Certain Maple Tree Seeds Can Kill Horses

This fall the British Equine Veterinary Association (BEVA) is warning of a perfect storm of conditions that might lead to an increased incidence of the muscle condition atypical myopathy (AM) in pastured horses in Great Britain. Atypical myopathy is a sudden onset disease that’s commonly fatal. Although veterinarians have recognized it for decades, researchers only identified the cause, hypoglycin A, in 2013.

The Risk

Hypoglycin A is most typically found in the sycamore tree (Acer pseudoplatanus), a type of maple. This is not the U.S. sycamore, Platanus occidentalis. However, a similar disease known as seasonal pasture myopathy (SPM) does occur in the United States due to consumption of Acer negundo, or box elder tree (also known as a boxelder maple or, in Canada, a Manitoba maple). This condition is most prevalent in the Midwestern United States and results from horses eating the tree’s seeds. Atypical myopathy also occurs due to seed consumption but might occur when dead wood or dead leaves are consumed, as well.

The level of the toxin varies from seed to seed, so the amount of seeds needed for the disease to occur can vary from fewer than 100 to several thousand. Symptoms can occur quickly and include weakened muscles, reluctance to walk, sudden stiffness and muscle tremors, sweating, depression, high heart rate, dark urine, collapse, and colic. The fatality rate is around 70%.

Due to the fruit’s winglike shape (each fruit contains two seeds), they can travel great distances, from a few hundred meters to as far as several kilometers. This means that even though you don’t have a tree in or near your pasture, your horses could still be at risk. In most situations horses aren’t particularly interested in eating these seeds, but sparse pastures cause horses to forage extensively and consume things they typically wouldn’t select. A lack of pasture combined with inadequate supplemental feed can result in an increased interest in seed consumption.

This year is a concern because British pastures are somewhat sparse after a dry summer, as is hay, which can lead to owners feeding less hay to make stores last longer. High winds associated with early storms have increased the risk of a considerable number of seeds falling at once and might also have carried seeds considerable distances.

Protecting Your Horse

Owners in Britain and the United States need to familiarize themselves with what these seeds look like and take necessary precautions, including:

  • Collecting seeds or fencing off areas where seeds are so horses can’t reach them.
  • Ensuring horses in sparse pastures receive adequate supplemental forage.
  • Monitoring horses carefully for clinical signs, which can occur up to four days after exposure.
  • Not overstocking pastures.
  • Ensuring horses are not drinking from water sources contaminated with seeds.
  • Limiting turnout while seeds are falling.

Spring is another time SPM and AM tend to occur, because seeds germinate in spring and the seedlings are another source of hypoglycin A. If you recognize clinical signs of disease, call your veterinarian immediately, because favorable outcomes are most likely with expedient veterinary intervention.