Equine Atypical Myopathy, also known as Atypical Myoglobinuria, is a frequently fatal condition affecting autumn and/or spring grazing horses. The disease’s clinical signs seem to appear after the occurrence of specific climatic conditions.

The condition was recognised in 1984 and since then, several clinical cases have been described in different countries in Europe. Belgium was first confronted with a major outbreak in 2000. In France, the first cases have been described in autumn 2002. Each year in Ireland, there have been from six to 12 cases diagnosed with perhaps many more occurring but not identified.

A typical story is of a horse starting to show stiffness and a reluctance to move. His muscles suddenly become weak to the point he can no longer remain standing. Then, as quickly as clinical signs set in, the horse dies. Just 48 hours earlier, the horse grazed happily in his pasture - an overgrazed field full of seed heads and dead leaves.

This story is typical of suspected cases of seasonal pasture myopathy (SPM), a highly fatal muscle disease described in the Midwestern United States and eastern Canada, and atypical myopathy (AM) in Britain and Europe.

Horses that develop Atypical Myoglobinuria are usually kept on sparse pastures with an accumulation of dead leaves, dead wood and trees in or around the pastures.

Unlike other muscle disorders, the disease’s clinical signs are not associated with exercise and include:

These clinical signs progress quickly with rapid respiratory rate within 48 hours and dyspnoea (difficulty breathing) and death within 72 hours in at least 75% of cases. The cause of death is a very specific metabolic block in the muscle’s ability to burn fat for fuel.

For decades the disease had baffled veterinary surgeons on both continents, who struggled to pinpoint and agree on a cause.

That changed in 2011 when Stephanie Valberg, DVM, Dipl. ACVIM, and a team of researchers from University of Minnesota (UM) and Iowa State University (ISU) started investigating SPM cases and found a link to box elder trees. She presented their findings in “Identification of the Cause of Seasonal Pasture Myopathy in Horses” at the 2014 American College of Veterinary Medicine Forum, held in Seattle, Washington.

Horses at SPM farms were less likely to receive supplemental hay or concentrate and were in proximity to the box elder trees.

Thus, the common features for this condition was little supplemental feeding and prolonged grazing of sparse pastures that were in close proximity to seed-laden box elder trees. Testing found that one amino acid, hypoglycin A, was “highly abundant” in the box elder seeds. This amino acid, also found in unripe fruit of the ackee tree (within the same tree family as box elder), is known to block the same specific enzyme in fat metabolism in humans as was seen in SPM-afflicted horses.

Most recently, scientists have found another related tree, the European sycamore maple tree (Acer pseudoplatanus), in northern European pastures where horses have died from Atypical Myoglobinuria. European sycamore maple seeds also contain hypoglycin A.

A paper just published in the March 2014 edition of the Equine Veterinary Journal by Votion et al has concluded that Atypical Myopathy in Europe, like seasonal pasture myopathy in North America, is highly associated with the toxic metabolite of hypoglycin A. MCPA-carnitine. This finding coupled with the presence of a tree of which whose seeds are known to also contain hypoglycin A indicates that ingestion of Acer pseudoplatanus (Sycamore tree seeds) is the probable cause of Atypical Myopathy. This finding has major implications for the prevention of Atypical Myopathy or Myoglobinuria.

Protecting your horses

Some horses do not develop Atypical Myopathy after years of living on affected pastures. This can make discussions with owners regarding prevention complicated.

Young horses and those new to an affected pasture appear to be at great risk if:

To address potential concerns of these muscle diseases developing in horses, removal of the box elder or European sycamore trees from affected pastures can be one approach, although, this might not always be feasible.

While box elders rarely live longer than 50 years, some of the sycamore maples on affected pastures are huge beautiful trees that owners may be very reluctant to remove.

In cases where the trees can’t be removed, it is recommended decreasing turnout time on affected pastures from October through mid-December and in the early spring.

Other important preventive measures could include providing additional forage if pastures are overgrazed, preventing over grazing of pastures through rotational grazing, and limiting turnout to less than 12 hours per day during autumn and early spring.

The condition has gone from a sporadic condition to more frequent large outbreaks throughout Europe. Climate change may have increased Sycamore tree seed production, seed dispersal by weather or toxin concentrations in seeds. It is also possible that increasing awareness of the disease has resulted in increasing reporting of Atypical myopathy, rather than an increased incidence.

There is no doubt that further research is needed to prove this link conclusively but the sycamore seed is the highly likely cause of this until now mysterious and more often than not fatal disease.

Michael Sadlier MVB MRCVS is a partner in Troytown/Grey Abbey Veterinary Hospital, Co Kildare, and is a member of the Equine Group of Veterinary Ireland.

Email: hq@vetireland.ie

Telephone: 01-4577976