Seasonal Pasture Myopathy Average Cost

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What is Seasonal Pasture Myopathy?

Although Boxelders and Sycamores are considered toxic to horses, there are some horses that seem to be immune or maybe just do not like to eat the seeds. Younger horses and those that are left out to pasture more than 12 hours per day are definitely more susceptible to this condition, although any horse can experience seasonal pasture myopathy. Even horses on pastures without Boxelder or Sycamore trees have been known to get this condition, probably because the helicopter seeds have been  transferred with the wind from other areas.

Seasonal pasture myopathy (SPM) is a fatal condition in horses that can be prevented by keeping your horses away from the Boxelder Maple tree (Acer negundo) and the  Sycamore tree (Acer pseudoplatanus). Studies show that horses found to have SPM had access to Boxelders or Sycamores and were even more susceptible when the fields were over grazed, and horses had more than 12-hour access to the trees. 

The seeds (helicopters) of the Box Elder and Sycamore are said to contain the toxin hypoglycin A, which destroys the muscles needed for standing, breathing, and even affects the cardiac muscles needed for the heart to beat properly. This condition can be fatal in a horse within just three days. If you believe your horse has been consuming the seeds of one of these trees, you should immediately take your horse to an equine veterinary professional, even if you have not noticed any symptoms.

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Symptoms of Seasonal Pasture Myopathy in Horses

The symptoms of SPM progress very fast. In fact, this disease is fatal within 72 hours if not discovered and treated immediately. These signs are the most common:

  • Weakened muscles
  • Apathy
  • Fatigue
  • Darkened mucous membranes
  • Muscle tremors
  • Refusal to exercise or move
  • Recumbency (inability to get up after lying down)
  • Rapid and irregular heart rate
  • Dark colored urine
  • Occasional choke
  • Breathing difficulty
  • Respiratory failure
  • Collapse
  • Coma
  • Death


There are two types of SPM, which are actually thought to be the same illness, but have different names.

  • Seasonal pasture myopathy (SPM)
  • Atypical myopathy (AM) is the European version of SPM

Causes of Seasonal Pasture Myopathy in Horses

The toxin, hypoglycin A, is known to be found in the seeds of Boxelder maple and Sycamore trees. After consumption, hypoglycin A is turned into methylene cyclopropyl acetic acid (MCPA), which prevents the absorption and metabolism of fatty acids. This causes increased weakness that progresses to coma and then death.

Diagnosis of Seasonal Pasture Myopathy in Horses

The veterinarian will note any recent illnesses and injuries as well as any medications or supplements your horse is on. The physical examination will include a detailed lameness assessment as well as vital signs and overall body condition. Diagnostic tests needed include routine blood tests, urinalysis, and a chemical analysis. In a horse with SPM, myofiber lipid is markedly changed and the urinalysis will show an abnormal amount of organic acids in the urine. A muscle biopsy will be done with a fine needle cytology extraction. This will most likely indicate acquired multiple acyl-CoA dehydrogenase deficiency (MADD) syndrome from riboflavin deficiency. In addition, the veterinarian will likely want to get radiographs (x-rays) and a bone scan.

Treatment of Seasonal Pasture Myopathy in Horses

Treating your horse for SPM depends on how long ago the seeds were consumed and how much was ingested. If your companion only ate a small amount and there are no symptoms, the veterinarian may not do anything but keep your horse for observation for 12-24 hours. If symptoms have not started, but you know your horse ate a great deal of seeds, the veterinarian will likely treat your horse immediately with intravenous (IV) fluids and charcoal therapy. Additionally, supportive treatments such as medication, oxygen, and electrolytes will be administered.


The veterinarian will administer activated charcoal with a nasal tube. This helps to absorb and expel the toxins so they are not absorbed by the intestinal tract. Intravenous (IV) fluids will also be given to flush the kidneys and prevent dehydration.

Oxygen Therapy

If needed, your horse will be given oxygen with a nasal cannula or face mask. This is especially calming and helpful if your horse is anxious.


Muscle relaxants, nonsteroidal anti-inflammatory drugs (NSAIDs), electrolytes, and possibly antibiotics will be added to the IV as needed. Vitamin C and other supplements may be given as well to help boost your horse’s immune system.

Recovery of Seasonal Pasture Myopathy in Horses

Unfortunately, close to 75% of horses diagnosed with SPM do not survive even with treatment. The best chance for survival is prevention, but if you suspect your horse may have SPM you should make a trip to the closest equine veterinary professional immediately.