What is Seasonal Pasture myopathy (SPM)?

  • An acute, highly fatal form of severe rhabdomyolysis affecting postural, respiratory, and cardiac muscles
  • A lipid storage myopathy caused by multiple acyl-CoA dehydrogenase deficiency (MADD), the same pathophysiologic cause of Atypical Myopathy (AM) in Europe; this causes a disruption of mitochondrial enzymatic pathways and leads to an accumulation of specific organic acids and enzyme-specific substrates (acyl-CoAs) that are transported in the bloodstream as acylcarnitines and eliminated in the urine as acylglycines, creating specific patterns of elevated serum acylcarnitines and urine organic acids
  • Caused by ingestion of hypoglycin A in box elder tree seeds in North America; AM is potentially caused by this as well
  • Possible contributing factors include antioxidant deficiency such as low selenium or vitamin E
  • One or multiple horses grazing on pasture can be affected
  • Cases have been documented in Eastern Canada, the Midwest, Texas, and the Southern USA but have likely gone unrecognized in many other areas of North America

What are the clinical signs?

  • Key differentials include acute onset colic, laminitis, and other causes of rhabdomyolysis
  • Mortality rate is 75-90%, even with treatment; true morbidity is unknown at this time
  • Initially: reluctance to move, muscular weakness, stiffness, fine muscle tremors, choke, dark urine
  • Later: Depression, sweating, congested mucous membranes, high heart rate and difficulty breathing
  • Progresses rapidly to lateral recumbency and death within 24-72 hours

Which horses are at risk?

Risk is highest in:

  • Horses exposed to box elder tree seeds
  • Horses pastured for over 12 hours daily
  • The fall season
  • Horses that do not receive supplemental hay while they are out on pasture, including horses of normal or excess body condition
  • Horses on sparse pasture with short grass
  • Horses on pasture with many trees and dead wood on the ground
  • Weeks when there has been heavy wind or rain
  • Horses that are on a pasture for their first season, such as a young horse or a horse that has recently moved to a farm

When does it occur?

  • In the Midwest, SPM usually occurs during wet cold fall weather before snow cover
  • Cases may occur during the spring and summer
  • The number of cases varies from year to year We investigated weather conditions in the Midwest for 14 previous cases of SPM:
    • 13 cases occurred from September to November, and 1 case during a particularly wet, cool May (mean temperature, 54oF).
    • 4 horses developed SPM 2 to 4 weeks prior to the first snowfall, 2 horses were evaluated 2 to 3 days after the first snowfall, and the remaining horses were evaluated 4 days to 6 weeks after the first snowfall
  • Heavy winds often occur within 5 days prior to horses becoming ill with SPM

How can I diagnose seasonal pasture myopathy?

  1. History and appropriate clinical signs in horses on pasture
  2. Rhabdomyolysis confirmed by marked elevations of serum muscle enzymes CK and AST and myoglobinuria
  3. Muscle biopsy or post mortem confirmation of severe myonecrosis of postural, respiratory muscles, and occasionally, cardiac muscle
  4. Excessive lipid staining in fresh intercostals, diaphragm or postural muscles (Oil red O) (fresh samples submitted to the Michigan State University Neuromuscular Diagnostic Laboratory).
  5. Assays of urine organic acids or serum acylcarnitines to confirm MADD can be submitted to the Baylor Metabolic Institute.  To access details on sample collection and submission contact this laboratory at http://www.baylorhealth.com/PH... .

How should I treat seasonal pasture myopathy?

  • There is no definitive treatment for SPM at this time. As such, early and aggressive supportive therapy is essential.
  • Whenever possible, immediate referral to a hospital is highly recommended.
  • Monitoring of cardiac function, electrolyte derangements, and kidney function is key.
  • Large volume intravenous fluids enriched with glucose, insulin, vitamin B2, vitamin C, and electrolytes, particularly calcium, is the mainstay of therapy.
  • Pain management using NSAIDs, lidocaine CRIs, and/or opioid derivatives is recommended.
  • Oral carnitine and vitamin E may provide additional benefit in these cases.

How should I manage co-grazing horses on pastures where a horse has had SPM?

  • Prohibit or minimize horses’ access to box elder trees and their seeds as best as possible in the fall.
  • Remove all horses from an affected pasture; there is a risk of developing SPM for up to 5 days after removing horses from the pasture where an SPM horse was just grazing
  • Monitor serum CK activity serially over the course of 1-2 weeks for potential new cases; consider therapy for any horse with abnormal serum CK activity
  • Do not put horses back on affected pastures until after the first three days of hard frost
  • If horses must be on pastures during an outbreak, select a pasture that is dry, has as few box elder seeds as possible, and limit horses’ turnout to less than 12 hours per day
  • Feed supplemental hay from feeders while horses are on the pasture; this includes feeding horses of normal and increased body condition
  • Ensure the horses have adequate nutrition, vitamin E, and a good vaccination and deworming protocol
  • Consider removing box elder trees that bear seeds from horse pastures

Who can veterinarians contact about this disease?

Dr. Stephanie Valberg: email at valbergs@cvm.msu.edu