigweed poisoning can be fatal, if not treated promptly.

The pigweed plant has the ability to accumulate nitrates.  If pigweed is ingested by a horse, the high concentration of nitrates in the plant will cause his potassium levels to increase resulting in  possible kidney failure.

Symptoms may include:

• Difficulty breathing
• Mucous membranes are bluish/gray in color
• Weakness
• Tremors
• Loss of coordination
• Reduced appetite
• Pregnant mares may abort their fetus
• Knuckling of fetlock joints
• Decreased growth rate in young horses
• Rapid heartbeat
• Diarrhea
• Colic
• Coma
• Secondary hypothyroidism (due to nitrates hindering iodine)

Causes of pigweed poisoning may be from:

• The horse eats the pigweed while foraging in an overgrown pasture
• The absence of other good plants to forage on; overgrazed pasture
• Pigweed is inadvertently mixed into the hay

Any time a potentially poisonous plant is ingested, the veterinarian must be consulted. A walk through of the pasture may reveal an area that has been heavily grazed; point this out to the vet so that he can identify the poisonous plants that may be present.

Clinical signs, including weakness and respiratory issues, may indicate a poisoning. The physical examination could reveal that your horse is experiencing tremors and his mucous membranes may have a bluish tinge. 

The veterinarian will want to perform blood tests in order to analyse the activity of the red blood cells; with pigweed poisoning he may see markers indicative of damage to the cells, thereby causing an inhibition of oxygen transport throughout the bloodstream. Potassium, serum creatinine, and blood urea nitrogen levels may be increased. Pigweed poisoning has been suspected to cause secondary hypothyroidism; the vet may suggest a thyroid test to check the T3 and T4 level for verification. A urinalysis can help determine nitrate levels and kidney function.

The veterinarian may start an IV intravenous to keep the horse hydrated and to be able to administer medications intravenously.  To counteract nitrate poisoning the veterinarian may administer a solution of methylene blue in isotonic saline.  Methylene blue will help restore the red blood cells ability to carry oxygen throughout the body. In addition, the vet may add glucose and insulin as hyperkalemia may be a concern. If hypothyroidism is diagnosed concurrently with the poisoning, this will be treated accordingly. Iodized salt should be made available in the future as it does have benefits against goiter caused by nitrate toxicity.

Horses that are treated in the early stages of pigweed poisoning have a good prognosis.  Follow up visits will be needed in order to retake bloodwork and a urinalysis in order to check nitrate and potassium levels.

To prevent pigweed poisoning from reoccurring, the toxic plant should be permanently removed from the pasture. The pigweed plant is not only toxic to animals, it also affect the nitrate level of the soil. The pasture needs to properly be maintained and efforts made to remove toxic plants.  Herbicides can help kill toxic weeds. Manually removing toxic plants from the roots is another option. 

A local agricultural expert can help you to identify toxic plants that can may be on your property. Nutritional grasses and legumes should be introduced in the pasture for the horse to forage on.